CalculynQuick answer
A 400 sq ft room with 8-foot ceilings, average sun and two people needs about 8,000 BTU.
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).
Formula
BTU = sq ft × 20 × (ceiling ÷ 8) × sun factor + extras
Assumptions & limitations
- Rule-of-thumb sizing; a professional load calculation (Manual J) is more precise.
Frequently asked questions
Is bigger better?
No. An oversized AC cycles on and off too often and doesn't remove humidity well.