BTU Calculator

Enter your room size to get the BTU per hour for a room air conditioner, with the ENERGY STAR chart and its adjustments.

How to calculate the BTU you need

A room air conditioner is rated in BTUs per hour, the amount of heat it can remove. ENERGY STAR explains how to pick one in three steps, and this calculator does them for you. We read the page in October 2026 at energystar.gov.

Step	What to do
1. Room area	rectangle: length × width. Triangle: length × width ÷ 2. Split odd shapes into rectangles and triangles
2. Chart	find the area in the chart below and read the BTUs per hour
3. Adjust	heavily shaded: −10%. Very sunny: +10%. Each person beyond two: +600 BTU. Kitchen: +4,000 BTU

The calculator reads measurements typed like 12'6" or 12 ft 6 in and accepts feet, inches, yards and meters. Use “Add another area” for an L-shaped room and the areas are added. The result is also shown in tons, where one ton is 12,000 BTU per hour. ENERGY STAR does not say in which order to apply the adjustments. This calculator applies the 10 percent change to the chart value first and then adds the fixed BTUs.

Example 1: a sunny room with four people

A room measures 18 ft by 15 ft, gets a lot of sun and has four people in it.

Step	Operation	Result
Room area	18 × 15	270 sq ft
Chart row 250 up to 300	from the chart	7,000 BTU/h
Very sunny	7,000 × 10%	+ 700 BTU/h
Two extra people	2 × 600	+ 1,200 BTU/h
Total	7,000 + 700 + 1,200	8,900 BTU/h
In tons	8,900 ÷ 12,000	0.74 tons

Example 2: a small kitchen

A kitchen measures 12 ft by 12 ft. That is 144 sq ft, which is in the 100 up to 150 row of the chart (5,000 BTU/h). The kitchen adds 4,000, so the unit needs 9,000 BTU/h, or 0.75 tons.

ENERGY STAR cooling capacity chart

The chart repeats the one on the ENERGY STAR page. The capacities are calculated for an 8 foot ceiling. The tons column is our conversion at 12,000 BTU per hour per ton.

Area to be cooled (sq ft)	Capacity needed (BTU/h)	Tons
100 up to 150	5,000	0.42
150 up to 250	6,000	0.50
250 up to 300	7,000	0.58
300 up to 350	8,000	0.67
350 up to 400	9,000	0.75
400 up to 450	10,000	0.83
450 up to 550	12,000	1.00
550 up to 700	14,000	1.17
700 up to 1,000	18,000	1.50
1,000 up to 1,200	21,000	1.75
1,200 up to 1,400	23,000	1.92
1,400 up to 1,500	24,000	2.00
1,500 up to 2,000	30,000	2.50
2,000 up to 2,500	34,000	2.83

The page does not say whether “up to 150” includes 150. This calculator reads each row as starting at its lower number, so a room of exactly 150 sq ft uses the 150 up to 250 row, the one with more capacity. It also counts 2,500 as part of the last row. Below 100 sq ft or above 2,500 sq ft the chart gives no figure, so the calculator shows a message and no number.

Why a bigger unit is not better

ENERGY STAR says an oversized room air conditioner is less effective and can leave the room damp and clammy, because it cools the air before it has removed the humidity. The most energy efficient unit is the one sized to the square footage of the space. To see which room sizes a given unit matches, or to convert BTU per hour to tons or kilowatts, use the AC size calculator. To measure an odd room, see the square footage calculator.

What this calculator does not do

It sizes one room air conditioner from the ENERGY STAR chart. It does not size a central system, a furnace, a heater, a garage or a shop, and it does not run a heat load calculation such as ACCA Manual J. The U.S. Department of Energy explains that a right-sized system starts from accurately determined heating and cooling loads, and that rules of thumb often lead to oversized systems. Nothing you type leaves your browser.

Frequently asked questions

How do I figure out how many BTUs I need?

Follow the three ENERGY STAR steps for a room air conditioner. Measure the floor area in square feet, find that area in the capacity chart to get the BTUs per hour, then adjust. Reduce the capacity by 10 percent if the room is heavily shaded, increase it by 10 percent if the room is very sunny, add 600 BTUs for each person beyond two, and add 4,000 BTUs if the unit is used in a kitchen.

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

In the ENERGY STAR chart, 12,000 BTUs per hour is the capacity for rooms from 450 up to 550 square feet, before any sun, people or kitchen adjustment. The chart is for room air conditioners and assumes an 8 foot ceiling.

How many BTUs do I need to cool a 20 by 20 room?

A 20 by 20 foot room is 400 square feet. The chart rows are written as ranges, such as 350 up to 400 and 400 up to 450. This calculator treats 400 as the start of the next row, so it gives 10,000 BTUs per hour before adjustments. If the room is very sunny it becomes 11,000.

What BTU do I need for my room size?

Use the chart on this page. It runs from 5,000 BTUs per hour for 100 up to 150 square feet to 34,000 for 2,000 up to 2,500 square feet. Type your dimensions into the calculator to get the row and the adjustments for you.

Does ceiling height change the BTUs?

ENERGY STAR says the capacities in its chart are calculated for an 8 foot ceiling. It does not publish a number for taller rooms, so this calculator does not invent one. ENERGY STAR only adds that with higher ceilings you may want a unit with a higher CADR. If your ceiling is higher, ask the seller or an HVAC professional.

Is this a heating or a whole-house calculation?

No. It follows ENERGY STAR guidance for sizing a room air conditioner, so it gives cooling capacity for one room. It does not size a furnace, a heater or a central system. A central system should be sized with a load calculation such as ACCA Manual J, which this page does not run.

What if my room is under 100 or over 2,500 square feet?

The ENERGY STAR chart starts at 100 square feet and stops at 2,500. Outside that range the calculator shows a message instead of a number, because there is no chart figure to use.