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BTU to Joules Converter

Use this free BTU to joules converter to convert energy between BTU, joules, kilojoules, calories, kilocalories and kilowatt-hours. Enter a value in any one unit and it instantly shows the same amount of energy in all the others — in both directions, with no page reload and no sign-up.

What is a BTU?

A BTU, or British thermal unit, is a unit of energy used mainly for heating and cooling. One BTU is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. That old-fashioned definition is why the conversion to joules is not a round number: 1 BTU equals 1,055.06 joules. Air conditioners, furnaces and water heaters are commonly rated in BTU — a small window air conditioner might be rated at 5,000 BTU, a typical home system at 36,000 BTU, and a large furnace at 80,000 BTU or more.

The joule is the standard scientific unit of energy. One joule is a small amount — about the energy needed to lift a small apple one metre — so everyday amounts are usually given in kilojoules (1,000 joules) or megajoules. Food energy is measured in calories: one small calorie is 4.184 joules, and one kilocalorie — the "calorie" printed on food labels — is 4,184 joules. The kilowatt-hour is the unit on electricity bills: one kilowatt-hour is the energy used by a 1,000-watt appliance running for one hour, and it equals 3,600,000 joules.

How the conversion works

All six units in this converter measure the same thing — energy — so converting between them is a matter of multiplying by a fixed factor. The converter first turns your input into joules, then turns those joules into every other unit. The exact factors it uses are: 1 BTU = 1,055.06 J; 1 kilojoule = 1,000 J; 1 calorie = 4.184 J; 1 kilocalorie = 4,184 J; and 1 kilowatt-hour = 3,600,000 J.

From those, the other useful factors follow. One kilowatt-hour equals 3,600,000 ÷ 1,055.06 = 3,412.14 BTU, which is the figure you will see on heating and electricity calculations everywhere. One BTU equals 1,055.06 ÷ 4.184 = 252.16 calories, or 0.25216 kilocalories — so a BTU is about a quarter of a food calorie. One kilocalorie equals 4,184 ÷ 1,055.06 = 3.97 BTU, and one kilojoule equals 1,000 ÷ 1,055.06 = 0.948 BTU.

Do not confuse energy with power. Energy is a total amount — a BTU, a joule, a kilowatt-hour. Power is a rate — how fast energy is used — measured in watts or in BTU per hour. An air conditioner rated at 12,000 BTU per hour removes 12,000 BTU of heat every hour it runs; run it for 5 hours and it has moved 60,000 BTU of heat, which is about 17.58 kWh. This converter handles energy units only. If you need to convert a power rating, such as BTU per hour to watts, use a power converter instead.

A practical place where these conversions matter is comparing heating costs. Gas is often billed by the therm, where 1 therm = 100,000 BTU = 29.3 kWh, while electricity is billed by the kilowatt-hour. Converting both to the same unit — joules or kilowatt-hours — is the only fair way to compare what a unit of heat actually costs from each fuel.

Worked example

A portable heater is rated at 1,000 BTU of heat output. How much energy is that in joules, and how many kilowatt-hours would 12,000 BTU be?

Step 1: BTU to joules — 1,000 BTU × 1,055.06 J per BTU = 1,055,060 joules. In kilojoules, that is 1,055,060 ÷ 1,000 = 1,055.06 kJ.
Step 2: BTU to kilocalories — 1,000 BTU × 0.25216 kcal per BTU = 252.16 kcal.
Step 3: Kilowatt-hours to BTU — 1 kWh × 3,412.14 BTU per kWh = 3,412.14 BTU.
Step 4: 12,000 BTU to kilowatt-hours — 12,000 ÷ 3,412.14 = 3.52 kWh. So a 12,000 BTU air conditioner running for one hour uses about 3.52 kWh of energy — and a 100 kWh electricity bill is the same as 341,214 BTU.

Try it yourself: enter 1 in the box, pick kilowatt-hours, and the converter shows 3,412.14 BTU and 3,600,000 joules. Enter 500 kilocalories and it shows about 1,984 BTU — close to the energy in a large meal.

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