What a BTU, a therm and a kilowatt-hour each measure
All three measure the same physical quantity — energy — and differ only in size and in which industry adopted them. Converting between them is exact, because each is defined as a fixed number of joules.
The British thermal unit is roughly the heat needed to raise one pound of water by one Fahrenheit degree. ‘Roughly’ matters, because water's specific heat varies slightly with temperature, which is why several BTUs exist. This calculator uses the International Table BTU of exactly 1,055.05585262 J, the value adopted for engineering and trade. The thermochemical BTU is 1,054.35 J and the 59 °F BTU is 1,054.804 J; the spread between them is under 0.07%, invisible on a heating load and material on a large gas contract.
The therm is the unit US and UK gas utilities bill in, because it is close to the energy in 100 cubic feet of natural gas. The US therm is defined as exactly 105,480,400 J, which is 100,000 BTU measured at 59 °F. In International Table BTU that comes to 105,480,400 ÷ 1,055.05585262 = 99,976.13 BTU, so a therm is 0.024% smaller than 100,000 IT BTU — near enough to quote as “100,000 BTU” on a bill, far enough to matter when a wholesale contract reconciles therms against MMBTU. The MMBTU is a million BTU and is the unit wholesale gas is traded in. Gas-industry notation uses Roman numerals, so M is a thousand and MM is a thousand thousand — the opposite of what an SI-trained reader expects, and the single most common misreading of a gas document.
The kilowatt-hour is one kilowatt sustained for one hour, so it is exactly 3.6 million joules. It is the electricity billing unit almost everywhere. Because it is defined from power and time rather than from a property of water, it is exact by construction.
Once everything is in joules the comparison is honest. One therm of gas carries 29.3 kWh of chemical energy, which is why gas has historically been so much cheaper per unit of heat than electricity, and why a heat pump delivering three units of heat per unit of electricity changes that comparison entirely.
Those exact figures are not house conventions. ISO 80000-5 (Quantities and units — Thermodynamics) fixes the quantity definitions, and NIST SP 811, the US guide to SI usage, tabulates the conversion factors this page uses: the International Table BTU at 1,055.05585262 J exactly and the kilowatt-hour at 3.6 MJ exactly. Because both are exact by definition rather than measured, a BTU→kWh conversion carries no uncertainty of its own — whatever error your answer has came in with the input.
Energy versus power, and why BTU/h is not BTU
The confusion that produces the most wrong answers on this subject is not a conversion factor. It is treating a rate as a quantity.
Energy is a total: joules, kWh, BTU, therms. Power is a rate: watts, kW, BTU/h, horsepower. A 12,000 BTU/h air conditioner is not storing 12,000 BTU; it is moving 12,000 BTU of heat every hour it runs. Run it for six hours and it has moved 72,000 BTU, which is 21.1 kWh of heat — not the same as 21.1 kWh of electricity consumed, because the machine is a heat pump and moves several units of heat per unit of electricity it draws.
The conversion between the two families is time. Divide energy by hours and you get average power; multiply power by hours and you get energy. That is what the delivered over field on this page does, and it is why one kilowatt-hour delivered in one hour is exactly one kilowatt, and 3,412.14 BTU/h.
Two rate figures worth carrying: 1 kW = 3,412.14 BTU/h, and one ton of refrigeration = 12,000 BTU/h = 3.517 kW of heat moved. The ton figure comes from the heat needed to melt one short ton of ice in 24 hours, which is where air-conditioning capacity ratings originally came from. For the mechanical side of the same equipment, our horsepower to kilowatts calculator handles motor and compressor ratings, and the electrical power calculator converts between volts, amps and watts on the supply side.
Worked example: an 85-therm gas bill in kWh
Your winter gas bill shows 85 therms. You want to know what that is in kWh so you can compare it against an electric heating quote.
- Into joules. 85 therms × 105,480,400 J/therm = 8,965,834,000 J, or 8,965.834 MJ.
- Out to kWh. 8,965,834,000 ÷ 3,600,000 = 2,490.51 kWh. The shortcut is 85 × 29.30011 = 2,490.51, using the therm-to-kWh factor directly.
- Into BTU. 8,965,834,000 ÷ 1,055.05585262 = 8,497,971 BTU, or 8.498 MMBTU — not 8.5 exactly, because of the 0.024% gap between the therm and 100,000 IT BTU.
- As an average rate. Over a 30-day billing period, 2,490.51 ÷ 720 h = 3.459 kW average, equal to 8,497,971 ÷ 720 = 11,803 BTU/h. That is the steady heat output your house needed on average across the month.
- Compare on cost. At $0.16/kWh, the same energy delivered as electric resistance heat would cost 2,490.5 × 0.16 = $398.48. A heat pump with a seasonal coefficient of performance of 3.0 would need a third of the electricity for the same delivered heat, so about $132.83 — the arithmetic being 398.48 ÷ 3.
Every number above is reproducible from the two factors 100,000 BTU per therm and 1,055.05585262 J per BTU. Nothing is estimated except the coefficient of performance, which is a machine property you should take from the equipment's own rating.
Energy unit equivalences
| One unit of | Joules | BTU | kWh | MJ |
|---|---|---|---|---|
| BTU (IT) | 1,055.05585262 | 1 | 0.000293071 | 0.001055056 |
| therm (US) | 105,480,400 | 99,976.13 | 29.3001111 | 105.4804 |
| MMBTU (106 BTU) | 1,055,055,852.62 | 1,000,000 | 293.0710702 | 1,055.0559 |
| kilowatt-hour | 3,600,000 | 3,412.141633 | 1 | 3.6 |
| watt-hour | 3,600 | 3.412141633 | 0.001 | 0.0036 |
| megajoule | 1,000,000 | 947.8171203 | 0.277777778 | 1 |
| gigajoule | 1,000,000,000 | 947,817.1203 | 277.777778 | 1,000 |
| kilocalorie (food Calorie) | 4,186.8 | 3.968320717 | 0.001163 | 0.0041868 |
| foot-pound (ft·lbf) | 1.3558179483 | 0.001285067 | 3.76616 × 10−7 | 1.35582 × 10−6 |
The kWh, joule, therm and kilocalorie factors are exact by definition. BTU and foot-pound are irrational in decimal and shown to the digits given.
MMBTU means a million, MBTU means a thousand
Gas and oil documentation inherited Roman-numeral notation, where M is 1,000. So MBTU is one thousand BTU and MMBTU is one thousand thousand — a million. An SI-trained reader sees the M in MMBTU and reasonably assumes mega, which happens to give the same answer, but then reads MBTU as a million and is out by a factor of 1,000. When a document uses both MCF and MMBTU, it is using the Roman convention throughout. When it uses MJ and GJ alongside, check every figure against a plausible physical magnitude before trusting it.
Assumptions and limits of a pure energy conversion
- Conversion says nothing about usable heat. A gas furnace at 80% annual efficiency delivers 0.8 × 29.30 = 23.44 kWh of heat per therm burned; the other 5.86 kWh goes up the flue.
- Electricity is not energy-equivalent to fuel at the meter. A kWh delivered to a resistance heater becomes one kWh of heat, while the same kWh driving a heat pump can move two to four times that much. Compare delivered heat, not metered energy.
- Gas is metered by volume, not energy. Utilities convert cubic feet or cubic metres to therms using a calorific value that changes with gas composition, so the therm figure on your bill already includes a measured heating value.
- Higher and lower heating value differ. US gas billing uses higher heating value, which counts the latent heat of the water vapour in the exhaust. Equipment efficiencies quoted against lower heating value are not directly comparable.
- The BTU has several definitions. This page uses the International Table BTU. Thermochemical and 59 °F BTUs differ by well under 0.1%, which only matters on wholesale quantities.
- Average power is not peak power. Dividing a month's energy by the hours in the month gives a mean, not the demand charge your supplier bills on, which is set by the highest short interval.
Catching a wrong-unit entry before it costs you
The one failure mode unique to a converter with eleven interchangeable units is picking the wrong one from a dropdown and getting an answer that still looks plausible. A misread MMBTU for BTU, or kWh for MWh, does not produce an obviously broken result — it produces a number that is wrong by a clean factor of a thousand or a million and reads as perfectly ordinary to anyone not holding the input in their head.
The fix is to keep one anchor ratio in mind rather than trust the dropdown labels alone: 1 kWh = 3,412.14 BTU, and 1 MMBTU = 293.07 kWh. Converting between kWh and BTU should move the number by roughly three orders of magnitude in one direction; converting between kWh and MMBTU should move it by roughly two and a half orders in the other. If your result differs from the input by something close to 1,000 or 1,000,000 instead, the units you selected are one step removed from the ones you meant — most often BTU where MMBTU was meant, or the reverse.
This matters most exactly where the stakes are highest: reconciling a contract quantity or checking a wholesale invoice, where the physical-plausibility check that catches a misplaced decimal on a household bill does not exist, because nobody has an intuition for what a correct MMBTU total looks like. Re-derive the expected order of magnitude from the anchor ratio before trusting an output you did not first estimate by hand.
Which unit belongs in which document
Use the unit the receiving document expects, and convert once at the boundary rather than repeatedly.
Utility bills. Electricity is kWh everywhere. Gas is therms or CCF in the United States, cubic metres and kWh in the United Kingdom and much of Europe, and gigajoules in Canada. If you are comparing tariffs across those systems, convert everything to kWh first, then compare price per kWh directly — the appliance energy cost calculator works in the same currency of kWh for individual loads.
HVAC. Equipment capacity is BTU/h or tons in North America and kW in the rest of the world. Heating and cooling loads from a Manual J or equivalent calculation come out in BTU/h. Remember that these are rates: sizing needs the rate, running-cost estimates need the rate multiplied by the run hours.
Fuel and generation. Wholesale gas trades in MMBTU or, in Europe, in MWh of gas. Diesel and propane are quoted per gallon or per litre and converted through their heating values; a generator's fuel burn per hour combined with its output tells you the real conversion efficiency, which the generator fuel consumption calculator works through.
Food and nutrition. The Calorie on a nutrition label is a kilocalorie, 4,186.8 J. A 2,000 Calorie daily intake is 8.37 MJ, or 2.33 kWh — roughly what a household kettle uses in an hour of continuous boiling, which is a useful reminder of how energy-dense fuel is compared with food.
Mechanical work. Foot-pounds and joules are work, the same physical quantity as heat since Joule's experiments established the mechanical equivalent. That equivalence is what lets a single conversion table cover a gas bill and a torque-times-angle calculation; the torque half of that is on our torque unit converter, and the pressure side of any thermodynamic cycle on the psi to bar calculator.