BTU to Watts Converter (and Tons of Refrigeration)

BTU/h, watts, kilowatts, tons of refrigeration and kcal/h, all five at once and all from exact constants — 1 BTU/h = 0.29307107 W and 1 ton = 12,000 BTU/h = 3,516.85 W — with a table of the common air-conditioner sizes.

Last updated 24 Sep 2026

BTU/h, watts, tons and kcal/h

Any one heat-flow unit → the other four

What are you starting from?Fill in the box for the unit you chose. The other four lock and show the same heat flow in their own units.

BTU per hour

Watts

Kilowatts

Tons of refrigeration

Kilocalories per hour

Supply voltage (V), for the equivalent heaterOnly used for the resistive-heater equivalent at the bottom: the current and resistance that would dissipate exactly this many watts.

The same heat flow written five ways, drawn as the resistive heater that would produce it: a resistor across the voltage you enter, dissipating exactly the watts on the left. The dots show the current it would draw. A heat pump moves several times this much heat for this much electricity — the drawing is the equivalence of the units, not of the machines.

3.517kWExample

12,000 BTU/h — the nominal capacity of a one-ton air conditioner

Exact conversions between heat-flow units

1 BTU_IT = 1,055.055 852 62 J exactly ⇒ 1 BTU/h = 1,055.055 852 62 ÷ 3,600 = 0.293 071 070 17 W
1 ton of refrigeration = 12,000 BTU/h exactly = 3,516.852 84 W
1 kcal_IT = 4,186.8 J exactly ⇒ 1 kcal/h = 1.163 W exactly
equivalent heater: I = P ÷ V, R = V² ÷ P

BTU_IT: the International Table British thermal unit, defined from the IT calorie of exactly 4.1868 J (ISO 31-4). Two other BTUs exist — thermochemical and 39 °F — differing by about 0.2%
BTU/h vs BTU: a BTU is an AMOUNT of energy; a BTU/h is a RATE. Air conditioners are always rated in BTU/h even when the box says “12,000 BTU”
ton: the rate of cooling that would freeze one short ton of water in 24 hours. Nothing to do with the mass of the machine
kcal/h: still used for boiler and chiller ratings in India and much of Europe

Worked example

12,000 BTU/h — the nominal capacity of a one-ton air conditioner
12,000 × 0.293 071 070 17 = 3,516.8528 W = 3.517 kW
In tons: 3,516.85 ÷ 3,516.852 84 = 1.0000 — exactly one, because the ton is DEFINED as 12,000 BTU/h
In kcal/h: 3,516.85 ÷ 1.163 = 3,023.95, which the trade rounds to 3,024
A resistive heater making the same heat on 230 V would draw 3,516.85 ÷ 230 = 15.29 A and measure 15.04 Ω
A real air conditioner does NOT draw 3.5 kW to deliver one ton of cooling — it moves the heat rather than making it, so at an EER of 3.5 it draws about 1 kW. The heater is the unit equivalence, not the machine.

Common air-conditioner sizes in every unit

Nominal sizes are exact multiples of 12,000 BTU/h by definition; a real unit’s tested capacity is usually within a few per cent of it, and falls as the outdoor temperature rises.
Nominal	BTU/h	Watts	kW	Tons	kcal/h
0.75 ton	9,000	2,638	2.638	0.750	2,268
1.0 ton	12,000	3,517	3.517	1.000	3,024
1.5 ton	18,000	5,275	5.275	1.500	4,536
2.0 ton	24,000	7,034	7.034	2.000	6,048
2.5 ton	30,000	8,792	8.792	2.500	7,560
3.0 ton	36,000	10,551	10.551	3.000	9,072
5.0 ton	60,000	17,584	17.584	5.000	15,120

The conversion factors, both ways

Every line here follows from two definitions — the IT calorie of 4.1868 J and the ton as 12,000 BTU/h — so none of them carries any rounding except where shown.
From	To	Multiply by	Exact?
BTU/h	W	0.293 071 070 17	Yes, from 1 BTU(IT) = 1,055.055 852 62 J
W	BTU/h	3.412 141 633	Yes, the reciprocal
ton	BTU/h	12,000	Yes, by definition
ton	W	3,516.852 84	Yes
kcal/h	W	1.163	Yes, from 1 kcal(IT) = 4,186.8 J
ton	kcal/h	3,023.949 1	Yes — the familiar 3,024 is rounded
hp	W	745.699 872	Mechanical horsepower; a different quantity, linked only because both are power

BTU, watts and tons

All five units on this page measure the same thing: the rate at which heat moves. Converting between them is arithmetic with exact constants, which is why this page keeps itself to that and sends the sizing question elsewhere.

BTU is not BTU/h. A British thermal unit is an amount of energy — roughly what it takes to warm a pound of water by one degree Fahrenheit. A BTU per hour is a rate, like a watt. Air conditioners and heaters are always rated as a rate, but the packaging almost always drops the “per hour”, so a box marked “12,000 BTU” means 12,000 BTU/h. The metric label does not have the problem: a unit sold as 3.5 kW is already a rate.

Which BTU? Three definitions are in use and they differ by about two parts in a thousand. This page uses the International Table BTU, which is the one every HVAC standard means: the IT calorie is exactly 4.1868 J by international agreement (ISO 31-4), and that fixes the IT BTU at exactly 1,055.055 852 62 J. Divide by 3,600 seconds and one BTU/h is 0.293 071 070 17 W. The thermochemical BTU (1,054.35 J) and the 39 °F BTU (1,059.67 J) exist but are not what your air conditioner is rated in.

Where the ton comes from. Before mechanical refrigeration, cooling was sold as ice. A ton of refrigeration is the rate of heat removal that would freeze one short ton — 2,000 lb — of water at 0 °C in 24 hours, using the latent heat of fusion of ice. That works out at almost exactly 12,000 BTU/h, and the modern definition simply makes it exactly 12,000 BTU/h, which is 3,516.85 W. It has nothing to do with how much the machine weighs, which is the single most common misunderstanding about the unit.

Cooling capacity is not electrical input. This is the number that trips people up. A one-ton air conditioner removes 3,516.85 W of heat from the room; it does not consume 3,516.85 W of electricity. It is a heat pump, so it moves heat rather than making it, and at an EER or ISEER of 3.5 it draws about 1 kW from the supply. The heater drawn on the diagram beside this page — 15.04 Ω across 230 V, 15.29 A — is what it would take to MAKE this much heat, which is a useful sanity check and a terrible way to cool a room.

For the size of unit a room needs, use the AC tonnage calculator, which this page deliberately does not duplicate; for horsepower and kilowatts, the hp to kW converter; for the current a given wattage draws at your voltage, the watts to amps calculator.

Frequently asked questions

How many watts is 12,000 BTU?

12,000 BTU/h is 3.517 kW of heat flow — exactly one ton of refrigeration. That is the cooling the unit delivers, not the electricity it uses: at an EER of 3.5 it draws about 1 kW.

How many BTU is 1 kW?

3,412.14 BTU/h. The reciprocal, 1 BTU/h = 0.293 071 07 W, follows from the International Table BTU of exactly 1,055.055 852 62 J.

What is a ton of refrigeration?

The rate of cooling that would freeze one short ton of water in 24 hours: exactly 12,000 BTU/h, or 3,516.85 W. It is a rate of heat removal and says nothing about the mass of the equipment.

Is BTU the same as BTU per hour?

No. A BTU is an amount of energy; a BTU per hour is a rate, comparable with a watt. Appliance ratings are always rates even when the label omits the “per hour”.

How many kcal/h is one ton of cooling?

3,023.95 kcal/h exactly, from 1 kcal(IT)/h = 1.163 W. The trade rounds it to 3,024, which is where the familiar figure comes from.

Related calculators

References

Bureau International des Poids et Mesures / ISO 31-4: the International Table calorie is exactly 4.1868 J, which fixes the International Table British thermal unit at exactly 1,055.055 852 62 J. Hence 1 BTU/h = 0.293 071 070 17 W, the figure quoted by RapidTables, BTU/hr to watts conversion, as 0.29307107 W.
Wikipedia, Ton of refrigeration: “The modern definition is exactly 12,000 BTU_IT/h”, i.e. 3.516853 kW. Retrieved 24 September 2026.
National Institute of Standards and Technology. NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B: conversion factors, including Btu(IT) = 1.055 055 852 62 × 10³ J and calorie(IT) = 4.1868 J exactly.