BTU to joules:
multiply by 1,055.056

How many joules are in a BTU?

One British thermal unit (International Table definition) is 1,055.055853 joules, or about 1.055 kilojoules. The BTU is the amount of heat traditionally associated with warming one pound of water by one degree Fahrenheit, and the joule is the SI unit of energy.

More than one BTU definition exists. The thermochemical and other versions differ from the International Table value in the third or fourth significant figure. The converter on this site uses the International Table BTU, so use the factor 1,055.055853 to match it.

The BTU to joule formula

1. 1Write the energy in BTU.
2. 2Multiply by 1,055.055853.
3. 3For large results, divide by 1,000 for kJ or 1,000,000 for MJ.
4. 4Check that the answer is a little over a thousand times the BTU figure.

Chart: BTU to joules

Rounded to three decimals. For a rough figure, remember that 1 BTU is just over 1 kJ, so 1,000 BTU is a little over 1 MJ.

Real uses: air conditioners and heaters

Air conditioners, heaters and boilers are often rated in BTU per hour, which is a rate of energy, not a total. A 12,000 BTU/h air conditioner moves 12,000 × 1,055.055853 = 12,660,670.2 joules every hour. Dividing by the 3,600 seconds in an hour gives about 3,516.85 joules per second, or roughly 3,517 watts of cooling.

A second check from the electricity side: one kilowatt-hour is 3,600,000 joules, which is 3,600,000 ÷ 1,055.055853 = about 3,412.14 BTU. This helps compare a gas appliance quoted in BTU with an electric one billed in kWh.

More examples: small and large amounts

Convert 5,000 BTU and you get 5,275,279.265 J, about 5.28 MJ. Convert a small amount, 2,500 J, and divide by 1,055.055853 to find 2.37 BTU. The factor works at any scale, though very small values are rarely quoted in BTU.

For power ratings, the useful link is the hour. One BTU per hour is 1,055.055853 J divided by 3,600 s, which is about 0.2931 W. A heater rated at 5,000 BTU/h therefore gives roughly 1,465 W of heat.

If you compare appliances, always confirm whether a figure is BTU or BTU per hour. A rating of 12,000 describes how fast heat moves, while a bill or a fuel quantity describes how much energy was used over time. Joules and kilowatt-hours belong with the second kind.

Common mistakes when converting BTU to joules

• Confusing BTU with BTU per hour, then reporting watts where joules were meant.
• Using 1,055 for every purpose. That is fine for estimates but adds a small error on large totals.
• Mixing BTU definitions from different sources, which differ slightly.
• Dividing instead of multiplying. A BTU is larger than a joule, so the joule figure must be bigger.