BTU Calculator
Enter room size, climate, and occupancy — get the recommended BTU rating for a window or portable air conditioner.
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Typical BTU ranges by room type
Before entering exact dimensions, this reference range (moderate climate, standard occupancy) is a useful sanity check against whatever the calculator returns:
| Room type | Typical size | Typical BTU |
|---|---|---|
| Bedroom | 150-250 sq ft | 5,000-6,000 |
| Living room | 300-450 sq ft | 7,000-10,000 |
| Kitchen | 150-250 sq ft | 8,000-10,000 (heat load pushes this up) |
| Open-plan living/dining | 500-700 sq ft | 12,000-14,000 |
How to use this calculator
Enter the room's square footage, pick the climate that best matches your area, and add occupants beyond 2 if the room regularly holds more people than that. Check "kitchen" if it's a kitchen — heat-producing appliances push the BTU requirement up meaningfully.
Why climate changes the answer so much
A room needs more cooling capacity per square foot in a hot, humid climate than a cool one — the baseline this calculator uses ranges from 18 BTU/sq ft (cool) up to 30 BTU/sq ft (hot), meaning the same room can need nearly double the BTU rating depending on climate alone. Occupants and kitchens add heat load on top of that baseline — each additional person beyond two adds roughly 380 BTU, and a kitchen adds a flat 4,000 BTU for appliance heat. See BTU vs tonnage explained if you're also comparing this room-level number against central/whole-house equipment specs.
Worked example
A 350 sq ft living room in a moderate climate, 2 occupants, not a kitchen: 350 × 20 BTU/sq ft = 7,000 BTU calculated, which rounds up to a standard 8,000 BTU unit.
Frequently asked questions
What happens if I undersize the BTU rating?
An undersized unit runs constantly without ever quite reaching the target temperature, which is both less comfortable and less efficient than a correctly sized unit cycling normally.
Is bigger always better?
No — an oversized unit cools the air quickly but shuts off before it's run long enough to properly dehumidify the room, leaving it cold and clammy. Matching the size to the room, not rounding up "to be safe," gives the best result.
Does this apply to central air or just window/portable units?
This is calibrated for a single room or window/portable unit. For whole-house central sizing, use the HVAC load calculator instead, which accounts for insulation and uses a whole-house baseline.
Does a room with lots of direct sunlight need more BTU?
Yes, meaningfully — a room with large south- or west-facing windows in direct sun can need 10-15% more capacity than this calculator's baseline accounts for. If that describes your room, treat the calculated result as a floor and consider rounding up to the next standard size rather than the exact number shown.
Rule-of-thumb estimate based on commonly published sizing guidance, not a substitute for a full Manual J load calculation. Reference only — consult an HVAC contractor for a precise recommendation.