Energy Converter

Convert between SI, electrical, and thermal energy units (joule, kilojoule, megajoule, watt-hour, kilowatt-hour, thermochemical calorie, IT calorie, kilocalorie, BTU, US therm, electronvolt) with NIST-grade exact factors anchored to the 2019 SI redefinition.

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この計算機はまだ日本語に翻訳されていません。

翻訳作業中のため、英語版を表示しています。

From

Value

kWh

Result

3,412.14 BTU

1 kWh= 3,412.141633 BTU

Decimals

All units

Unit	Value
Joule (J)	3,600,000
Kilojoule (kJ)	3,600
Megajoule (MJ)	3.6
Watt-hour (Wh)	1,000
Kilowatt-hour (kWh)	1
Calorie (thermochemical) (cal_th)	860,421
Calorie (IT) (cal_IT)	859,845
Kilocalorie (food) (kcal)	860.421
British thermal unit (IT) (BTU)	3,412.14
US therm (therm)	0.0341214
Electronvolt (eV)	2.24694E25

Food-label calories are kilocalories (kcal). 1 kcal = 1000 cal_th — use the kilocalorie row when reading nutrition labels.

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更新日: 2026年7月18日

Energy converter for joules, kWh, calories, kcal and BTU.

An energy converter changes a value from one energy unit to another — joules, kilowatt-hours, calories, kcal or BTU — by routing each through the joule. It keeps look-alikes apart: 1 kWh is 3.6 MJ or 860.421 kcal, and a food Calorie is 1000 small calories, not one.

What Is an Energy Converter?

A food Calorie is not one calorie — it is a thousand of them. The "Calorie" printed on a Nutrition Facts label, with a capital C, is a kilocalorie: 1000 small calories, or 4184 joules. A single small calorie, written lowercase, is only 4.184 joules — the heat needed to warm one gram of water by one degree Celsius. That thousand-fold gap hiding inside one everyday word is why energy trips people up, and it is exactly the mix-up an energy converter is built to prevent.

Energy is one physical quantity written in many different sizes. The joule in a physics problem, the kilowatt-hour on your electricity bill, the kilocalorie on a cereal box, the BTU on a furnace nameplate and the electronvolt in a particle detector all measure the same thing — how much work can be done or heat released. The joule sits at the centre because it is the SI unit built from the others: one joule is the work done when a force of one newton acts through one metre, the same joule a watt delivers in a second. This tool routes every value through that base unit — it scales your input up to joules, then back down into whatever unit you asked for, so any pair of units is only two steps apart with no special cases.

The units fall into four families. The SI family — joule, kilojoule, megajoule — is the scientific backbone and covers mechanical work. The electrical family — watt-hour and kilowatt-hour — is what utilities meter: 1 kWh is 3.6 MJ, 3600 kJ, or 860.421 kcal. The thermal family — calorie, kilocalorie, BTU and therm — turns up in food, heating and HVAC. And the electronvolt covers atomic and particle physics, where energies are far too small to write comfortably in joules.

What separates a careful energy converter from a throwaway one is how it treats the calorie. Two calorie definitions are in daily use: the thermochemical calorie at 4.184 joules, which underpins food labels and chemistry, and the International Table calorie at 4.1868 joules, used in steam tables. They sit less than 0.1% apart — invisible on a menu, but worth knowing for a precise heat calculation. The bigger trap is the capital letter: because the food "Calorie" is a kilocalorie, a European or Australian kilojoule figure has to be divided by 4.184 — not 4184 — to reach food Calories, or you land a thousand-fold off.

How to Use the Energy Converter

Choose your starting unit under From, choose the unit you want under To, then enter a value. The converted figure appears at once and updates on every keystroke, and clicking it copies the number. Both menus carry the full unit list, so you set the from-unit and the to-unit independently rather than being tied to one fixed pairing. A precision control lets you keep the automatic display or fix the number of decimals, which helps when a factor such as 860.421 kilocalories per kilowatt-hour runs to several places.

Every conversion pivots through one base unit, the joule, so doing it by hand is two moves: scale the input into joules, then scale back out. Kilowatt-hours to megajoules — 1 kWh is 3.6 MJ, so 12 kWh is 12 × 3.6 = 43.2 MJ. Kilocalories to kilojoules — 1 kcal is 4.184 kJ, so a 2000-kcal daily diet is 2000 × 4.184 = 8368 kJ, which is 8.368 MJ. Reverse either one by dividing instead of multiplying: 3600 kJ collapses back to 1 kWh, and 4.184 kJ back to 1 kcal. Whichever pair you pick, it is one scale in and one scale out.

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The Energy Conversion Formula

E_t = E_s × f_s ÷ f_t

$E_s$ = The energy you start from, in the source unit
$f_s$ = Joules in one source unit — its factor to the joule (4184 for a kilocalorie, 3,600,000 for a kilowatt-hour)
$f_t$ = Joules in one target unit — its factor to the joule
$E_t$ = The converted energy, in the target unit

Every energy conversion is a single multiplication, because all energy units share the same zero: zero of anything is zero joules. Give each unit one number — how many joules it equals — and any pair of units is two scalings apart. Scale the input up to joules with the source factor, then scale back down with the target factor. There is no offset to add, which is why energy converts the same clean way whether the units are mechanical, electrical, thermal or atomic.

Each family anchors to the joule differently. The SI units step by powers of ten: 1 kilojoule = 1000 J and 1 megajoule = 1,000,000 J. The electrical units come from watts times seconds: 1 watt-hour = 3600 J and 1 kilowatt-hour = 3,600,000 J, which is 1000 W × 3600 s. The thermal units carry their own definitions: 1 thermochemical calorie = 4.184 J, 1 kilocalorie = 4184 J, 1 BTU = 1055.05585262 J (shown rounded as 1055.06), and 1 US therm = 100,000 BTU. The atomic unit, the electronvolt, is 1.602176634 × 10⁻¹⁹ J, fixed by the 2019 SI redefinition of the elementary charge.

Every one of those factors to the joule is an exact definition, not a measurement, so the converter treats them all as exact. What varies is whether two units share a tidy ratio. A kilowatt-hour is exactly 3.6 megajoules and a kilocalorie is exactly 4.184 kilojoules, so those land on clean finite numbers. But a kilowatt-hour in kilocalories (860.421) or a kilojoule in kilocalories (0.239006) is the rounded face of a fraction that never terminates — 3,600,000 ÷ 4184 does not divide evenly. The definitions stay exact; the only rounding you ever see is in the decimals the converter displays, which the precision control lets you extend.

Common Energy Conversions

Conversion	Result
1 kilowatt-hour	3.6 megajoules
1 kilowatt-hour	3600 kilojoules
1 kilowatt-hour	860.421 kilocalories
1 megajoule	0.277778 kilowatt-hours
1 kilocalorie	4184 joules
1 kilocalorie	4.184 kilojoules
1 kilojoule	0.239006 kilocalories
1 calorie	4.184 joules
1 BTU	1055.06 joules
1 watt-hour	3600 joules
1 US therm	105.506 megajoules
1 electronvolt	1.602177 × 10⁻¹⁹ joules

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Worked Energy Conversions

Nutrition label: 8368 kJ in food Calories

A meal listed as 8368 kJ on a European or Australian label is 2000 food Calories. Divide kilojoules by 4.184, the thermochemical calorie: 8368 ÷ 4.184 = 2000 kcal, which a US label would print as 2000 Calories. Reverse it by multiplying — 2000 × 4.184 = 8368 kJ — and a smaller 900 kJ snack works out to 900 ÷ 4.184 = 215 Calories. Divide by 4184 instead of 4.184 and you would report a 2000-Calorie meal as 2 Calories, the classic thousand-fold slip.

Electricity bill: 12 kWh in megajoules

Twelve kilowatt-hours off your meter is 43.2 MJ. Multiply by 3.6, since 1 kWh is exactly 3.6 MJ: 12 × 3.6 = 43.2 MJ, or 43,200 kJ. To go back, divide by 3.6 — so 43.2 MJ returns to 12 kWh. Utilities meter energy in kWh, while some gas bills quote megajoules, so this is the pair that reconciles an electricity reading with a gas one on the same statement.

Heating: a 60,000 BTU furnace in joules

A furnace rated 60,000 BTU per hour releases about 63.3 MJ of heat each hour. Multiply BTU by 1055.06 joules: 60,000 × 1055.06 = 63,303,600 J, which is 63.3 MJ or 17.6 kWh. One BTU on its own is roughly 1.055 kJ. US heating and air-conditioning gear is rated in BTU per hour, so converting to kWh lets you compare a furnace directly against an electric heat pump quoted in kilowatts.

Food versus electricity: 1 kWh in kilocalories

One kilowatt-hour carries 860.421 kilocalories of energy. Divide the 3,600,000 joules in a kWh by 4184: 3,600,000 ÷ 4184 = 860.421 kcal — roughly 43% of a 2000-kcal daily food intake. Put another way, the energy in a single unit on your electricity bill would feed an adult for close to half a day, which is why 860.421 is a rounded figure: 3,600,000 ÷ 4184 never divides evenly.

Battery: a 100 Wh laptop battery in joules

A 100 Wh battery — the airline carry-on limit for lithium cells without approval — stores 360,000 joules. Multiply watt-hours by 3600, since 1 Wh is 3600 J: 100 × 3600 = 360,000 J, or 360 kJ. That same 100 Wh is 0.1 kWh (1000 Wh make a kWh), so the battery holds about a tenth of one unit on your electricity bill. Note the battery is rated in watt-hours of energy, not watts of power — the two are not interchangeable.

Common Energy Conversion Mistakes

Reading a food Calorie as a small calorie. The "Calorie" on a Nutrition Facts label (capital C) is a kilocalorie — 1000 small calories, or 4184 joules — while a lowercase calorie is only 4.184 joules. A "300-Calorie" snack really holds 300 kcal, so a European label of 2200 kJ is 526 food Calories (2200 ÷ 4.184); dividing by 4184 instead — as if the figure were joules — undercounts it a thousand-fold.
Treating a kilowatt-hour as power. A kilowatt (kW) is a rate — how fast energy is used — while a kilowatt-hour (kWh) is an amount: the energy used when 1 kW runs for one hour. A 2 kW heater left on for 3 hours uses 6 kWh. Your bill charges kWh, which is energy; the nameplate lists kW, which is power. Mixing the two is the classic energy-versus-power slip.
Assuming every calorie is 4.184 joules. Two definitions coexist: the thermochemical calorie (4.184 J), used on food labels and in chemistry, and the International Table calorie (4.1868 J), used in steam tables. They differ by less than 0.1% — harmless for nutrition, but worth pinning down before a precise heat-load or efficiency calculation.
Confusing watt-hours with watts. A watt-hour (Wh) is a stored amount of energy; a watt (W) is the rate it flows. A 100 Wh battery running a 10 W device lasts about 10 hours — you divide the energy in Wh by the power in W. Quoting a battery in watts, or a motor in watt-hours, is that same energy-versus-power error scaled down.
Rounding a BTU to a round thousand joules. One BTU is 1055.06 joules, not 1000, and a US therm is exactly 100,000 BTU — about 105.5 MJ or 29.3 kWh. Treating a BTU as a kilojoule shaves roughly 5% off a heating figure, which stacks up fast across the tens of thousands of BTU on a furnace or gas bill.

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Energy Converter — Frequently Asked Questions

What is the difference between a calorie and a Calorie?

A small calorie (lowercase) is 4.184 joules — the heat that warms one gram of water by one degree Celsius. A food Calorie (capital C) is a kilocalorie: 1000 small calories, or 4184 joules. So a 2000-Calorie diet means 2000 kilocalories.

How do I convert kWh to MJ?

Multiply by 3.6. So 1 kWh is 3.6 MJ and 10 kWh is 36 MJ. The two are linked by fixed SI prefixes, so the factor never changes — reverse it by multiplying megajoules by 0.277778.

How do I convert kilocalories to kilojoules?

Multiply by 4.184. So 100 kcal is 418.4 kJ, and a 2000-kcal daily diet is 8368 kJ (8.368 MJ). To go the other way, multiply kilojoules by 0.239006 to get kilocalories. The 4.184 factor is the thermochemical calorie used on food labels.

How do I convert kilojoules to Calories on a food label?

Divide the kilojoules by 4.184, because a food Calorie is a kilocalorie. So a 2000 kJ portion is 2000 ÷ 4.184 = 478 Calories, and a 900 kJ snack is about 215 Calories. European and Australian labels list kJ, while US labels list Calories, so this is the conversion that lets you compare them.

How many calories are in a kilowatt-hour?

One kilowatt-hour equals 860.421 kilocalories — about 860,421 small calories. That is a little under half of a typical 2000-kcal daily food intake, which shows how much energy a single unit on your electricity bill really carries.

How do I convert joules to kWh?

Divide the joules by 3,600,000, since 1 kWh is exactly 3.6 million joules. So 7,200,000 J is 2 kWh and 1,800,000 J is 0.5 kWh. To reverse it, multiply kilowatt-hours by 3,600,000 to get joules.

What is 1 BTU in joules?

One BTU is 1055.06 joules, roughly 1.055 kilojoules (the exact NIST value is 1055.05585262 J). The BTU shows up mostly on US furnace and air-conditioner nameplates, where heating and cooling capacity is quoted in BTU per hour.

How many joules are in a watt-hour?

One watt-hour is exactly 3600 joules, because it is one watt sustained for 3600 seconds. So a 60 Wh battery holds 60 × 3600 = 216,000 J, and 1000 watt-hours make one kilowatt-hour (3,600,000 J).

What is a therm and how many joules is it?

A US therm is exactly 100,000 BTU, which is about 105.5 megajoules or 29.3 kWh. It is a natural-gas billing unit, so a gas statement quoted in therms can be lined up against an electricity bill in kWh once both are on the same scale.

Is a kilowatt-hour a measure of power?

No — a kilowatt-hour measures energy, not power. A kilowatt (kW) is the rate at which energy is used; a kilowatt-hour (kWh) is the amount used when 1 kW runs for one hour. Your electricity bill charges for kWh, while an appliance nameplate states its kW draw. Confusing the two is the most frequent energy mistake.

Which calorie does this converter use, thermochemical or IT?

Both are available. The thermochemical calorie (4.184 J) matches food labels and chemistry references, while the International Table calorie (4.1868 J) appears in steam tables and some HVAC work. They differ by less than 0.1% — negligible for nutrition, but useful to distinguish for precise heat calculations.

What is an electronvolt used for?

The electronvolt (eV) measures the tiny energies of atoms and subatomic particles, where a joule would be an unwieldy string of zeros. One eV is 1.602177 × 10⁻¹⁹ joules, fixed by the 2019 SI redefinition of the elementary charge, and it is the working unit of atomic, nuclear and particle physics.

How accurate is this energy converter?

It uses the standard defined factors — 1 kWh is 3.6 MJ, 1 kcal is 4184 J, 1 cal is 4.184 J, and 1 BTU is 1055.05585262 J. Every unit here is linked to the joule by an exact definition, so the only rounding you ever see comes from how many decimals you choose to display.

Is this energy converter free to use?

Yes, and it needs no sign-up. Every unit group is covered — SI joules, electrical watt-hours, thermal calories and BTU, and the electronvolt for physics work — so one page handles energy problems from a food label to a particle collision. An embeddable version is available for HVAC, energy-audit, nutrition and physics sites.

How big is 1 kWh in everyday terms?

1 kWh is 3.6 MJ, or 860.421 kcal — a little under half an adult's 2000-kcal daily food energy. In practical terms it is roughly what a 1000 W microwave draws in one hour, or a 10 W LED bulb over 100 hours.

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