AC Size Calculator

Convert an air conditioner size between tons, BTU per hour and kilowatts, and see which room sizes it matches.

How to convert tons, BTU per hour and kilowatts

Air conditioner size is the cooling capacity, the heat the unit removes in an hour. In the United States it is usually given in BTU per hour or in tons. One ton is 12,000 BTU per hour, which the National Institute of Standards and Technology lists as 3,516.853 watts. NIST also gives 1 BTU per hour as 0.2930711 watts. We read both factors in the NIST Guide to the SI, Appendix B.8 in October 2026.

To get	Formula
Tons	BTU per hour ÷ 12,000
BTU per hour	tons × 12,000
Kilowatts of cooling	BTU per hour × 0.2930711 ÷ 1,000
BTU per hour from kilowatts	kilowatts × 1,000 ÷ 0.2930711

The kilowatts here are cooling capacity, the heat removed. They are not the electric power the unit draws, which is a different number on the nameplate.

Example 1: a 2.5 ton unit

Step	Operation	Result
BTU per hour	2.5 × 12,000	30,000 BTU/h
Kilowatts of cooling	30,000 × 0.2930711 ÷ 1,000	8.79 kW
ENERGY STAR room chart	30,000 BTU/h row	1,500 up to 2,000 sq ft

The ENERGY STAR chart is for room air conditioners. A 2.5 ton central system does not serve one 1,500 sq ft room, and how many tons a whole house needs depends on a load calculation, not on the chart.

Example 2: a 13,000 BTU unit

A size that is not in the chart gives two neighbors. 13,000 ÷ 12,000 is 1.083 tons, and 13,000 × 0.2930711 ÷ 1,000 is 3.81 kW. The chart has a 12,000 BTU/h row (450 up to 550 sq ft) and a 14,000 BTU/h row (550 up to 700 sq ft), so the calculator shows both and does not make up a row for 13,000.

Common air conditioner sizes

Some sizes from the ENERGY STAR chart for room air conditioners with their conversions. The full chart, with the adjustments for sun, people and a kitchen, is in the BTU calculator.

BTU per hour	Tons	Cooling kW	Room area in the ENERGY STAR chart
6,000	0.50	1.76	150 up to 250 sq ft
8,000	0.67	2.34	300 up to 350 sq ft
10,000	0.83	2.93	400 up to 450 sq ft
12,000	1.00	3.52	450 up to 550 sq ft
18,000	1.50	5.28	700 up to 1,000 sq ft
24,000	2.00	7.03	1,400 up to 1,500 sq ft
30,000	2.50	8.79	1,500 up to 2,000 sq ft
34,000	2.83	9.96	2,000 up to 2,500 sq ft

What size AC does a whole house need?

Search results are full of shortcuts such as a number of BTUs per square foot or a number of square feet per ton. This page does not use them, because we found no official source for them. The U.S. Department of Energy says that to right-size an HVAC system you must first accurately determine the heating and cooling loads, and that these loads are then used with sizing calculations based on recognized industry standards. It names ACCA's Manual J, Eighth Edition, as the method contractors use for homes, and warns that rules of thumb are too often used and result in excessively oversized systems (DOE Building Science Education, checked October 2026). This calculator does not run Manual J. For a central system, ask a contractor for a load calculation.

Why not just buy a bigger unit?

For a room air conditioner, ENERGY STAR says a unit that is too large is less effective and may leave the room feeling damp and clammy, because it cools the room before it can remove the humidity (ENERGY STAR, Room Air Conditioners). To size a single room from its dimensions, use the BTU calculator.

Frequently asked questions

How many BTUs are in a ton of air conditioning?

One ton of refrigeration is 12,000 BTU per hour. NIST lists it as 3,516.853 watts of cooling capacity. So tons = BTU per hour ÷ 12,000, and BTU per hour = tons × 12,000.

How many tons is a 24,000 BTU air conditioner?

It is 2 tons, because 24,000 ÷ 12,000 = 2. An 18,000 BTU unit is 1.5 tons, a 12,000 BTU unit is 1 ton and a 30,000 BTU unit is 2.5 tons.

How many kilowatts is a ton of air conditioning?

One ton is 3.517 kilowatts of cooling capacity, which is the heat the unit removes. It is not the electricity the unit consumes. The calculator converts with 1 BTU per hour = 0.2930711 watts, the NIST factor.

What size air conditioner do I need for my house?

This calculator cannot tell you. The U.S. Department of Energy says a right-sized system starts from accurately determined heating and cooling loads, with a method such as ACCA Manual J, and that rules of thumb often lead to oversized systems. Ask an HVAC contractor for a load calculation, or use the BTU calculator for a single room.

How many square feet does a 12,000 BTU or 1 ton air conditioner cover?

In the ENERGY STAR chart for room air conditioners, 12,000 BTU per hour matches rooms from 450 up to 550 square feet, before sun, people and kitchen adjustments. That chart is for room units and assumes an 8 foot ceiling. It does not apply to a central system for a whole house.

Is a bigger air conditioner better?

No. ENERGY STAR says a room air conditioner that is too big is less effective, because it cools the room before it has removed the humidity, which can leave the room damp and clammy. It adds that the most energy efficient unit is the one sized to the space.

What is the difference between this calculator and the BTU calculator?

The BTU calculator starts from the room. You type its size, the sun, the people and the kitchen, and it gives the BTU per hour you need. This calculator starts from the unit. You type a size in tons, BTU per hour or kilowatts, and it converts it and shows which room sizes that capacity matches in the ENERGY STAR chart.

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