BTU to Tons Converter (Cooling Capacity)

Background.

A ton of air conditioning is not a weight. It is a rate of heat removal, and the number behind it is exact: one ton of refrigeration is 12,000 BTU per hour, by definition. NIST's own unit tables print the definition inside the unit's name — 'ton of refrigeration (12 000 Btu(IT)/h)' — which means this conversion is not a measured approximation like inches to centimetres was before 1959. It is a matter of what the word means.

The name is a fossil. Before mechanical refrigeration, cooling was sold as ice, and a plant's capacity was quoted as the tons of ice it could replace in a day. One short ton of ice absorbs about 288,000 BTU as it melts; spread over 24 hours that is 12,000 BTU per hour, and the unit outlived the ice trade by more than a century. It is now simply the unit central air conditioning is sold in across North America, while window units, portable units and mini-splits are sold in BTU per hour, and the rest of the world uses kilowatts.

That mixture is why this page exists. A homeowner comparing a '3-ton' central system against an '18,000 BTU' mini-split and a '10 kW' European datasheet is looking at three numbers in three units for the same physical quantity, and the arithmetic that reconciles them is trivial only once you know which multiplier belongs to which. Three tons is 36,000 BTU per hour is 10.55 kilowatts is 36 MBH. All four describe the same machine.

Two distinctions cause most of the confusion, and both are stated beside the result rather than buried below it. The first is BTU against BTU per hour. A BTU is a quantity of energy — about 1,055 joules, the heat needed to raise a pound of water by one degree Fahrenheit. BTU per hour is a rate, a power. Air conditioners are rated in the rate, always, and the '12,000 BTU air conditioner' on a shelf label is an abbreviation of 12,000 BTU per hour that has been repeated so often it looks like a unit in its own right. The second is thermal against electrical power. A 3-ton air conditioner removes 10.55 thermal kilowatts from the house; it does not consume 10.55 electrical kilowatts to do it. A modern unit draws roughly a third of that, and how much less is exactly what SEER2 and EER measure.

The page also reports which nominal residential product class the result falls into, and it is honest about the status of that ladder. North American residential split systems are built at 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons, with 4.5 conventionally skipped. That is what manufacturers make, not something a standards body publishes, so it is labelled as market product classes and a value that falls outside the range is reported as outside it rather than rounded onto the nearest rung. It also comes with the caveat every nominal rating deserves: a 3-ton system is rated at the standard test conditions of AHRI 210/240, and its real capacity falls as the outdoor temperature rises — which is the whole reason ACCA Manual S makes you check equipment against expanded performance data at your own design conditions rather than against the badge on the box.

What is btu to tons converter?

A ton of refrigeration is a unit of power — a rate of heat flow — equal to exactly 12,000 BTU(IT) per hour, or about 3,516.85 watts. It measures how fast a machine removes heat, not how much it weighs and not how much electricity it uses. NIST Special Publication 811 lists it in its table of conversion factors as 'ton of refrigeration (12 000 Btu(IT)/h) → 3.516 853 × 10³ W'.

A BTU, or British thermal unit, is a unit of energy: 1,055.056 joules in the International Table definition NIST uses, roughly the heat required to raise one pound of water by one degree Fahrenheit. BTU per hour is that energy divided by time, which makes it a power. Every air-conditioning capacity rating is in BTU per hour even when the 'per hour' is dropped from the label.

MBH means one thousand BTU per hour — the M is the Roman numeral for a thousand, not the SI prefix mega. It is the unit boiler and commercial HVAC literature is normally printed in, and one ton is 12 MBH.

Nominal tonnage is the marketing size class of a piece of equipment: a '3-ton' condenser. Its rated capacity is measured under the standard test conditions of AHRI Standard 210/240, and actual delivered capacity varies with outdoor temperature, indoor wet-bulb temperature and airflow. Nominal size is a label for shopping, not a guarantee of output at your design conditions.

Thermal capacity is distinct from electrical input power. The tons or kilowatts on this page describe heat removed from the space; the electricity consumed to remove it is a different, smaller number, and their ratio is what SEER2, EER2 and COP express.

How to use this calculator.

1. 01Pick the direction — whichever unit you already have. Both directions return all four units, so the choice only decides which field you type into.
2. 02Enter the capacity. Central systems are usually quoted in tons; window units, portable units and mini-splits in BTU per hour; European and commercial datasheets in kilowatts.
3. 03Read the converted result. Tons, BTU per hour, kilowatts and MBH all describe the same machine.
4. 04Check the nominal size line if you are shopping for a central system. It tells you the smallest standard class at or above your figure, and says plainly when your figure is outside the range residential split systems are built in.
5. 05Remember the kilowatt figure is thermal, not electrical. It is heat removed from the house, not power drawn from the panel — for running cost you need the unit's SEER2 or EER rating as well.
6. 06Do not use this page to size equipment. Converting a capacity says nothing about what capacity your building needs; that is a Manual J load calculation followed by a Manual S selection.

The formula.

The core conversion is a single division and it is exact. Tons of refrigeration equals BTU per hour divided by 12,000, and BTU per hour equals tons times 12,000. There is no measurement uncertainty in that factor, because the ton of refrigeration is defined as 12,000 BTU(IT) per hour — NIST Special Publication 811 puts the definition inside the unit's own name in its table of conversion factors. Forty-two thousand BTU per hour is therefore exactly 3.5 tons, and 3 tons is exactly 36,000 BTU per hour, with nothing rounded on the way.

The kilowatt conversion is not exact, and it is worth knowing why. NIST publishes the International Table BTU per hour as 2.930711 × 10⁻¹ watts, to seven significant figures. That figure is not a defined integer ratio because the International Table BTU is itself defined through the International Table calorie, so it carries a finite number of published digits rather than being exact by construction. Forty-two thousand BTU per hour therefore becomes 42,000 × 0.2930711 = 12,308.99 watts, or 12.31 kilowatts.

Those two NIST entries cross-check each other, which is a useful thing to be able to verify rather than assume. NIST separately publishes 'ton of refrigeration → 3.516853 × 10³ W'. Multiplying the BTU-per-hour factor by the definitional 12,000 gives 12,000 × 0.2930711 = 3,516.8532 watts, which agrees with NIST's own published ton figure across all seven of its significant digits. That agreement is asserted in this calculator's test suite, so the two constants cannot drift apart silently.

MBH is the simplest of the four: thousands of BTU per hour, so BTU per hour divided by 1,000. The M is the Roman numeral, not the SI mega prefix, which is a genuine trap — MBH is a thousand BTU per hour, not a million. One ton is 12 MBH.

The nominal size class is a lookup rather than a calculation: the smallest entry in the residential ladder at or above the converted tonnage. The ladder runs 1.5, 2, 2.5, 3, 3.5, 4 and 5 tons, with 4.5 conventionally absent from the market, so 4.2 tons steps to 5 rather than to a 4.5-ton machine that nobody builds. The comparison is made on the unrounded tonnage, so a capacity fractionally above a rung selects the next one up even when it displays as sitting on the edge. Values below 1.5 tons and above 5 tons are reported as outside the range, with a note about what equipment covers those sizes instead, rather than being forced onto the nearest rung.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology, "Guide for the Use of the International System of Units (SI)", NIST Special Publication 811, Appendix B.8 "Factors for Units Listed Alphabetically". Retrieved 2026-07-29. Source of all three constants used here, read directly from the table: "ton of refrigeration (12 000 Btu(IT)/h)" → 3.516 853 × 10³ W, which carries the definitional 12,000 inside the unit's own name; "British thermal unit(IT) per hour (Btu(IT)/h)" → 2.930 711 × 10⁻¹ W; and "British thermal unit(IT) (Btu(IT))" → 1.055 056 × 10³ J. The first two entries cross-check each other and that agreement is asserted in this calculator's test suite.
2. [2]National Institute of Standards and Technology, "Guide for the Use of the International System of Units (SI)", NIST Special Publication 811 (2008 edition), full document. The introduction to Appendix B explains the distinction this page relies on between units whose conversion factors are exact by definition and those published to a finite number of significant figures — which is why the 12,000 BTU/h per ton is exact while the 0.2930711 W per BTU/h is not.
3. [3]Air-Conditioning, Heating, and Refrigeration Institute, AHRI Standard 210/240, "Performance Rating of Unitary Air-conditioning and Air-source Heat Pump Equipment". The standard defines the test conditions under which a nominal cooling capacity is rated, which is the basis for this page's statement that a nominal tonnage is a rating at standard conditions rather than a promise of output at any given design condition. The standard is sold by AHRI and was not opened; it is cited as the governing rating document.
4. [4]Wes Davis, Air Conditioning Contractors of America, "Reviewing HVAC Designs for Compliance with ACCA Manual S", ICC Building Safety Journal, January–February 2009. Read directly from the PDF on 2026-07-29. Source of this page's statement that cooling equipment must be verified against the manufacturer's expanded performance data at the design conditions used for the Manual J load — the article works exactly that check on a nominal 2.5-ton and a nominal 3-ton unit and shows the nominal size failing while the larger one passes, which is why nominal tonnage is presented here as a shopping label rather than a capacity guarantee.