Unit & Measurement Tables
Energy Conversion Table
Energy is the unit family with the most aliases. The same quantity of heat is a joule in physics, a calorie on a food label, a BTU on an air conditioner, a therm on a gas bill and a kilowatt hour on an electricity bill. None of these are different kinds of energy — they are different trades measuring the same thing with the instrument they happen to own.
Last updated
Two definitional traps sit underneath the arithmetic. First, the calorie on a nutrition label is a kilocalorie: 1,000 small calories, the amount of heat needed to raise one kilogram of water by one degree. Confusing the two puts a 250 kcal snack at 250 calories when it is really 250,000. Second, the BTU has several variants — the IT BTU used here is 1,055.055 85 J, while the thermochemical BTU is slightly different — and the difference is small enough to hide inside a rounding error but large enough to matter in bulk gas contracts.
The tables below use the exact definitions: 4.184 J per thermochemical calorie, 3.6 MJ per kWh, 1,055.055 852 62 J per IT BTU and 105,480,400 J per US therm. Every cell is computed at build time, so the figures stay consistent with each other.
Kilojoules to kilocalories, BTU, watt hours and kilowatt hours
| Kilojoules | Kilocalories | BTU | Watt hours | Kilowatt hours |
|---|---|---|---|---|
| 1 | 0.239 | 0.95 | 0.278 | 0.000278 |
| 5 | 1.195 | 4.74 | 1.389 | 0.001389 |
| 10 | 2.39 | 9.48 | 2.778 | 0.002778 |
| 50 | 11.95 | 47.39 | 13.889 | 0.013889 |
| 100 | 23.901 | 94.78 | 27.778 | 0.027778 |
| 250 | 59.751 | 236.95 | 69.444 | 0.069444 |
| 500 | 119.503 | 473.91 | 138.889 | 0.138889 |
| 1,000 | 239.006 | 947.82 | 277.778 | 0.277778 |
| 2,500 | 597.514 | 2,369.54 | 694.444 | 0.694444 |
| 5,000 | 1,195.029 | 4,739.09 | 1,388.889 | 1.388889 |
| 10,000 | 2,390.057 | 9,478.17 | 2,777.778 | 2.777778 |
A food label's 250 kcal is 1,046 kJ. The kilojoule figure is always the larger number, which is why a 2,000 kcal diet shows as about 8,400 kJ on an Australian label.
Kilowatt hours to megajoules, kilocalories, BTU and therms
| Kilowatt hours | Megajoules | Kilocalories | BTU | Therms |
|---|---|---|---|---|
| 0.5 | 1.8 | 430 | 1,706 | 0.0171 |
| 1 | 3.6 | 860 | 3,412 | 0.0341 |
| 2 | 7.2 | 1,721 | 6,824 | 0.0682 |
| 5 | 18 | 4,302 | 17,061 | 0.1706 |
| 10 | 36 | 8,604 | 34,121 | 0.3412 |
| 20 | 72 | 17,208 | 68,243 | 0.6824 |
| 50 | 180 | 43,021 | 170,607 | 1.7061 |
| 100 | 360 | 86,042 | 341,214 | 3.4121 |
| 250 | 900 | 215,105 | 853,035 | 8.5304 |
| 500 | 1,800 | 430,210 | 1,706,071 | 17.0607 |
| 1,000 | 3,600 | 860,421 | 3,412,142 | 34.1214 |
One kWh is 3.6 MJ exactly. US gas bills are quoted in therms and electricity bills in kWh, so the 29.31 kWh per therm figure is the one to remember when comparing heating costs.
BTU to kilojoules, kilowatt hours and therms
| BTU | Kilojoules | Kilowatt hours | Therms |
|---|---|---|---|
| 1,000 | 1,055.1 | 0.293 | 0.01 |
| 5,000 | 5,275.3 | 1.465 | 0.05 |
| 10,000 | 10,550.6 | 2.931 | 0.1 |
| 20,000 | 21,101.1 | 5.861 | 0.2 |
| 50,000 | 52,752.8 | 14.654 | 0.5 |
| 100,000 | 105,505.6 | 29.307 | 1 |
| 500,000 | 527,527.9 | 146.536 | 5 |
| 1,000,000 | 1,055,055.9 | 293.071 | 10 |
A 12,000 BTU air conditioner is a one-ton unit and draws about 3.5 kWh per hour of running. 100,000 BTU is exactly one therm, the standard unit on a US gas bill.
Kilocalories to kilojoules, watt hours, BTU and foot-pounds
| Kilocalories | Kilojoules | Watt hours | BTU | Foot-pounds |
|---|---|---|---|---|
| 1 | 4.2 | 1.16 | 3.97 | 3,086 |
| 10 | 41.8 | 11.62 | 39.66 | 30,860 |
| 50 | 209.2 | 58.11 | 198.28 | 154,298 |
| 100 | 418.4 | 116.22 | 396.57 | 308,596 |
| 250 | 1,046 | 290.56 | 991.42 | 771,490 |
| 500 | 2,092 | 581.11 | 1,982.83 | 1,542,980 |
| 1,000 | 4,184 | 1,162.22 | 3,965.67 | 3,085,960 |
| 2,000 | 8,368 | 2,324.44 | 7,931.33 | 6,171,920 |
| 2,500 | 10,460 | 2,905.56 | 9,914.17 | 7,714,900 |
The calorie on a nutrition label is a kilocalorie — 1,000 of the small calories a physicist means. This table uses the label convention throughout.
How to Use These Tables
- Food energy: multiply kilocalories by 4.184 to get kilojoules. A 500 kcal meal is 2,092 kJ, which is the number an EU label would print.
- Electricity: multiply kWh by 3.6 to get megajoules, or by 3,412 to get BTU. A 1,500 W heater running four hours uses 6 kWh, which is 21.6 MJ.
- Gas: divide therms by 0.02931 to get kWh, or multiply kWh by 0.03412 to get therms. 100 therms is 2,931 kWh of heat.
- Cooling: divide BTU by 12,000 to get tons of cooling capacity. A 36,000 BTU system is a three-ton unit.
- Mechanical work: multiply foot-pounds by 1.35582 to get joules, then compare with the electrical input to find the efficiency of the machine.
Common Questions
Is a calorie the same as a kilocalorie?
No, and the labels make it worse by calling kilocalories "calories". A dietary Calorie with a capital C is 1,000 small calories, or 4.184 kJ. Everything on this page uses the dietary convention for kcal.
How many kWh are in a therm?
29.31 kWh. A therm is 105.48 MJ and a kWh is 3.6 MJ, so 105.48 ÷ 3.6 = 29.31. This is the conversion to use when comparing gas heating with electric heating.
Why does a kWh equal 3.6 MJ?
Because a watt is one joule per second. An hour is 3,600 seconds, and a kilowatt is 1,000 watts, so 1,000 × 3,600 = 3,600,000 J. The factor is exact, not an approximation.
What is a BTU?
The heat needed to raise one pound of water by one degree Fahrenheit, about 1,055 J. It survives in US heating and cooling because equipment is sized in BTU per hour, and 12,000 BTU/h is one ton of cooling.
Which BTU definition does this table use?
The International Table BTU, 1,055.055 852 62 J, which is the one used in US engineering practice and in the ASHRAE handbooks. The thermochemical BTU differs by about 0.03%.
How do I convert joules to calories?
Divide by 4.184. A 1,000 J burst of work is 239 calories, which is a quarter of a dietary Calorie — a reminder of how much energy a food calorie really is.
More Unit & Measurement Tables
Sources
- NIST Special Publication 811 — units for energy, work and heat — National Institute of Standards and Technology
- NIST Guide to the SI, Chapter 5 — non-SI units accepted for use — National Institute of Standards and Technology
- Energy Information Administration — units and calculators explained — U.S. Energy Information Administration
Filed with Unit Conversion calculations.