BTU Converter (BTU, therm, ft·lbf)

Understanding the BTU converter — concept and principle

The British thermal unit (BTU) and the therm are energy units still used in North American and British heating, gas billing and air-conditioning. This converter changes a quantity between BTU, therms, MMBtu, kilowatt-hours, joules, kilocalories and foot-pounds, so you can compare a gas bill in therms with an electricity bill in kWh, or read a furnace rating in SI units.

How it is calculated

Every unit is first turned into joules, then divided by the size of the target unit:

result = value × (joules per "from" unit) ÷ (joules per "to" unit).

• 1 BTU (IT) = 1,055.05585262 J
• 1 US therm = 105,480,400 J (100,000 BTU at 59 °F, about 99,976 IT BTU)
• 1 EC therm = 105,506,000 J
• 1 MMBtu = 1,000,000 BTU
• 1 kWh = 3,600,000 J; 1 kcal (IT) = 4,186.8 J; 1 ft·lbf = 1.3558179483 J

The principle

A BTU was originally the heat needed to warm one pound of water by one degree Fahrenheit. That amount depends slightly on the starting temperature, so several BTU definitions exist; the International Table (IT) BTU fixes it exactly in joules. The US therm is defined from the 59 °F BTU, which is why one therm is not exactly 100,000 IT BTU. Since all of these units measure the same physical quantity, energy, conversion is a single multiplication through the joule, the SI unit.

Worked example

Default: 1 US therm to kWh.

1. Convert to joules: 1 × 105,480,400 J = 105,480,400 J.
2. One kWh is 3,600,000 J.
3. 105,480,400 ÷ 3,600,000 = 29.30011111 kWh.
4. The same energy is 99,976.13 IT BTU, 25,193.56 kcal and 0.09998 MMBtu.

How to read the result

About 29.3 kWh per therm lets you compare fuel prices: divide the price per therm by 29.3 to get a price per kWh of gas energy, then compare it with electricity after allowing for appliance efficiency. Air-conditioner ratings in "BTU/h" are power, not energy: 12,000 BTU/h (one "ton" of cooling) is about 3.5 kW. Natural gas is often sold by MMBtu on markets; 1 MMBtu is roughly 10 therms.

Common mistakes

• Treating BTU/h (power) as BTU (energy); you must multiply power by hours to get energy.
• Assuming 1 therm = 100,000 IT BTU exactly; the difference is small but shows up in precise work.
• Mixing up MMBtu (million BTU) with "MBtu", which some industries use for one thousand BTU.
• Comparing gas and electricity per kWh without accounting for boiler or heat-pump efficiency.

FAQ

Frequently asked questions

How many BTU are in one kWh?

About 3,412.14 IT BTU.

How many therms are in a cubic meter of natural gas?

It depends on the gas heating value; a typical figure is around 0.36 therm, but use your supplier's number.

What does a 9,000 BTU air conditioner mean?

It removes about 9,000 BTU of heat per hour, roughly 2.6 kW of cooling.

Sources: NIST Special Publication 811 (2008), Appendix B · BIPM, The International System of Units (SI Brochure), 9th edition