How to calculate btu (ac size)
BTU = sq ft × 20 × (ceiling ÷ 8) × sun factor + extras
Worked example. Recommended size: 8,000 BTU.
Example inputs — Room size (sq ft): 400; Ceiling height (ft): 8; Sunlight: Average; People who usually use the room: 2; Is it a kitchen?: No. Starting values are illustrative.
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
A common rule of thumb is 20 BTU per square foot with 8-foot ceilings, adjusted for sun, extra people (+600 BTU each beyond two) and kitchens (+4,000 BTU).
Assumptions & limitations
- Rule-of-thumb sizing; a professional load calculation (Manual J) is more precise.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
Is bigger better?
No. An oversized AC cycles on and off too often and doesn't remove humidity well.