Joules to BTU:
multiply by 0.000947817

How many BTU are in a joule?

One joule equals 0.0009478171203 BTU. That is less than one thousandth of a BTU, because the British thermal unit is a much bigger amount of energy: 1 BTU is 1,055.055853 joules.

The converter on this site uses the International Table (IT) BTU. Other BTU definitions exist, and they differ from the IT value by a small fraction of a percent, which matters only for precise engineering work.

The joule to BTU formula

Because the factor is so small, it is easier to think of joules and BTU as being about a thousand to one. Divide joules by 1,000 as a first guess, then correct it slightly: 5,000 J ÷ 1,000 is 5, and the exact answer of 4.74 is about 5 percent lower.

1. 1Write the energy in joules. If you have kilojoules, multiply by 1,000 first.
2. 2Multiply by 0.0009478171203, or divide by 1,055.055853.
3. 3Round to a sensible number of decimals for the size of the answer.
4. 4Check that the BTU figure is a little under one thousandth of the joules.

Chart: joules to BTU

The BTU column is long because one joule is such a small amount of energy. For larger amounts, convert kilojoules or megajoules first and divide by the BTU size.

Where joules and BTU meet in practice

Heating and cooling equipment in the US and some other markets is rated in BTU per hour, while the science behind it, including most textbooks, works in joules and watts. An air conditioner rated 12,000 BTU per hour moves 12,000 × 1,055.055853 = 12,660,670 joules each hour.

A second example: one kilowatt-hour is 3,600,000 joules, and 3,600,000 × 0.0009478171203 = 3,412.14 BTU. That is why a 1 kW electric heater adds about 3,412 BTU of heat every hour it runs.

Quick sanity checks for joules to btu

Pair the units in your head: 1,000 J is 0.948 BTU, and 1,055 J is one BTU. So 2,110 J is almost exactly 2 BTU.

For larger energies, convert through kilojoules. A 500 kJ amount is 500,000 J, and 500,000 × 0.0009478171203 = 473.91 BTU.

If the BTU result is not a little smaller than the joules divided by 1,000, something went wrong in the arithmetic.

Common mistakes when converting J to BTU

• Multiplying by 1,055 instead of 0.000948. That factor is for BTU to joules, so a joule figure would come out over a million times too large.
• Confusing BTU with BTU per hour. The first is energy, the second is power, so only the second compares with watts.
• Dropping zeros. 0.0009478 and 0.009478 differ by a factor of ten, and the second gives 10 times too much energy.
• Mixing definitions. The IT BTU used here is 1,055.056 J, so values from other definitions will not match exactly.