Room AC BTU Calculator: What Size Air Conditioner Do I Need?
Find the right cooling capacity for a window or portable air conditioner based on room size, using the ENERGY STAR sizing chart and its adjustments for sun, occupancy and kitchens.
How the calculation works
This calculator uses the room air conditioner sizing chart published by ENERGY STAR. You find the room’s square footage in the chart’s area column and read the cooling capacity needed in BTUs per hour, then apply ENERGY STAR’s adjustments.
- Area = room length × room width.
- Base capacity from the chart. For example, ENERGY STAR lists 5,000 BTU for 100 up to 150 sq ft, 7,000 BTU for 250 up to 300 sq ft, 10,000 BTU for 400 up to 450 sq ft and 12,000 BTU for 450 up to 550 sq ft. The chart continues up to 34,000 BTU for 2,000 up to 2,500 sq ft.
- Adjustments listed by ENERGY STAR:
- Reduce capacity by 10 percent if the room is heavily shaded.
- Increase capacity by 10 percent if the room is very sunny.
- Add 600 BTU for each person beyond two who regularly uses the room.
- Add 4,000 BTU if the unit is used in a kitchen.
ENERGY STAR notes the capacities assume an 8-foot ceiling, and that rooms with higher ceilings may need more capacity.
Worked example
A 15 × 20 ft living room is 300 sq ft. The chart gives 7,000 BTU for 250 up to 300 sq ft.
| Situation | Capacity |
|---|---|
| Normal sun, two people | 7,000 BTU |
| Very sunny, two people | 7,000 × 1.1 = 7,700 BTU |
| Normal sun, four people | 7,000 + 2 × 600 = 8,200 BTU |
Round to the nearest unit size available. A unit somewhat larger than the estimate is common; a much larger unit cools the air quickly but shuts off before removing humidity, leaving the room cool and clammy.
Why size matters
An undersized air conditioner runs constantly on hot days and never quite reaches the set temperature. An oversized one short cycles, wastes energy and dehumidifies poorly. Sizing by the chart and adjustments avoids both problems for a single room.
Installation and upkeep
- Install the unit where it is not blocked by curtains or furniture and, for window units, in a window that is not in direct afternoon sun if you have the choice.
- Seal gaps around the unit to keep warm air out.
- Clean or replace the unit’s filter regularly; ENERGY STAR’s maintenance guidance stresses that dirty filters increase energy costs and can damage equipment.
When this calculator is not enough
The chart is for room air conditioners cooling a single room. It does not size central air conditioning or heat pumps for a whole house. Whole-home systems should be sized with a professional load calculation that accounts for insulation, windows, duct losses and climate; picking a central system by floor area alone often leads to oversizing. Rooms with very high ceilings, large west-facing windows or poor insulation may also need more capacity than the chart suggests.