BTU Calculator

How this BTU estimate works

The starting rule of thumb for room air conditioners is 20 BTU per square foot of living space with a standard 8 ft ceiling. This calculator adjusts that base for your room:

Adjusted BTU = area × 20 × sun factor × insulation factor × (ceiling height ÷ 8, if over 8 ft)
Sun factor: 0.9 shady, 1.0 average, 1.1 very sunny. Insulation factor: 0.9 good, 1.0 average, 1.15 poor. The range shown is the adjusted figure ±10%, rounded to the nearest hundred.

General estimate only — not professional sizing advice. A proper whole-house or mini-split sizing uses a Manual J load calculation, which accounts for windows, doors, local climate, ductwork, occupancy, and heat-generating appliances. Use this result to compare window units, portable units, and single-room options — and get a contractor's load calculation before buying central equipment. Kitchens typically need about 4,000 BTU extra, and rooms regularly holding more than two people about 600 BTU per extra person; this tool doesn't add those, so nudge up a size if they apply.

Bigger is not better. An oversized unit cools fast but shuts off before it dehumidifies, leaving the room cold and clammy — and it costs more up front. If your estimate lands between two standard sizes, the smaller one is usually the better choice unless the room is a sun-baked or kitchen space.

Frequently asked questions

How many BTU for a bedroom? A typical 150–250 sq ft bedroom with average sun and insulation lands around 6,000–8,000 BTU — the most common window-unit sizes exist precisely because of bedrooms.

What about tons? Central AC is sized in tons, where 1 ton = 12,000 BTU per hour. A 24,000 BTU estimate here equals a 2-ton unit — but a single room off a central system is not sized room by room this way.

Portable AC ratings look higher — why? Single-hose portable units are inefficient (they exhaust conditioned air), so their real-world cooling is well below the box rating. The newer DOE "SACC" rating is the honest number; compare SACC to the estimate above.