Understanding your result
The headline is the capacity in tons, because tons are the shorthand American contractors, quotes and model numbers use; the sub-line gives the same figure in BTU per hour and kilowatts. The three stats repeat BTU/hr, kW and watts, which are the three figures most likely to appear on a label or in a spec sheet.
The table lists every unit, including kcal/h and the Asian HP class, and adds BTU per day, which is the capacity multiplied by 24. That last row is useful when comparing an air conditioner’s output with a daily heat gain, or when reading a dehumidifier or chiller specification that quotes a daily figure.
If the capacity does not land on a half-ton, the notes name the two stock sizes it sits between. That matters when a spec sheet from another country gives a figure such as 7 kW: the central AC size calculator rounds loads to those stock sizes, and the AC running cost calculator accepts either tons or BTU/hr, so a converted figure can be used directly.
How we calculate this
Everything is first converted to BTU per hour and then to each of the other units, so the rounding of one step cannot leak into another. The factors are the standard ones: 1 ton = 12,000 BTU/hr by definition, 1 kWh = 3,412.14 BTU (the thermal equivalent of a kilowatt-hour), 1 BTU = 0.252 kcal. The HP factor is the convention of the Asian room air conditioner market, where one “HP” is a capacity class of about 2.5 kW (8,500 to 9,000 BTU/hr); it is a label, not a physical unit, and the converter uses 9,000 BTU/hr and says so in a note. All of these factors are collected on the unit conversions page.
Capacity is heat removed. To turn it into electricity consumed, divide by the efficiency: BTU/hr ÷ (SEER2 × 1,000) gives kW drawn over the season, as the efficiency ratings table explains, and that is what the running cost calculator does.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Ton of refrigeration | 12,000 BTU/hr | Definition: 2,000 lb of ice × 144 BTU/lb latent heat ÷ 24 h |
| BTU per kilowatt-hour | 3,412.14 | Thermal equivalent of a kWh (EIA uses 3,412) |
| kcal per BTU | 0.252 | Definition (1 BTU = 1,055.06 J; 1 kcal = 4,184 J) |
| Air-conditioner “HP” class | 9,000 BTU/hr (about 2.5 kW) | Room air conditioner labelling convention in Japan and South-East Asia; some makers use 8,500 |
| Stock sizes | Half-ton steps, 1.5 to 5 tons | US residential split-system capacities |
Assumptions last reviewed October 7, 2026.
The converter changes units; it does not rate equipment. A unit’s real capacity at your conditions is lower than its label at high outdoor temperatures and for heat pumps in cold weather, and a label in BTU/hr says nothing about efficiency. The guide to SEER2, HSPF2, AFUE and COP explains the ratings that do.
Two worked examples
A 36,000 BTU/hr label
A US condensing unit whose model number contains “36”, meaning 36,000 BTU/hr nominal.
- Tons: 36,000 ÷ 12,000 = 3 tons
- Kilowatts: 36,000 ÷ 3,412.14 = 10.6 kW (10,551 W)
- kcal/h: 36,000 × 0.252 = 9,072 kcal/h
- HP class: 36,000 ÷ 9,000 = 4 HP
- Per day: 36,000 × 24 = 864,000 BTU
Three tons is a stock size, so no note appears. If this unit is replacing one marked “30”, the household is going up a half ton, and the size calculator is the place to check whether the house needs it.
A 7 kW split system from a European catalogue
A spec sheet gives cooling capacity as 7.0 kW.
- BTU/hr: 7 × 3,412.14 = 23,885 BTU/hr
- Tons: 23,885 ÷ 12,000 = 1.99 tons
- Watts: 7,000 W of cooling
- kcal/h: 23,885 × 0.252 = 6,019 kcal/h
- HP class: 23,885 ÷ 9,000 = 2.65 HP
The note points out that 1.99 tons is, for every practical purpose, a 2-ton unit (24,000 BTU/hr), which is how a US contractor would describe it. The 7 kW figure is heat removed; if the same sheet gives power input as 2.1 kW, the unit’s COP is 7 ÷ 2.1 = 3.3.
Where to find your inputs
The capacity. On US equipment it is encoded in the model number in thousands of BTU/hr (18, 24, 30, 36, 42, 48, 60) and printed on the nameplate on the outdoor unit. On equipment sold elsewhere, the nameplate gives cooling capacity in kW, sometimes as a range for inverter units (use the rated or nominal figure, not the maximum).
Which figure to convert. Convert the cooling capacity, not the power input. Both may be in kW on a non-US label; the capacity is the larger number, usually three to four times the input.
Horsepower labels. If a catalogue gives only HP, use the HP option and then check the result against any kW or BTU figure elsewhere on the page; makers differ slightly on what one HP means.
Common mistakes
- Dividing by 3,412 to get electricity use. That gives the capacity in kW, not the draw. Divide BTU/hr by SEER2 × 1,000 for the seasonal electrical draw.
- Treating AC horsepower as mechanical horsepower. One mechanical HP is 2,544 BTU/hr; one AC “HP” is about 9,000. Mixing them up makes a unit look three and a half times smaller than it is.
- Reading the heating figure as the cooling figure. Heat pump labels give both; the model number usually follows the cooling capacity.
- Using the maximum of an inverter range. Inverter units quote a range such as 2.0 to 8.5 kW; the nominal figure, often near the middle, is the one to compare with a fixed-speed unit.
- Expecting a label to say what a house needs. The converter changes units. The central AC size calculator works out the load.
Questions people ask
- How many BTU are in a ton of air conditioning?
- 12,000 BTU per hour. The unit comes from ice. A ton of refrigeration is the cooling produced by melting one US short ton (2,000 lb) of ice over 24 hours; ice absorbs 144 BTU per pound, so 2,000 × 144 ÷ 24 = 12,000 BTU per hour. A 3-ton air conditioner therefore removes 36,000 BTU of heat per hour, which is 864,000 BTU a day if it ran nonstop.
- How many kW is a 3-ton air conditioner?
- 10.55 kW of cooling capacity, because 36,000 BTU/hr ÷ 3,412 BTU per kWh = 10.55. That is the heat it removes, not the electricity it uses; a 3-ton unit with a SEER2 of 15 draws about 2.4 kW of electricity while running. Outside North America capacity is usually given in kW, so a 10.5 kW split system in Australia or Europe is a 3-ton unit.
- Why is the kW on the label different from the kW the unit draws?
- Two different quantities share the unit. Cooling capacity in kW is the heat removed; power input in kW is the electricity consumed. The ratio of the two is the COP or EER. A 10.5 kW (3-ton) unit with a COP of 3.5 draws 3 kW. Labels outside the US give both; US labels give capacity in BTU/hr and efficiency as SEER2 and EER2, from which the draw follows.
- What is "horsepower" on an air conditioner?
- In Japan, Hong Kong, the Philippines and much of South-East Asia, room air conditioners are classed by a nominal "horsepower" that has become a size label rather than a measurement. One HP corresponds to roughly 2.5 kW or 9,000 BTU/hr of cooling, so a "1.5 HP" unit is about 13,500 BTU/hr and a "2 HP" unit about 18,000. It is not the 2,544 BTU/hr of a mechanical horsepower. Always check the kW or BTU figure on the label.
- What is a kcal/h rating?
- Kilocalories per hour, a capacity unit still used on some Japanese and European equipment and in older engineering tables. One BTU is 0.252 kcal, so 12,000 BTU/hr is 3,024 kcal/h, and a unit rated at 2,500 kcal/h is about 9,900 BTU/hr or 2.9 kW. Multiply kcal/h by 1.163 to get watts.
- Are tons used for heating capacity too?
- Heat pumps are sold by their cooling tonnage, and the same unit's heating capacity is quoted in BTU/hr at a given outdoor temperature, usually 47 °F and 17 °F, because it falls as the air gets colder. Furnaces and boilers are rated in BTU/hr input and output, never in tons. The BTU, watts and kW converter in the heating group handles those.