Concrete Calculator

Work out how many bags of concrete and how many cubic yards a slab, round pad, footing, wall, column, post hole or set of stairs needs — with an editable waste allowance, real published bag yields and a stated bags-versus-ready-mix threshold.

What are you pouring?

Length

Use decimals — 10 ft 6 in is 10.5.

Width

Thickness

How many of these?

Waste allowance

Bag size

Price per bag (optional)

USD

Ready-mix, price per cubic yard (optional)

USD

80 lb bags needed

62

0.60 cu ft per bag · includes a 10% allowance · rounded up to whole bags

Concrete needed

1.36 cu yd

36.67 cu ft · 1.04 m³

1.23 cu yd before the 10% allowance

Dry mix to carry

4,960 lb

2.5 short tons · 62 bags × 80 lb

1.36 cu yd — either works, and this is where the money is decided. 62 × 80 lb bags is 4,960 lb to carry and mix; NRMCA advises against ready-mix loads under 4 cu yd and suppliers commonly add a short-load fee, so get a quote before you buy the bags.

Includes a 10% allowance. NRMCA advises 4–10% over the plan dimensions for waste, over-excavation and form movement.

Mixed and cured, that is about 5,133 lb of concrete at 140 lb per cu ft — the dry mix plus the water you add at the mixer.

How many bags, by bag size

Bag size	Yield per bag (cu ft)	Bags needed	Total dry weight (lb)
40 lb	0.30	123	4,920
50 lb	0.375	98	4,900
60 lb	0.45	82	4,920
80 lb	0.60	62	4,960

For 36.67 cu ft including the 10% allowance. Yields are the manufacturers' published figures, not the bag weight divided by a density.

Bags per cubic yard

Bag size	Published yield (cu ft)	Bags per cu yd	Source
40 lb	0.30	90	QUIKRETE and Sakrete
50 lb	0.375	72	QUIKRETE only
60 lb	0.45	60	QUIKRETE and Sakrete
80 lb	0.60	45	QUIKRETE and Sakrete

27 cu ft divided by the published yield, rounded up. Two of the four biggest concrete calculators divide the finished weight by the bag weight instead, which charges you for the mixing water: their charts print 101, 81, 67 and 51.

Coverage by thickness

Thickness (in)	Sq ft per 80-lb bag	Sq ft per cu yd	80-lb bags per 100 sq ft
2	3.6	162	28
3	2.4	108	42
4	1.8	81	56
5	1.44	64.8	70
6	1.2	54	84
8	0.9	40.5	112
12	0.6	27	167

80-lb bags throughout, so these figures stay quotable. For another size, divide your cubic feet by its published yield in the table above.

Bags or a ready-mix truck?

Under 1 cu yd — fewer than 45 80-lb bags. Buy bags. A ready-mix truck's minimum load is more concrete than the job needs, and you pay for what you do not use.
1 to 4 cu yd — either. NRMCA advises against ordering loads under 4 cubic yards, and suppliers commonly charge a short-load fee on top of the yardage. The fee is regional, so the crossover is a phone call rather than a number this page can print.
4 cu yd and up — order ready-mix. That is 180 80-lb bags, 14,400 lb of dry mix, 7.2 short tons to carry.

The 4-cubic-yard floor is NRMCA CIP 31's own figure. Bag counts here are 80-lb bags at their published 0.60 cu ft yield.

How this is calculated

Slab: 10 ft × 10 ft × 4 in ÷ 12 = 33.33 cu ft.

Round slab: π × (8 ft ÷ 2)² × 4 in ÷ 12 = 33.33 cu ft.

Footing: 20 ft × 16 in ÷ 12 × 8 in ÷ 12 = 33.33 cu ft.

Wall: 20 ft × 4 ft × 8 in ÷ 12 = 33.33 cu ft.

Column: π × (12 in ÷ 12 ÷ 2)² × 4 ft = 33.33 cu ft.

Post hole: π × (12 in ÷ 12 ÷ 2)² × 30 in ÷ 12 = 33.33 cu ft, less a 3.5 in post at 0.00 cu ft = 33.33 cu ft.

Stairs: 4 ft × 7 in ÷ 12 × [11 in ÷ 12 × 4(4−1)÷2 + 12 in ÷ 12 × 4] = 33.33 cu ft. 4 risers means 0 treads plus the landing.

× 1 identical pours = 33.33 cu ft.

+ 10% allowance: 33.33 × 1.10 = 36.67 cu ft.

36.67 cu ft ÷ 0.60 cu ft per bag = 61.11, rounded up to 62 bags.

Different sizes? Run each one and add the bag counts — this page does one shape at a time.

Volume is never rounded up. The 10% allowance is added first (33.33 → 36.67 cu ft), then the bag count is rounded up once; the weight is the 62 whole bags you carry, not the fraction.

Bag yields are the published figures from the QUIKRETE Concrete Mix No. 1101 and Sakrete High Strength data sheets; the 4–10% ordering allowance is NRMCA CIP 31. Both are listed under the calculator.

Working in metres? The volume converter turns cubic yards and cubic feet into cubic metres.

Concrete calculator. Bags, cubic yards and the weight for a slab, post hole, footing or stairs.

A concrete calculator turns the dimensions of a pour into the cubic yards it takes and the whole bags you have to buy. This one covers seven shapes, adds an editable waste allowance before it divides by the manufacturers' published bag yields, and says whether the job is a bag job or a ready-mix load.

Bags, Cubic Yards, and Why Two Calculators Disagree About the Same Slab

Concrete is sold two ways and the arithmetic behind each one is different. Ready-mix comes by the cubic yard, so the volume of the pour is the order. Bagged mix comes by the bag, and the count is the volume divided by the published yield printed on the bag — 0.60 cu ft for an 80 lb bag, 0.45 for a 60 lb, 0.375 for a 50 lb and 0.30 for a 40 lb, which is 45, 60, 72 and 90 bags to the cubic yard. Sakrete states that order of operations on its own data sheet: multiply length by width by thickness in inches, divide by 12, then "divide by the yield in the chart above to determine the numbers of bags needed." It is a volume divided by a volume, with no weight anywhere in it.

A 10 × 10 ft slab 4 in thick is 33.33 cu ft, 1.23 cu yd, and 56 whole 80-lb bags. That 56 is the figure almost every page on the subject prints. Concrete Network states that "for a 4-inch thick, 100-square-foot slab, you'll need 56 bags"; howmuchconcrete.io answers the identical slab with "56 bags" and "1.23 cubic yards". It is the correct answer to a pour where the subgrade is dead flat, the forms never move and nothing spills. Add the 10% allowance this page opens with and the same slab reads 36.67 cu ft, 1.36 cu yd and 62 bags — six more bags, 4,960 lb to carry instead of 4,480.

Where other bag counts go wrong is the step before that division. Omni Calculator's concrete-slab page sets a density of 150 lb/cu ft and prints 101, 81, 67 and 51 bags to the cubic yard for the 40, 50, 60 and 80 lb sizes, against the published 90, 72, 60 and 45. Checked in August 2026, that is about 12% more bags than the manufacturers say the job needs — six extra 80-lb bags on every cubic yard — and the cause is structural rather than a typo. The route that produces it divides a finished weight by a dry bag weight: 27 cu ft at 150 lb/cu ft is 4,050 lb, and 4,050 ÷ 80 is 50.6, which rounds to the 51 the chart shows. The mixing water is in the numerator and missing from the denominator. Keep the two consistent and the published figure comes back: a cubic yard at the 140 lb/cu ft that QUIKRETE's own ASTM C138 line gives weighs 3,780 lb, one 80 lb bag makes 0.60 cu ft of that weighing about 84 lb, and 3,780 ÷ 84 is 45 exactly.

The allowance is not padding either. NRMCA's CIP 31 says to "order about 4% to 10% more concrete than is estimated from a volumetric calculation of the plan dimensions", and CIP 8 gives the reason in one line: an ⅛-inch error in the depth of a 4-inch slab is already a 3% shortage. Of the five general-purpose calculators checked for this page, only Omni Calculator gives you a field for it; the rest say something similar in prose and leave the arithmetic to the reader. Inch Calculator writes "Add 5%-10% additional material to account for settling, spillage, and waste" with no field to do it in. CalculatorSoup writes "Add 5-10% for waste, spillage, uneven subgrade and over-excavation". Concrete Network writes "Never order the exact amount of needed concrete." QUIKRETE's own calculator notes that its yields "do not allow for uneven substrate, waste, etc." and that "all calculations are rounded up to the next highest whole bag". Here the allowance is a field with 10% already in it, and the order is fixed: the allowance goes on the volume first, then the bag count is rounded up once, at the end.

The shape that separates this page from the rest is the post hole. A 4×4 post measures 3.5 in on a side, not 4, and it stands in the hole you are filling. In a 12-in hole 30 in deep the hole is 1.96 cu ft, the post occupies 0.21 cu ft of it — 10.8%, about the size of the entire waste allowance — and 1.75 cu ft is what you actually mix. None of the pages checked subtracts it. calculator.net's hole and column block asks for a diameter, a depth and a quantity, with nowhere to declare a post; CalculatorSoup, Inch Calculator and Concrete Network never raise the post at all; the two fence-post guides ranking for the query fill the cylinder solid. MudMixer's article opens by saying a 4×4 in a 10–12 in hole "typically needs 1–2 bags of 60 lb concrete, or one 80 lb bag", while its own table further down gives 3–4 eighty-pound bags for a 12-in hole 30 in deep. The spread inside one page is a factor of three, and the reason is that a rule of thumb has to leave the depth out.

Seven shapes share the form: slab or patio, round slab or pad, footing or trench, wall or stem wall, column or tube form, post hole with the post subtracted, and stairs. Each takes its own two to five dimensions plus a count for repeated pours, and they all share the allowance, the bag size and the two optional price fields. The cubic-yard card carries the metric equivalent underneath it — 1.36 cu yd reads 1.04 m³ — and the volume converter handles the rest if your supplier quotes in meters.

Twelve Real Pours, From One Post Hole to a 24-Foot Driveway

The pour	Concrete needed	Before the allowance	80-lb bags	Dry mix	Verdict
Post hole, 12 in wide × 30 in deep, no post	0.08 cu yd	0.07 cu yd	4	320 lb	Buy bags
Column or 12 in tube form, 4 ft tall	0.13 cu yd	0.12 cu yd	6	480 lb	Buy bags
Six post holes, 12 in × 30 in, 4×4 subtracted	0.43 cu yd	0.39 cu yd	20	1,600 lb	Buy bags
Round pad, 8 ft across, 4 in	0.68 cu yd	0.62 cu yd	31	2,480 lb	Buy bags
Footing, 20 ft long, 16 in wide, 8 in deep	0.72 cu yd	0.66 cu yd	33	2,640 lb	Buy bags
Stairs, 4 risers, 4 ft wide	0.90 cu yd	0.82 cu yd	41	3,280 lb	Buy bags
Sidewalk, 3 × 30 ft, 4 in	1.22 cu yd	1.11 cu yd	55	4,400 lb	Either
Slab, 10 × 10 ft, 4 in	1.36 cu yd	1.23 cu yd	62	4,960 lb	Either
Shed pad, 10 × 12 ft, 4 in	1.63 cu yd	1.48 cu yd	74	5,920 lb	Either
Patio, 12 × 12 ft, 4 in	1.96 cu yd	1.78 cu yd	89	7,120 lb	Either
Wall, 20 ft long, 4 ft high, 8 in thick	2.17 cu yd	1.98 cu yd	98	7,840 lb	Either
Driveway, 24 × 24 ft, 4 in	7.82 cu yd	7.11 cu yd	353	28,240 lb	Order ready-mix

Where Bags Stop Being the Cheaper Way to Buy Concrete

Two numbers decide this and only one of them is arithmetic. NRMCA's CIP 31 supplies the volume: "Avoid ordering small clean-up loads, less than 4 cubic yards (2.5 cubic meters)." A cubic yard is 45 eighty-pound bags. Four cubic yards is 180 bags, 14,400 lb of dry mix, 7.2 short tons through your hands and into a mixer. None of the general-purpose calculators checked states a threshold at all, even though NRMCA publishes the figure.

Under 1 cu yd — buy bags. Fewer than 45 of them. A truck's minimum load is more concrete than the job needs and you pay for the part that goes home in the drum. Six post holes at 12 in × 30 in come to 0.43 cu yd and 20 bags. A 12-in tube form 4 ft tall is 0.13 cu yd and 6 bags. A 20 ft footing 16 in wide and 8 in deep is 0.72 cu yd and 33 bags. All three are bag jobs on any reading.

1 to 4 cu yd — the band where the money is decided. A 10 × 10 slab is 1.36 cu yd: 62 bags, 4,960 lb, 2.5 short tons. A 12 × 12 patio is 1.96 cu yd and 89 bags. A 20 ft stem wall 4 ft high and 8 in thick is 2.17 cu yd and 98 bags. Every one of those is possible by hand and none of them is a pleasant afternoon. Ready-mix is cheaper per yard in this band, and then the supplier adds a short-load fee for being under its minimum. The fee is regional, the sites that print an amount for it name no source — SlabCalc's own table is headed "2025–2026, national averages" with no survey behind it — and CIP 31, the document that names the 4-yard floor, publishes no fee at all. So the page tells you the fee exists and to get a quote, rather than printing a number it cannot stand behind.

The two optional price fields turn that into a subtraction you can act on. Put $6.00 a bag and $180.00 a cubic yard against the default slab and a note appears on each of the first two cards: ≈ $372.00 and ≈ $244.44. The $127.56 between them is the whole budget the truck has for delivery and the short-load fee. If your plant's fee lands under it, the truck wins; over it, the bags do. One phone call closes the question, and neither figure includes the mixer rental or the two afternoons.

4 cu yd and up — order ready-mix. A 24 × 24 ft driveway at 4 in is 7.82 cu yd, which as bags is 353 of them, 28,240 lb, 14.1 short tons; the note on the bag card says as much on screen. Mixing that in batches also invites the failure the allowance exists to prevent. Inch Calculator ties the two together in one sentence: ordering enough "will ensure you can complete your project in a single pour without needing to order a short load, pay crew overtime, and risk a cold joint if you come up short."

The Three Numbers Behind Any Bag Count

n = \left\lceil \frac{V \times (1 + w)}{y} \right\rceil

$V$ = The plan volume in cubic feet, straight from the shape's dimensions and multiplied by the number of identical pours. A 10 × 10 ft slab at 4 in is 33.33 cu ft.
$w$ = The waste allowance as a fraction. The field starts at 10%, and NRMCA CIP 31 puts the useful band at 4% to 10% over the plan dimensions.
$y$ = The manufacturer's published yield per bag in cubic feet: 0.60 for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb, 0.30 for 40 lb. A volume, never a weight divided by a density.
$n$ = Whole bags. The ceiling is applied once, at the end, which is what QUIKRETE's own calculator does when it rounds up to the next highest whole bag.

Every shape reduces to a volume in cubic feet before anything else happens. A rectangular pour is length by width by thickness, with the thickness in inches divided by 12:

V = L \times W \times \frac{T}{12}

which gives 10 × 10 × 4 ÷ 12 = 33.33 cu ft for the default slab, and 20 × 4 × 8 ÷ 12 = 53.33 cu ft for a 20 ft stem wall 4 ft high and 8 in thick. A round pad, a tube form and a post hole are the same idea on a circle:

V = \pi \left(\frac{D}{2}\right)^{2} \times \frac{T}{12} \qquad V_{\text{hole}} = \pi \left(\frac{d}{24}\right)^{2} \times \frac{h}{12}

An 8 ft pad at 4 in is 16.76 cu ft. A 12-in hole 30 in deep is 1.96 cu ft, and a 4×4 post standing in it takes (3.5 ÷ 12)² × (30 ÷ 12) = 0.21 cu ft back out, leaving 1.75 cu ft to mix.

Stairs are the shape where the number in the field is not the number in the formula. Four risers means four rises but only three treads, because the fourth tread is the landing you gave its own depth to:

V = W \times \frac{R}{12} \left[ \frac{T_{r}}{12} \cdot \frac{n(n-1)}{2} + \frac{P}{12} \cdot n \right]

At 4 ft wide, 7-in rises, 11-in treads, a 12-in landing and four risers that comes to 22.17 cu ft, which is 0.90 cu yd and 41 bags once the allowance and the rounding are done.

Then three steps in a fixed order, and the order is the part that gets reversed elsewhere:

1. The allowance goes on the volume. 33.33 × 1.10 = 36.67 cu ft. Nothing is rounded here — the cubic-yard card prints 1.36 and keeps 1.23 as a note beside it, so you can see what the allowance bought.

2. The bag count is the ordered volume divided by the published yield. 36.67 ÷ 0.60 = 61.11.

3. That one division is rounded up, once. 61.11 becomes 62 bags, and the weight card reports the 62 whole bags you carry, 4,960 lb, not the 61.11 the division produced.

Two weights come out of the same pour and they are not interchangeable. The dry mix is what you lift: bags × bag size, 4,960 lb for that slab, 2.5 short tons. The finished concrete is the ordered volume at 140 lb per cu ft, the ASTM C138 unit weight on QUIKRETE's data sheet: 36.67 × 140 = 5,133 lb. The difference is the water, and it comes out of a hose. On a small pour the two can invert — one 12-in post hole with no post subtracted takes 4 whole bags, 320 lb of dry mix, to fill 2.16 cu ft that finishes at 302 lb — because the fourth bag was bought for a hole that only wanted 3.6 of them.

One more relation is worth keeping, because it settles the bag-size question on its own. Divide each published yield by its bag weight and all four give 0.0075 cu ft per pound of dry mix. Bag size therefore changes how many times you bend over, not how much concrete arrives: the same 10 × 10 slab is 123 forty-pound bags weighing 4,920 lb, 98 fifties at 4,900 lb, 82 sixties at 4,920 lb, or 62 eighties at 4,960 lb. The tonnage moves by less than 1.5% across the whole range, and every bit of that is the rounding up to whole bags.

Six Pours, Read Off the Live Calculator

The default 10 × 10 slab: 62 bags, and the 56 you will see everywhere else

Nothing needs touching for a complete answer. The page opens on Slab, patio or driveway at 10 × 10 ft, 4 in thick, one pour, a 10% allowance and 80 lb bags. Three cards report:

80 lb bags needed: 62 — subtitled 0.60 cu ft per bag, includes a 10% allowance, rounded up to whole bags
Concrete needed: 1.36 cu yd — 36.67 cu ft, 1.04 m³, with 1.23 cu yd before the 10% allowance noted underneath
Dry mix to carry: 4,960 lb — 2.5 short tons, 62 bags × 80 lb

Set the allowance to 0 and the same slab drops to 1.23 cu yd, 56 bags and 4,480 lb, with the subtitle changing to no allowance added and a line quoting CIP 8's ⅛-inch example back at you. The six bags between those two states are the whole argument of this page: 56 is the geometry, 62 is the order.

The table under the cards runs the same slab through the other bag sizes — 123 forty-pound, 98 fifty-pound, 82 sixty-pound, 62 eighty-pound. Its interesting column is the weight, because it barely moves: 4,920, 4,900, 4,920, 4,960 lb. Same concrete, 61 more trips from the pallet if you buy the small bags.

Six fence posts with the 4×4 taken out of each hole

Choose Post hole, post subtracted; 12 in diameter, 30 in deep; square post, 3.5 in; six of them. The results:

80 lb bags needed: 20
Concrete needed: 0.43 cu yd — 11.56 cu ft, 0.39 cu yd before the allowance
Dry mix to carry: 1,600 lb — 0.8 short tons

Two lines with the results do work no competing page does: "Each 3.5 in post displaces 0.21 cu ft — 10.8% of the hole. Already subtracted." and "For 6 identical post holes." That 10.8% is the same size as the entire waste allowance, so a page that fills the hole solid and then adds 10% has spent the allowance on the post and has nothing left for the spill.

For a hole with nothing standing in it — a pier, or a post you will set later — pick No post, fill the hole. One 12-in hole 30 in deep is then 1.96 cu ft, 2.16 with the allowance, 0.08 cu yd, 4 bags, 320 lb.

What the ranking pages give for that same hole: Everything About Concrete's table says 4 bags for a 12-in hole 24–30 in deep, MudMixer's table says 3–4 eighty-pound bags for 12 in × 30 in, and the summary at the top of that same MudMixer article says one 80 lb bag. None of the three mentions that a 4×4 is 3.5 in.

A 12-inch tube form 4 ft tall: six bags

Sonotube is the everyday name for it and Column, pier or tube form is the control. Give the diameter in inches and the height in feet: 12 and 4. The math panel prints the working as π × (12 ÷ 12 ÷ 2)² × 4 = 3.14 cu ft, which becomes 3.46 cu ft with the allowance, 0.13 cu yd, 6 bags, 480 lb.

Ten percent of 3.14 cu ft is 0.31 cu ft, just over half a bag, which is why the allowance rarely moves the count on a pour this small: it has to exceed whatever fraction of the last bag was already going spare before it buys another one. Scale changes that fast. On the driveway further down, the same 10% is 0.71 cu yd — more concrete than the entire six-post-hole job.

$372.00 of bags against $244.44 of yardage, on the same slab

Both price fields are optional and empty by default; the page asserts no price for concrete anywhere, because bag prices and yardage move by region and by month. Type $6.00 a bag and $180.00 a cubic yard against the default 10 × 10 slab and a note appears under each of the first two cards:

≈ $372.00 at $6.00 a bag — the 62 bags
≈ $244.44 at $180.00 a cubic yard — the 1.36 cu yd

Read the gap rather than the two totals. $127.56 is everything the truck has left to spend on delivery and a short-load fee before the bags become the cheaper way to buy the same concrete. At 1.36 cu yd the load is well under any plant's minimum, so a fee is in play, which is exactly what the verdict line says: get a quote before you buy the bags.

The cost column in the bag-size table applies the single price you typed to all four rows, so it compares bag sizes only if your store charges the same per bag across sizes. Where it does not, run the size you are actually buying.

A 24 × 24 driveway: 353 bags, 14.1 short tons

Slab, 24 × 24 ft, 4 in thick, and the page changes its advice rather than just its numbers:

80 lb bags needed: 353, with the note "That is 28,240 lb of bagged mix, about 14.1 short tons to carry. Ready-mix is the cheaper way to move it."
Concrete needed: 7.82 cu yd — 211.20 cu ft, 5.98 m³, 7.11 cu yd before the allowance
Dry mix to carry: 28,240 lb — 14.1 short tons

The verdict line reads "7.82 cu yd — order ready-mix. 353 × 80 lb bags is 28,240 lb, about 14.1 short tons, and 4 cu yd is the small-load floor NRMCA names." Above that floor the fee question disappears: the load is a normal order and the yardage is the whole conversation. The allowance is still doing its job, and at this size it is worth 0.71 cu yd on its own — a mistake in the depth of a driveway this size is measured in wheelbarrows.

Stairs: four risers is three treads and a landing

Stairs, 4 risers, 7-in rise, 11-in tread, 4 ft wide, 12-in landing: 22.17 cu ft, 0.90 cu yd, 41 bags, 3,280 lb, 0.82 cu yd before the allowance.

The math panel states the trap in its last clause — 4 risers means 3 treads plus the landing. Count four treads and you have added a tread's worth of concrete that will never be poured. Treat the landing as a fourth tread and you lose the difference between the 12-in landing you specified and an 11-in tread. The formula handles it with a triangular number: each step down carries the ones above it, so the tread contribution is n(n−1)÷2 rather than n.

Stairs are also the shape where the allowance earns its keep. CIP 31 lists form movement alongside waste and over-excavation as the reason for the 4% to 10% band, and stair forms are the flimsiest thing on a small job: the error surface is every riser face and the landing, not just the bottom of one slab.

Driving the Form: Seven Shapes, Eleven Fields at Most

Twenty-eight inputs are declared and no more than eleven are ever on screen — eight on the simplest shape — because choosing a shape hides the dimensions the other six need. There is no calculate button — every card moves as you type.

1. Pick the shape at What are you pouring?: slab, patio or driveway; round slab or pad; footing or trench; wall or stem wall; column, pier or tube form; post hole with the post subtracted; or stairs. The fields under it change to match.

2. Enter the dimensions that shape asks for, two to five of them. Lengths, widths and heights are in feet and take decimals, which the hint spells out — 10 ft 6 in is 10.5. Thicknesses, diameters and depths are in inches, because that is how they are specified on a job.

3. For a post hole, set Post shape and Post size. Square and round are both there, and the size is the actual dimension rather than the nominal one: a 4×4 is 3.5 in, a 6×6 is 5.5 in, and the hint under the field says so. Choosing No post — fill the hole turns the subtraction off and fills the cylinder solid.

4. Set How many of these? for repeated identical pours — six post holes, three piers, two flights. The count multiplies the volume before the allowance goes on, and a line with the results confirms what it counted.

5. Edit the Waste allowance if 10% is not right for this job. A formed slab on a compacted base sits near the bottom of the CIP 31 band; rough excavation, trench footings and a subgrade nobody screeded sit at the top. Set it to 0 and the bag subtitle changes to no allowance added, and a line quotes CIP 8's ⅛-inch example — which is the honest answer to why 0 is rarely the right number.

6. Choose the Bag size you can actually buy: 40, 50, 60 or 80 lb. The headline card renames itself to match, and the bag-size table highlights the row you picked.

7. The last two fields, Price per bag and Ready-mix, price per cubic yard, are optional and start at 0. Leave them at 0 and every money surface on the page stays hidden — no totals, no blank cells, no placeholder dashes.

Three cards report the answer: bags, cubic yards, dry weight. Under them come the lines, as the shape and the settings call for them: the bags-or-truck verdict for your volume with its arithmetic, the allowance note, the post displacement, the count of identical pours, the finished weight, the rounding, and the source line for the yields. Below those sit five blocks. How many bags, by bag size renders open, because it answers a purchase question rather than a reference one. Bags per cubic yard, Coverage by thickness, Bags or a ready-mix truck? and How this is calculated are closed until you open them; their contents are in the page either way, so a find-in-page search still reaches them.

Where a Concrete Estimate Goes Wrong

Ordering the plan volume. The 33.33 cu ft a 10 × 10 slab measures is not the 36.67 cu ft you order, and the 56 bags that follow from it is the figure most pages print. CIP 31 asks for 4% to 10% over the plan dimensions and CIP 8 explains why in one line: an ⅛-inch error in the depth of a 4-inch slab is already a 3% shortage. Coming up short mid-pour costs a short load, a second mixing session, or a cold joint.
Typing the nominal post size. A 4×4 is 3.5 in and a 6×6 is 5.5 in. Entering 4 for a 4×4 subtracts about 30% more than the post really occupies, so the bag count lands short — the direction of error you least want on a job where the posts have to be set in one go.
Deriving bags from a weight. A cubic yard weighs about 3,780 lb at the ASTM C138 unit weight, and dividing that by the 80 lb printed on the bag gives 47.25 where the published yield gives 45. Do it at 150 lb/cu ft, as Omni Calculator's chart does, and you get 51. The bag holds 80 lb of dry mix and makes 0.60 cu ft of concrete weighing about 84 lb; the gap is water you add on site.
Rounding the volume before the bags. The order is allowance, divide, round once. Rounding 33.33 cu ft up to 34, or pushing 1.23 cu yd to 1.5 because a supplier sells in half yards, and then adding 10% on top, stacks two safety margins into a number nobody chose. This page never rounds the volume and never rounds the yardage to a supplier increment, because increments differ by plant — that one is a question for the order desk.
Assuming 50 lb is an industry size. QUIKRETE publishes a 50 lb bag at 0.375 cu ft; Sakrete's High Strength data sheet has no 50 lb row at all. That row here rests on one manufacturer, and the two brands also disagree about the 90 lb bag — 0.675 cu ft against 0.66 — which is why 90 lb is not offered.
Thinking the big bags are more concrete per pound. Every published yield works out to 0.0075 cu ft for each pound of dry mix, so 123 forty-pound bags and 62 eighty-pound bags cover the same slab to within one bag's rounding. Bag size is a decision about your back and about the price per cubic foot at your store, not about coverage.
Running one shape and calling it the job. This page does one shape at a time and the math panel says so: run each part and add the bag counts. A shed pad plus its four piers is two runs. Adding the bag counts after each shape has been rounded up is the safer arithmetic, because every rounding is a real bag sitting on the pallet.
Reading the dry weight as the finished weight. 62 bags is 4,960 lb through your hands and 5,133 lb in the forms. The gap matters twice: for the trip home, where 2.5 short tons is more than one pickup load, and for the mixer, which is rated in cubic feet of wet mix rather than pounds of dry.

What This Calculator Leaves Alone

Most of the gaps below are deliberate, and naming them is more use than letting a reader discover one halfway through an order.

No price of its own. Neither price field carries a default and no cost appears anywhere until you type one. Bag prices and yardage move by region, by season and by plant, and a shipped figure would be stale before it was useful. What the page does instead is arithmetic on your numbers: the bag total, the yardage total, and the difference between them.

No short-load fee figure. The fee is real and the page says so, but CIP 31 — the document that names the 4-cubic-yard floor — publishes no amount, and the amounts quoted elsewhere are uncited and inconsistent. SlabCalc labels its tables "2025–2026, national averages" without naming a survey behind them. A regional charge with no primary source is a phone call, not a printable number.

No structural design. Slab thickness, reinforcement, rebar spacing, PSI, mix design, admixtures, cure time, control joints, subbase depth and frost depth are code and engineering questions. This page takes the thickness you give it and converts it to volume; it does not tell you whether 4 in is enough for what will park on it.

No labor, mixer rental or delivery. The dry weight and the bag count are the honest proxies for effort: 4,960 lb over an afternoon reads differently once it is written down as 2.5 short tons.

One shape at a time, and no supplier increment. A pour with a thickened edge, a step, or two different thicknesses is two or three runs added together. The yardage is printed exactly as calculated rather than rounded up to a plant's half-yard or quarter-yard increment, because the increment varies and only the order desk knows it.

Imperial in, metric only as a read-out. Feet, inches, cubic yards, pounds and short tons are the inputs and the primary outputs; cubic meters appear as a subtitle on the volume card. A metric version would need its own sourced yield table, since the 20 kg and 25 kg retail bags sold elsewhere are different products with different published yields. For the conversion on its own, the volume converter does it.

Standard concrete mix only. The yields behind every bag count are those of QUIKRETE Concrete Mix No. 1101 and Sakrete High Strength Concrete Mix. Fast-setting mixes, high-early-strength mixes, crack-resistant blends and the expanding foam products sold for fence posts all behave differently and some are not measured in cubic feet at all. Read the bag before you rely on a count.

Concrete Bags, Yards and Ordering — Frequently Asked Questions

How many bags of concrete do I need for a 10x10 slab?

For a 4-inch 10 × 10 ft slab, 62 eighty-pound bags with the standard 10% allowance, or 56 with no allowance at all. In the other sizes it is 123 forty-pound, 98 fifty-pound or 82 sixty-pound bags. The volume is 1.36 cu yd, or 1.23 before the allowance.

How many 80 lb bags of concrete make a cubic yard?

45. A cubic yard is 27 cu ft and an 80 lb bag yields a published 0.60 cu ft. The other sizes work out to 60 bags of 60 lb, 72 of 50 lb, and 90 of 40 lb.

How many bags of concrete do I need per fence post?

The depth decides it, which is why the rules of thumb disagree so widely. A 12-in hole 30 in deep holds 1.96 cu ft, so with nothing standing in it that is 4 eighty-pound bags including the allowance. Six of those holes with a 4×4 set in each come to 0.43 cu yd and 20 bags for the whole run, because the 3.5-in post takes 0.21 cu ft — 10.8% — out of every hole. Deepen the same hole or widen it and the count moves fast: volume goes with the square of the diameter.

How many bags of concrete fill a 12-inch Sonotube 4 feet deep?

Six 80 lb bags. The form holds 3.14 cu ft, 3.46 with a 10% allowance, which is 0.13 cu yd and 480 lb of dry mix. Pick Column, pier or tube form, then give the diameter in inches and the height in feet.

How much extra concrete should I order for waste?

NRMCA's CIP 31 says to "order about 4% to 10% more concrete than is estimated from a volumetric calculation of the plan dimensions", and the allowance field here starts at 10%. Stay near the bottom of that band for a formed slab on a compacted base, and near the top for trench footings, rough excavation or a subgrade nobody screeded. CIP 8 gives the reason to use any allowance at all: an ⅛-inch error in the depth of a 4-inch slab is already a 3% shortage. The three causes CIP 31 lists — waste, over-excavation and form movement — all push the requirement the same way, upward.

Is it cheaper to buy bags or order a ready-mix truck?

Under a cubic yard, bags — that is fewer than 45 eighty-pound bags, and a truck's minimum load is more concrete than the job needs. Above 4 cu yd, ready-mix, which is the small-load floor CIP 31 names and 180 bags of hand mixing. Between the two it is a subtraction. At $6.00 a bag and $180.00 a cubic yard, a 10 × 10 slab is $372.00 in bags against $244.44 of yardage; the $127.56 between them is what the delivery and the short-load fee have to stay under for the truck to win. The fee is regional, so your supplier is the only source for it.

How much does an 80 lb bag of concrete weigh once it is mixed?

About 84 lb. The bag makes 0.60 cu ft of concrete, and concrete weighs about 140 lb per cu ft by the ASTM C138 figure on QUIKRETE's data sheet. The extra 4 lb is the water you add at the mixer. The card labeled Dry mix to carry reports the 80 lb side, and the finished-weight line with the results reports the other.

Why do concrete calculators disagree about the number of bags?

Because two different methods are in use. Dividing the volume by the yield printed on the bag gives 45, 60, 72 and 90 bags per cubic yard for the 80, 60, 50 and 40 lb sizes. Dividing the weight of a cubic yard of finished concrete by the dry weight of a bag gives a bigger number, because the mixing water is counted as though it came in the bag: Omni Calculator's chart prints 51, 67, 81 and 101 off a 150 lb/cu ft density, about 12% high. The published yields are the figures the manufacturers stand behind, so they are what this page divides by.

How many bags of concrete cover 100 square feet?

At 4 in thick, 56 eighty-pound bags — one bag covers 1.80 sq ft. At 6 in it is 84 bags at 1.20 sq ft each, at 2 in it is 28, and at 8 in it is 112. The coverage table on this page is pure yield with no allowance in it, so add yours on top; the calculator does that for you once you enter the actual dimensions.

How many bags of concrete are on a pallet?

It varies by brand and by store, so the label is the only reliable answer. The useful rule of thumb comes from Sakrete: "one pallet of bags is about a cubic yard. Though it actually takes 45 bags of 80-pound mix to hit a cubic yard dead on." Plan by the yield rather than by the pallet, then ask the yard how their pallets are made up.

Does the fence post itself change how much concrete I need?

Yes, by more than most estimates allow for. A 3.5-in post in a 12-in hole 30 in deep displaces 0.21 cu ft of the hole's 1.96 cu ft, which is 10.8% — the same order of magnitude as a full waste allowance. Choose Post hole, post subtracted and give the actual post size, and the displacement comes out before the allowance goes on.

Is this concrete calculator free to use?

Yes, with nothing to sign up for. All seven shapes, all four bag sizes, the bag-size comparison, the two reference tables and both explanatory panels are open to anyone, and the calculator can be embedded on another site. The manufacturer data sheets and the NRMCA bulletins behind the numbers are listed under the tool.

How accurate is the bag count?

The arithmetic is exact: your dimensions, your allowance, and a published yield, with one rounding up to whole bags at the end. The uncertainty lives in the ground rather than in the sum — an over-excavated trench, a subgrade that dips, or forms that bow will all take more concrete than the plan dimensions say, which is what the allowance covers. The yields apply to standard concrete mix; fast-setting and specialty products publish their own.

What do I do with the bag count once I have it?

Take the count and the bag size to the store as one figure, and buy all of it in one trip: a pour that stops halfway while somebody drives back for four more bags is how cold joints happen. Check the verdict line before you buy anything, because above a cubic yard the yardage may cost less than the bags even after a fee. Read the dry weight as a logistics number — 4,960 lb is 2.5 short tons and more than one pickup load. And if you land in the 1-to-4-cubic-yard band, phone the plant with the cubic-yard figure before you commit, since that is the one number the order desk actually wants.

Sources & References

Content verified by the Smart Calculators Team