Room air conditioner BTU calculator
Estimate the cooling capacity (BTU/hr) for a window or room air conditioner from the room's size, using the U.S. Department of Energy's 20-BTU-per-square-foot guideline plus the ENERGY STAR occupancy adjustment. For a single room — central-AC sizing needs a pro's Manual J load calculation.
3,600 BTU/hr
Cooling capacity
- Room area
- 180 sq ft
- Equivalent tons
- 0.30 tons
What this assumes
- Capacity = 15 ft × 12 ft = 180 sq ft × 20 BTU + 0 × 600 BTU = 3600 BTU/hr.
- Base rate is the DOE's 20 BTU per square foot for a room air conditioner.
- Adjust further: sunny room +10%, heavily shaded −10%, kitchen +4,000 BTU (ENERGY STAR).
Coverage rates & sources
Every number this calculator uses is a published engineering constant — not an estimate we made up. Here is exactly what it assumes and where each value comes from.
- Cooling load: 20 BTU/hr per ft²Source: U.S. DOE — Room Air Conditioners (energy.gov)
- Extra-occupant load: +600 BTU/hr per person over 2Source: ENERGY STAR — room air conditioner sizing guidance (energystar.gov)
- Sunny / shaded adjustment: +10% sunny, −10% heavily shadedSource: ENERGY STAR — room air conditioner sizing guidance (energystar.gov)
- Kitchen adjustment: +4,000 BTU/hrSource: ENERGY STAR — room air conditioner sizing guidance (energystar.gov)
- Cooling ton: 12,000 BTU/hrSource: Refrigeration unit definition (1 ton = 12,000 BTU/hr)
Before you buy
- Room / window units only. Central air conditioning is sized by a Manual J whole-house load calculation an HVAC pro performs — square footage alone is not enough.
- Bigger is NOT better: an oversized AC short-cycles, leaves the room clammy (it cools before it dehumidifies), and wastes energy. Size close to the estimate, not above it.
- Apply the remaining ENERGY STAR adjustments yourself: sunny room +10%, heavily shaded −10%, kitchen +4,000 BTU. Insulation, ceiling height, and window area also matter.
- Confirm the final size and the electrical circuit with an HVAC pro or the unit's manufacturer.
This is a planning estimate, not a substitute for a pro's on-site measurement. For load-bearing, structural, or code-regulated work, confirm quantities with a licensed contractor.
Frequently asked
Is it bad to buy an air conditioner that's too big?
Yes. An oversized room AC cools the air quickly and shuts off before it removes enough humidity, leaving the room cold and clammy, and it short-cycles, which wastes energy and wears the unit. ENERGY STAR recommends sizing to the room, not buying the biggest unit.