How do BTUs, tons and kilowatts convert for heating and cooling?
A ton of cooling, a therm of gas and a kilowatt-hour all count heat. The conversions, and the one mix-up that trips most people up.
Published 3 min read
In short
One kilowatt-hour equals 3,412 Btu and one therm equals 100,000 Btu. One ton of cooling capacity equals 12,000 Btu per hour, which is about 3.52 kW.
Btu and kWh measure an amount of energy; Btu per hour, tons and kilowatts measure a rate. Equipment size is a rate; your bill is an amount.
Heating and cooling in the United States are counted in a mix of units: British thermal units on equipment labels, tons on air conditioners, kilowatt-hours on the electric bill and therms or cubic feet on the gas bill. They all describe heat, and they convert with a handful of fixed numbers.
What are the basic conversions?
| Unit | Equals | Source |
|---|---|---|
| 1 Btu | The heat needed to raise one pound of water by 1 °F | EIA |
| 1 kWh | 3,412 Btu | EIA |
| 1 therm | 100,000 Btu | EIA |
| 1,000 cubic feet (Mcf) of natural gas | 1.037 MMBtu, or 10.37 therms (2025 U.S. average) | EIA |
| 1 ton of cooling | 12,000 Btu per hour | ACHR News |
| 1 ton of cooling | about 3.52 kW | 12,000 ÷ 3,412 |
MMBtu means one million Btu. The gas conversion changes slightly from year to year because the energy content of delivered gas varies; EIA publishes the average.
What is the difference between Btu and Btu per hour?
This is the mistake that causes most confusion. Btu and kWh are amounts of energy. They are what you pay for: a month’s gas or electricity. Btu per hour (Btu/h), tons and kilowatts (kW) are rates: how fast a system can deliver or remove heat. They describe the size of equipment.
A system delivering heat at 60,000 Btu/h for one hour delivers 60,000 Btu. A 3 kW electric heater running for two hours uses 6 kWh. Product listings sometimes drop the “per hour” and say “36,000 BTU” when they mean 36,000 Btu/h. For furnaces, also check whether a figure is input, the fuel burned, or output, the heat delivered.
Where does the “ton” come from?
The ton comes from the days of ice. It is the heat absorbed by melting 2,000 pounds of ice at 32 °F in 24 hours, which works out to 12,000 Btu per hour. A “3-ton” air conditioner removes heat at a nominal 36,000 Btu/h.
The nominal size is a label, not a performance figure. Actual capacity changes with outdoor temperature and indoor conditions, which is why sizing decisions use the manufacturer’s performance data at specific conditions.
How do common equipment sizes convert?
| Nominal size | Btu per hour | Kilowatts |
|---|---|---|
| 1.5 tons | 18,000 | 5.28 |
| 2 tons | 24,000 | 7.03 |
| 2.5 tons | 30,000 | 8.79 |
| 3 tons | 36,000 | 10.55 |
| 3.5 tons | 42,000 | 12.31 |
| 4 tons | 48,000 | 14.07 |
| 5 tons | 60,000 | 17.58 |
Kilowatts here are thermal capacity, heat moved per unit of time, not the electricity the unit draws. A heat pump moving 10.55 kW of heat uses much less electrical power than that; the ratio between the two is its coefficient of performance.
How do I convert temperatures?
- °C = (°F − 32) × 5 ÷ 9. For example, 5 °F is −15 °C and 47 °F is 8.3 °C.
- °F = °C × 9 ÷ 5 + 32.
- A temperature difference converts without the 32: a difference of 18 °F is a difference of 10 °C.
How do I use these for my bills?
To compare fuels, put both on the same footing: cost per million Btu. For electricity, multiply the price per kWh by 293.07, the number of kWh in one MMBtu. For gas, multiply the price per therm by 10. Then divide each by the efficiency of the equipment that turns it into heat. Our guide to heat pump and gas furnace running costs walks through it with 2026 prices.