🗓 Updated July 22, 2026 · EcoBack editorial team
Too few BTU and your room never gets cool; too many is wasted money and needlessly loud. Enter your room details and the calculator gives you the right cooling capacity plus a unit sized to match.
BTU by room size — quick reference
Even without the calculator, these are the common guide values for a portable air conditioner by room size (at normal ceiling height and average sun exposure).
| Room size | Recommended BTU | Typical room | Matching model |
|---|---|---|---|
| up to 15 m² | 7,000–9,000 BTU | small bedroom, office | Comfee (value) |
| 15–25 m² | 9,000–12,000 BTU | bedroom, home office | De'Longhi Pinguino (sweet spot) |
| 25–35 m² | 12,000–14,000 BTU | living room | Klarstein (large rooms) |
| over 35 m² | 14,000+ BTU / monobloc | open-plan space | Klarstein (high-BTU) |
Models compiled from public reviews, not tested by us. For high ceilings, south/top-floor rooms or several people, size one step up — the calculator above does this automatically.
How the BTU calculator works
The base is about 340 BTU per m², adjusted for ceiling height, sun exposure, people (each person beyond two adds heat) and heat sources such as an open kitchen. It's a proven rule of thumb — the exact figure also depends on insulation and windows. For the full breakdown see our guide to how many BTU you need by room size.
Frequently asked questions
How many BTU do I need for my room?
As a rule of thumb, about 340 BTU per square metre. Rough guide: up to 15 m² needs 7,000–9,000 BTU, 15–25 m² needs 9,000–12,000 BTU, 25–35 m² needs 12,000–14,000 BTU, and over 35 m² needs 14,000+ BTU. Add capacity for high ceilings, strong sun, several people or an open kitchen.
What happens if the air conditioner has too few BTU?
An underpowered unit runs at full load constantly, never reaches the target temperature and still uses a lot of power. Size to your room or slightly above.
Is more BTU always better?
No. An oversized unit short-cycles on and off, cools unevenly, is louder and costs more to buy. Matching the room is best.