BTU Conversion Calculator: BTU/h to Watts, kW, Tons, Therms and HP

BTU vs BTU/h

A British thermal unit (BTU) is an amount of energy: roughly the heat needed to warm one pound of water by 1 °F. Equipment capacity is a rate, energy per unit time, so heating and cooling equipment is rated in BTU per hour (BTU/h). Labels and sales sheets often drop the "/h" and just say "BTU", but a 24,000 BTU air conditioner means 24,000 BTU/h.

This distinction matters when you convert. BTU/h converts to watts or kilowatts (power). BTU converts to watt-hours or kilowatt-hours (energy). Gas bills are in therms (energy), while a burner's input is in therms per hour or BTU/h (power).

Conversion factors

BTU/h = Value × Factor
Watts = BTU/h ÷ 3.412  ·  Tons = BTU/h ÷ 12,000  ·  Therms/h = BTU/h ÷ 100,000  ·  hp = BTU/h ÷ 2,544

Energy versions: 1 kWh = 3,412 BTU, 1 therm = 100,000 BTU, and one hundred cubic feet of natural gas (1 CCF) holds roughly 103,000 BTU, slightly more than a therm.

Worked examples

1. Air conditioner: 12,000 BTU/h ÷ 3.412 = 3,517 W, or 3.517 kW, which is exactly 1 ton. That is the calculator's default result. Note this is cooling output, not electrical input; a unit with an EER of 12 draws about 12,000 ÷ 12 = 1,000 W. See the COP calculator.
2. Space heater: a 1,500 W electric heater gives 1,500 × 3.412 = 5,118 BTU/h. Electric resistance heat converts power to heat one-for-one.
3. Furnace: 80,000 BTU/h input is 0.8 therm per hour. At 96% AFUE, about 76,800 BTU/h reaches the house.
4. Mini-split rated 5.3 kW: 5.3 × 3,412 = about 18,100 BTU/h, roughly a 1.5-ton unit.

Where each unit shows up

• BTU/h: US furnaces, air conditioners, heat pumps, water heaters and boilers. Sizing tools such as the BTU calculator for AC and heating BTU calculator give results in BTU/h.
• Tons: central air conditioners and heat pumps; one ton is 12,000 BTU/h.
• kW: metric equipment ratings, electric heaters and most of the world outside North America.
• Therms: natural gas billing; useful for comparing fuels in the gas vs electric heating guide.
• Horsepower: motor and compressor shaft power. Compressor horsepower is not the same as cooling tons, though older rules of thumb treated them as roughly equal.

Input vs output ratings

Before converting, check whether a number is an input (fuel or electricity consumed) or an output (heat delivered). Gas furnaces are labelled with both; the output is input × efficiency. Heat pumps deliver more heat than the electricity they use, so their kW of electrical input is far below their kW of heating output. Mixing the two is the most common conversion mistake, and it leads to wrong running-cost estimates. To turn conversions into dollars, use the heating cost calculator.

Quick mental math: 1 kW is about 3,400 BTU/h, and 1 ton is about 3.5 kW. Those two figures cover most everyday conversions within a percent or two.

Converting energy, not power

The calculator converts rates. For bills and fuel comparisons you often need quantities of energy instead. The same factors apply without the "per hour":

These fuel values are typical higher heating values; your utility or supplier may list slightly different figures. To compare the cost of heat, divide the price of each fuel unit by its BTU content and then by the appliance efficiency: AFUE for a furnace, COP for a heat pump. For example, electricity at 15 cents per kWh costs about $44 per million BTU with resistance heat, and roughly a third of that from a heat pump running at COP 3.

Frequently asked questions