Compound Interest Calculator
Calculate how your investments grow over time with compound interest and regular contributions.
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USD
USD
%
Monthly
Final balance
$107,144
Total contributions
$70,000
Interest earned
$37,144
Yearly breakdown
| Year | Balance | Contributions | Interest |
|---|---|---|---|
| 1 | $16,955 | $16,000 | $955 |
| 2 | $24,413 | $22,000 | $1,458 |
| 3 | $32,411 | $28,000 | $1,997 |
| 4 | $40,986 | $34,000 | $2,575 |
| 5 | $50,182 | $40,000 | $3,195 |
| 6 | $60,042 | $46,000 | $3,860 |
| 7 | $70,614 | $52,000 | $4,573 |
| 8 | $81,952 | $58,000 | $5,337 |
| 9 | $94,108 | $64,000 | $6,157 |
| 10 | $107,144 | $70,000 | $7,036 |
Táto kalkulačka slouží iba pre informativní účely. Nepredstavuje finanční poradenství.
Compound interest calculator. Final balance and interest from a lump sum plus monthly deposits.
A compound interest calculator projects your final balance and interest earned from a starting sum, monthly deposits, rate and time. It handles regular contributions and any compounding frequency, so you can see how much of your total comes from deposits versus interest.
What Is Compound Interest?
Put $3,000 a month into an account earning 7% a year, compounded monthly, and after 30 years you are looking at roughly $3.68 million. Only about $1.08 million of that is money you actually paid in — 3,000 dollars a month across 360 months — while the other $2.6 million is pure interest. That is the whole point of compound interest: your interest starts earning its own interest, so the part your deposits never funded ends up more than double what you contributed.
Compound interest is interest that gets added back to your balance, so the next round of interest is calculated on a bigger number. Simple interest never does this — it only ever pays on your original deposit, so the yearly amount stays flat. The gap looks small at first, then runs away from you. Take a one-time $100,000 at 5% for 10 years: with annual compounding it becomes $162,889, and with monthly compounding $164,701. Stretch the timeline and the effect turns dramatic — that same $100,000 at 8%, compounded yearly, with nothing else added, grows to about $1,006,266 over 30 years, more than ten times what you started with.
Understanding this changes the questions you ask. Instead of asking how much you will end up with, you start asking how early you can begin, how long you can leave it alone, and what return is realistic — because time in the market and the annual rate move the final number far more than whether interest compounds monthly or yearly. One honest caveat: every figure here assumes a fixed, steady return. Real markets rise and fall, fees and taxes take a cut, and inflation quietly lowers what your future balance can actually buy. Treat the results as a projection for planning, not a promise.
How to Use the Compound Interest Calculator
This compound interest calculator does the formula work for you — fill in five fields and read the result. Here is what each one means:
1. Starting amount (principal). The single lump sum you begin with, like 100,000 dollars. If you are only making regular deposits and have nothing upfront, set this to 0.
2. Monthly contribution. The fixed amount you add every month, such as 3,000 dollars. Leave it at 0 if you are only investing a one-time lump sum.
3. Annual interest rate. Your expected yearly return as a percentage, for example 5% or 7%. This is the most sensitive input, so it pays to be conservative rather than optimistic.
4. Number of years. How long you plan to stay invested — 10 years, 20 years, 30 years. This is usually the biggest lever of all.
5. Compounding frequency. How often interest is added back: yearly, quarterly or monthly. Monthly is the closest match for most funds and accounts that reinvest earnings.
Hit calculate and the tool works out the compound growth on your lump sum and the future value of your monthly deposits at the same time, then shows three numbers: your final balance, your total contributions (the cash you actually put in), and your interest earned (the balance minus what you contributed). A quick example: start with 10,000 dollars, add 1,000 dollars a month at 6% compounded monthly for 20 years, and you reach about 497,453 dollars — of which 250,000 dollars is your own money and roughly 247,453 dollars is interest. When you read the result, do not just stare at the big final number; look at how large the interest slice is, because that share is what compounding actually did for you.
Compound Interest and Annuity Formula
- = Final balance (principal plus all interest)
- = Principal (the initial lump-sum amount)
- = Annual interest rate as a decimal (5% = 0.05)
- = Compounding periods per year (monthly = 12, quarterly = 4, annual = 1)
- = Investment length in years
The lump-sum part uses the standard compound interest formula above: your final balance equals the principal, multiplied by one plus the periodic rate, raised to the number of periods. When you also add a fixed amount each period (PMT), the future-value-of-an-annuity term is added on top:
This calculator applies each contribution at the start of the period (an annuity-due), which is why the deposit term carries the extra (1 + r/n) factor. The compounding frequency n nudges the result upward: common values are yearly (n=1), quarterly (n=4) and monthly (n=12). But the nudge is small. A one-time 100,000 dollars at 5% over 10 years lands at 162,889 dollars with yearly compounding and 164,701 dollars with monthly — a difference of only about 1,811 dollars. The lesson: starting early and picking a realistic rate matter far more than agonizing over yearly versus monthly compounding.
Compound Interest Scenarios at a Glance
| Starting amount | Monthly deposit | Rate | Years | Compounding | Final balance | Total contributions | Interest earned |
|---|---|---|---|---|---|---|---|
| $100,000 | $0 | 5% | 10 | Annual | $162,889 | $100,000 | $62,889 |
| $100,000 | $0 | 5% | 10 | Monthly | $164,701 | $100,000 | $64,701 |
| $0 | $5,000 | 5% | 10 | Monthly | $779,646 | $600,000 | $179,646 |
| $10,000 | $1,000 | 6% | 20 | Monthly | $497,453 | $250,000 | $247,453 |
| $100,000 | $0 | 8% | 30 | Annual | $1,006,266 | $100,000 | $906,266 |
| $0 | $3,000 | 7% | 30 | Monthly | $3,681,262 | $1,080,000 | $2,601,262 |
| $100,000 | $0 | 6% | 12 | Annual | $201,220 | $100,000 | $101,220 |
Worked Compound Interest Examples
Lump Sum Plus Deposits: $10,000 and $500 a Month at 7%
Say you open an account with $10,000 and add $500 every month, expecting a 7% average annual return compounded monthly. After 10 years the calculator projects a final balance of $107,143.85. Of that, $70,000 is money you put in yourself — the $10,000 starting deposit plus 120 monthly contributions of $500 — and the remaining $37,143.85 is interest. In other words, roughly a third of your ending balance was built by compounding rather than by your own deposits. This is the classic pattern for a medium-term goal like a house deposit or a college fund: your steady contributions do most of the heavy lifting in the early years, while interest gradually takes over as the balance grows larger.
Pure Compounding: $10,000 Left Alone for 20 Years
A one-time deposit shows compounding at its purest, with no fresh money clouding the picture. Drop $10,000 into an account earning 5% a year, compounded annually, and leave it completely untouched for 20 years. The balance grows to $26,532.98. You never added another cent, yet the account earned $16,532.98 in interest — more than the $10,000 you originally deposited. That is the plain compound-interest formula at work: the balance equals the principal multiplied by one plus the annual rate, raised to the number of years. Notice how the growth is slow at first and steepest near the end, which is the signature curve of compounding and the reason patience pays off.
The Long-Horizon Flip: $100 a Month for 30 Years
Here is the scenario that surprises people most. Start with just $1,000, add a modest $100 a month, assume a 6% average annual return compounded monthly, and stay the course for 30 years. The final balance reaches $106,976.34. Across those three decades you contributed $37,000 of your own money, which means $69,976.34 of the balance is pure interest. Read that again: the interest earned is nearly double everything you actually paid in. That crossover — where compounding produces more than your contributions ever did — is the single strongest argument for starting young and leaving your money invested. Delay those same deposits by even a decade and the ending balance shrinks sharply, because the earliest years are the ones compounding leans on most.
Does Frequency Matter? $5,000 at 8% for One Year
Does compounding frequency actually change your return? Put $5,000 into an account at 8% for a single year and compare the two settings side by side. With monthly compounding the balance ends at $5,415.00, an interest gain of $415. With annual compounding — where interest is credited just once at year-end — it ends at $5,400.00, an interest gain of $400. Monthly compounding wins, but only by $15 over the whole year. The gap widens over longer horizons, yet it stays small next to the effect of the rate and the time invested. The practical takeaway: prefer more frequent compounding when everything else is equal, but never let it distract you from the two inputs that truly move the needle — your annual rate and your number of years.
5 Common Compound Interest Mistakes
- Treating compound interest like simple interest. Simple interest only ever pays on your original deposit, while compound interest pays on the whole balance, including past interest. A one-time 100,000 dollars at 5% compounded monthly reaches about 164,701 dollars in 10 years; simple interest would add a flat 5,000 dollars a year, or 50,000 dollars total, for just 150,000 dollars — an undercount of nearly 15,000 dollars that widens every year.
- Ignoring how often interest compounds. The same 100,000 dollars at 5% for 10 years grows to 162,889 dollars with annual compounding but 164,701 dollars with monthly. The gap here is only about 1,811 dollars, but when you compare two products, make sure they use the same compounding frequency — otherwise you are not comparing like with like.
- Forgetting that inflation and tax eat into the real return. The calculator shows nominal dollars. That 3,681,262 dollars from investing 3,000 dollars a month at 7% for 30 years will not buy in 30 years what it buys today, and dividends or capital gains may be taxed along the way. For retirement planning, lean toward a conservative rate and mentally discount the future balance for inflation.
- Assuming the return is fixed and guaranteed. The projection uses one steady rate, but real markets swing up and down, and some years are negative. Treating an average annual return as a number you will hit every single year is the most dangerous misread — the result is an illustration of long-term averages, not a promised payout.
- Counting your own contributions as interest. Final balance equals total contributions plus interest earned. In the 3,000 dollars a month, 7%, 30-year example, the roughly 3,681,262 dollars final balance includes 1,080,000 dollars you paid in yourself; the genuine interest is 2,601,262 dollars. Judge results by the interest slice, not the headline total.
Compound Interest Questions Answered
What's the difference between simple and compound interest?
Simple interest pays only on your original deposit, so the amount stays flat each year. Compound interest pays on the balance including past interest, so it accelerates. Over 10 years a one-time 100,000 dollars at 5% reaches about 164,701 dollars compounded monthly.
What is the Rule of 72?
The Rule of 72 estimates how long money takes to double: divide 72 by your annual return. At 6%, 72 ÷ 6 = 12 years, and a one-time 100,000 dollars does grow to about 201,220 dollars in 12 years — almost exactly double. It stays accurate for rates between roughly 2% and 15%, and is most precise near 8%.
Is monthly or annual compounding better?
Monthly compounding earns a little more because interest starts compounding sooner. But the gap is small: a one-time 100,000 dollars at 5% for 10 years gives 162,889 dollars annually versus 164,701 dollars monthly — about 1,811 dollars. Time and rate matter far more.
How much does investing $3,000 a month grow to in 30 years?
At a 7% average annual return compounded monthly, 3,000 dollars a month becomes about 3,681,262 dollars — roughly 3.68 million — after 30 years. You would pay in 1,080,000 dollars of your own money, and the remaining 2,601,262 dollars is interest, more than double what you contributed. That assumes the return holds up over the full period, so treat it as a projection, not a guarantee.
How much can a lump sum grow with no extra contributions?
It depends on rate and time. A one-time 100,000 dollars at an 8% return compounded annually, with nothing added, grows to about 1,006,266 dollars over 30 years — more than ten times the starting amount, with roughly 906,266 dollars of that being interest. The later years produce the steepest growth, which is the signature shape of a compounding curve.
How much interest does a $10,000 deposit earn in 20 years?
A one-time 10,000 dollars deposit at 5% compounded annually grows to about 26,533 dollars over 20 years, earning roughly 16,533 dollars in interest with nothing added. Because compounding accelerates, most of that gain lands in the later years — the balance climbs faster the longer you leave it alone.
Can compound interest earn more than I contribute?
Yes, given enough time. Investing 1,000 dollars up front plus 100 dollars a month at 6% compounded monthly for 30 years reaches about 106,976 dollars. You pay in only 37,000 dollars, so roughly 69,976 dollars — nearly double your contributions — is interest. That crossover, where interest overtakes everything you paid in, is exactly why long time horizons matter so much.
Is this compound interest calculator free, and does it handle regular contributions?
Yes — it is free to use with no signup, and it handles both a starting lump sum and regular monthly contributions. Enter your monthly deposit, rate, years and compounding frequency, and it calculates the lump-sum growth and the future value of your deposits together, then breaks out your final balance, total contributions and interest earned.
How accurate are the results, and how should I read them?
The math uses the standard compound interest and future-value-of-an-annuity formulas, so it is precise for the inputs you give it. The catch is that it assumes a fixed return — real markets fluctuate, and fees, taxes and inflation all reduce what you keep. Enter a conservative rate, and mentally discount the nominal final balance for inflation to gauge real buying power.
I have my final balance — what should I do with that number?
Look first at the interest-earned share of the total: the larger it is, the more compounding did the work and the longer you gave it. If that share is small, your timeline is probably too short or your rate too low, so consider extending the years or raising your monthly contribution. Run the numbers twice — once with a conservative rate and once with an optimistic one — to get a realistic range instead of a single false-precise figure.
What matters more, a big starting amount or a long time horizon?
Time usually wins. A one-time 100,000 dollars at 8% compounded annually reaches about 1,006,266 dollars over 30 years with nothing added, but steady deposits of 3,000 dollars a month at 7% reach about 3,681,262 dollars in the same 30 years. Consistent contributions plus a long runway generally beat a large upfront sum, because compounding needs time to build momentum.