AC BTU Calculator
Enter the size of your room to see the cooling capacity to look for in a window or wall air conditioner.
Look for about 6,000 BTU/hr
180 sq ft, calculated for an 8-foot ceiling. Total before rounding: 6,000 BTU/hr.
This is an estimate, not a professional calculation. See the disclaimer and the methodology.
How it is calculated
Step 1: multiply the room length by its width, in feet, to get the area in square feet. The ENERGY STAR chart then gives the capacity for that area. It lists bands such as "150 up to 250 square feet", which means 150 is included and 250 is not.
Step 2: adjust. A heavily shaded room takes 10 percent off the chart value. A very sunny room adds 10 percent. Each person beyond two who regularly uses the room adds 600 BTU per hour. A kitchen adds 4,000 BTU per hour. ENERGY STAR does not say whether the percentages apply to the chart value or to the running total. This calculator applies them to the chart value and then adds the people and kitchen amounts, in the order ENERGY STAR lists them.
Step 3: ENERGY STAR advises rounding the total down to the nearest thousand BTU per hour and looking for a unit with that capacity. The headline result does that, and the total before rounding is shown under it.
The chart is calculated for an 8-foot ceiling. ENERGY STAR gives no adjustment for taller ceilings, climate, insulation or windows, and neither does this calculator. The chart is meant for window and wall room air conditioners. It is not a sizing method for central air conditioning, heat pumps or mini splits.
Worked examples
A 12 by 15 foot bedroom
12 × 15 = 180 square feet. The chart gives 6,000 BTU/hr for 150 up to 250 square feet. With normal sun and two people nothing is added, so look for a 6,000 BTU/hr unit.
A sunny 14 by 18 foot living room with four people
14 × 18 = 252 square feet, which the chart sets at 7,000 BTU/hr. Very sunny adds 10 percent (700 BTU/hr) and two extra people add 1,200 BTU/hr. The total is 8,900 BTU/hr, which rounds down to 8,000 BTU/hr.
A 10 by 12 foot kitchen
120 square feet is 5,000 BTU/hr on the chart. The kitchen adds 4,000 BTU/hr, so look for a 9,000 BTU/hr unit.
Frequently asked questions
How many BTUs do I need per square foot?
ENERGY STAR does not use a single BTU-per-square-foot figure. It publishes a chart of capacity by room area, for example 6,000 BTU per hour for 150 up to 250 square feet and 12,000 for 450 up to 550. The ratio changes from band to band.
What size air conditioner do I need for a 300 square foot room?
On the ENERGY STAR chart, 300 square feet falls in the 300 up to 350 band, which is 8,000 BTU per hour. A 250 to 299 square foot room is in the band below, at 7,000.
Why does the result round down?
ENERGY STAR advises rounding the total down to the nearest thousand BTU per hour when picking a unit. The calculator shows both the rounded number and the exact total.
Can I use this for a central air conditioner, a heat pump or a mini split?
No. The chart is for room air conditioners. Whole-home and ductless systems are sized from a heat load calculation that looks at climate, insulation, windows and more. This calculator does not do that.
What is a BTU?
A BTU, or British thermal unit, is a measure of heat. An air conditioner rated in BTU per hour tells you how much heat it can remove from a room each hour.
What if my room has a high ceiling?
The chart assumes an 8-foot ceiling. ENERGY STAR gives no ceiling adjustment, so this calculator has none either. For a much taller room, treat the result as a starting point and ask an HVAC professional.
Related calculators
Sources and last update
- ENERGY STAR, Room Air Conditioners: sizing chart and adjustments
- ENERGY STAR, "How to choose the right sized window AC" (rounding advice)
Last updated: November 3, 2026. See methodology and sources.