Convert BTU to joules or joules to BTU, with a selectable BTU definition for closer textbook or HVAC matches.
Table of contents
How to use our BTU to Joules Converter
- Choose What do you want to convert? so the visible amount row matches your starting unit.
- Enter BTU amount (BTU) or Joule amount (J); commas are okay, and negative values are allowed for relative energy changes.
- Pick BTU definition; use International Table BTU, common unless your source specifically says thermochemical BTU or Btu_th.
- If needed, use Number display and Digits after decimal point to make the answer easier to copy or compare.
- After you calculate, check Conversion factor used and Quick unit note; if your source says BTU per hour, you need a power conversion instead of this energy conversion.
Definitions
BTU amount (BTU): Energy measured in British thermal units. BTU is a unit of energy, not a rate.
Joule amount (J): Energy measured in joules. A joule is an SI unit of work or energy [3].
BTU definition: The exact BTU type used to choose the joules-per-BTU factor.
International Table BTU: The default BTU choice in this converter, using 1055.056 J per BTU.
Thermochemical BTU: A slightly different BTU definition, using 1054.350 J per BTU.
Converted energy: The main answer after changing the starting energy into the target unit.
Conversion factor used: The number of joules matched to 1 BTU for the selected BTU definition.
Kilojoule (kJ): 1000 joules.
Megajoule (MJ): 1000000 joules.
Watt-hour (Wh): Energy equal to 3600 joules, because 1 watt is 1 joule per second and 1 hour is 3600 seconds.
Common mistakes and quick fixes
Mistake: Typing a BTU per hour value into BTU amount (BTU).
Fix: Use BTU amount (BTU) only for energy, and use Quick unit note to catch rate units like BTU/h.
Mistake: Leaving Joule amount (J) filled while What do you want to convert? is set to BTU to joules.
Fix: Only the visible amount row is used, so switch What do you want to convert? if joules are your starting value.
Mistake: Comparing Converted energy to a source that used a different BTU definition.
Fix: Match BTU definition to the source, then compare Conversion factor used.
Mistake: Rounding too early with Digits after decimal point when checking homework or a table.
Fix: Use more digits until the final step, then round the final Converted energy if needed.
Mistake: Reading Same energy in watt-hours as watts.
Fix: Same energy in watt-hours is energy in Wh, not power; power needs time, such as hours or seconds.
Limitations & Key Assumptions / Boundary Conditions
- The default factor is International Table BTU: 1 BTU = 1055.056 J. Thermochemical BTU uses 1054.350 J per BTU, so small differences from other sites can be normal.
- This converter handles energy only. BTU per hour, watts, and tons of cooling are power or rate units, not plain BTU energy amounts.
- Negative entries are allowed only as signed energy changes, such as heat removed. They do not mean a stored object has less than zero energy.
- Very large or very small results may be rounded by the selected Number display and Digits after decimal point settings.
- The watt-hour result is an energy comparison. It does not estimate an electric bill unless you also know price per kWh and device efficiency.
Methodology
Core conversion
The calculator first chooses the BTU-to-joule factor from BTU definition. International Table BTU uses 1055.056 J per BTU, and thermochemical BTU uses 1054.350 J per BTU. These are standard energy conversion factors [2].
joules = btu_amount * btu_to_joule_factor
btu = joule_amount / btu_to_joule_factor
The reverse conversion uses the same factor as the forward conversion, so converting from BTU to joules and then back to BTU returns the starting amount except for displayed rounding.
Extra energy units
When the target is joules, the calculator also shows the same energy in kJ, MJ, and Wh so large joule values are easier to read.
kilojoules = joules / 1000
megajoules = joules / 1000000
watt_hours = joules / 3600
Mini example
For 100 BTU with International Table BTU selected, the calculation is 100 * 1055.056 = 105505.6 J. The same energy is 105.5056 kJ, 0.1055056 MJ, and 29.3071111111 Wh before display rounding.
Input handling
The visible amount is the only amount used. If What do you want to convert? is BTU to joules, Joule amount (J) is ignored. If it is Joules to BTU, BTU amount (BTU) is ignored. Commas and spaces are removed before parsing, but blank values, non-number text, and BTU per hour style entries are blocked with an error.