Room Cooling BTU Calculator
Sizing a room air conditioner correctly matters: an undersized unit never cools the room, while an oversized one cycles on and off without removing humidity. The U.S. ENERGY STAR program suggests a base of about 20 BTU per hour for each square foot of living space, with adjustments for sun, occupancy, and kitchens. Enter your room dimensions and conditions to estimate the cooling capacity you need.
Room cooling BTU formula
Area = length * width
Base BTU = area * BTU per sq ft
Sun adjustment = base BTU * (sunny % / 100)
Occupant add = occupants over 2 * 600
Total BTU = base BTU + sun adjustment + occupant add + kitchen add
Tons = total BTU / 12,000
ENERGY STAR recommends increasing capacity by 10 percent for a very sunny room, reducing it by 10 percent for a shaded room, adding 600 BTU for each regular occupant beyond two, and adding 4,000 BTU for a kitchen. One ton of cooling equals 12,000 BTU per hour.
Worked example
A 15 by 12 foot bedroom is 180 square feet. Base cooling = 180 times 20 = 3,600 BTU per hour. With no extra occupants, no sun adjustment, and no kitchen, the adjusted load stays at 3,600 BTU per hour, or 0.30 tons. A 5,000 BTU window unit would comfortably cover it.
ENERGY STAR sizing notes
- ENERGY STAR base guidance is roughly 20 BTU per hour per square foot of room area.
- Add about 600 BTU for each person who is regularly in the room beyond the first two.
- Increase capacity by about 10 percent if the room is very sunny; reduce by 10 percent if heavily shaded.
- Add about 4,000 BTU if the unit is cooling a kitchen, which generates extra heat.
- One ton of air conditioning equals 12,000 BTU per hour.
Room Cooling BTU Calculator: frequently asked questions
How many BTU do I need to cool a room?
As a starting point, ENERGY STAR suggests about 20 BTU per hour for each square foot of room area. A 180 square foot room needs roughly 3,600 BTU per hour before adjustments for sun, occupants, and kitchen heat.
What happens if my air conditioner is too big?
An oversized air conditioner cools the air quickly but shuts off before it removes enough humidity, leaving the room cold and clammy. Frequent short cycling also wears the equipment and wastes energy. Right-sizing matters.
How do I convert BTU to tons of cooling?
Divide BTU per hour by 12,000. So 36,000 BTU per hour equals 3 tons of cooling. Window units are usually rated in BTU, while central systems are rated in tons.
Do I need to adjust for a sunny room?
Yes. ENERGY STAR suggests increasing capacity by about 10 percent for a room that gets a lot of direct sun, and decreasing it by about 10 percent for a heavily shaded room.
Sources and methodology
- U.S. ENERGY STAR: Room Air Conditioners (base 20 BTU per sq ft and adjustment guidance).
- One ton equals 12,000 BTU per hour is a fixed engineering unit conversion.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.