BTU explained: heating & cooling

By Francesco Zinghinì

The BTU — British Thermal Unit — is the unit that underlies almost every number in this site: system capacity, fuel energy, sizing loads and efficiency all trace back to it. Yet it is routinely misused, because people conflate an amount of energy with a rate of energy. This guide sorts that out and shows how the BTU ties the whole subject together.

The definition

One BTU is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. It is a small unit — roughly the heat from a single wooden match. For scale, it takes about 1,000 BTU to boil a cup of water from room temperature. A BTU is an amount of energy, like a gallon is an amount of water.

BTU vs BTU per hour: the critical distinction

Here is the mistake almost everyone makes at least once. Air conditioners and furnaces are not rated in BTU; they are rated in BTU per hour (BTU/h), a rate. Saying "a 36,000 BTU air conditioner" is shorthand for "a 36,000 BTU-per-hour air conditioner" — it removes 36,000 BTU of heat every hour it runs. Confusing the two is like confusing "gallons" with "gallons per minute." Capacity is always a rate; energy consumed over time is an amount. Keep them separate and the rest is easy:

• Capacity (a rate): tons and BTU/h. 1 ton = 12,000 BTU/h.
• Fuel energy (an amount): 1 therm = 100,000 BTU; 1 kWh = 3,412 BTU.
• Power (a rate): 1 kW = 3,412 BTU/h.

BTU in cooling

On the cooling side, capacity is usually spoken in tons but the underlying unit is BTU/h. A 3-ton system moves 36,000 BTU/h of heat out of your house. Sizing works out how many BTU/h your home gains on a hot day and matches equipment to it — the rule-of-thumb estimate multiplies floor area by a climate-zone BTU-per-square-foot band, as in the AC-size calculator. Convert freely with the tons ↔ BTU ↔ kW tool. See what a ton of cooling is for where the 12,000 comes from.

BTU in heating

Heating uses the same unit. A furnace’s output is rated in BTU/h — the heat it delivers to the house — while its input is the fuel energy it consumes, also measured in BTU/h. The ratio of output to input is efficiency: a furnace with 100,000 BTU/h input and 80,000 BTU/h output is 80% AFUE. Because fuels are sold by energy content in BTU, the BTU is what lets you compare gas, propane, oil and electricity on a level field — see heat pump vs gas furnace for delivered cost per million BTU.

BTU, energy and dollars

Because every fuel has a known energy content in BTU, you can normalize any heating source to the same unit. One therm of natural gas holds 100,000 BTU; a gallon of propane about 91,500 BTU; a gallon of #2 heating oil about 138,500 BTU; a kilowatt-hour of electricity 3,412 BTU. Divide the price you pay by the usable energy (energy content × efficiency) and you get delivered dollars per million BTU — the only fair way to compare fuels. That is exactly what the $/MMBTU-by-fuel calculator does, always using the price you enter from your own bills.

A conversion cheat-sheet

Because everything reduces to the BTU, a handful of fixed factors let you move between every unit you will meet. None of these ever change — they are definitions and stable physical constants, which is exactly why calculators built on them never need maintenance:

• 1 ton = 12,000 BTU/h — cooling capacity.
• 1 kW = 3,412 BTU/h and 1 kWh = 3,412 BTU — electric power and energy.
• 1 therm = 100,000 BTU — natural gas billing unit.
• Natural gas ≈ 1,037 BTU/cu ft — if your meter reads in cubic feet or CCF.
• Propane ≈ 91,500 BTU/gal; #2 heating oil ≈ 138,500 BTU/gal.
• ~400 CFM per ton — design airflow.

With just these you can answer questions that sound complicated: a 60,000 BTU/h furnace output is 0.6 therms of delivered heat per hour; a 2-ton AC moves 800 CFM; 10 kWh of electric heat delivers 34,120 BTU. The fuel energy-content table collects the heating denominators, and the tons ↔ BTU ↔ kW converter handles the capacity side.

BTU and airflow

Finally, cooling BTU/h implies air movement. To remove 12,000 BTU/h (one ton) an air conditioner circulates about 400 CFM of air across the coil — the 400-CFM-per-ton rule. So a 36,000 BTU/h system moves roughly 1,200 CFM, which sets duct and register sizing (duct CFM tool). Once you see that capacity, fuel energy, power and airflow are all just BTU expressed in different ways — per hour, per therm, per kilowatt-hour — the whole field stops feeling like a jumble of unrelated numbers and starts looking like one unit wearing different hats.