How Many BTU Do I Need?
Start from roughly 20 BTU per square foot of room (the ENERGY STAR/DOE baseline for average insulation and 8-foot ceilings), then adjust for a kitchen, heavy sun, extra occupants, or taller ceilings. For a portable AC, apply this table to the SACC rating, not the larger ASHRAE BTU number printed on the box.
The one thing to know: The baseline rule (ENERGY STAR/DOE) is roughly 20 BTU per square foot of room for a well-insulated space with 8-foot ceilings, but real sizing adjusts that baseline: add roughly 600 BTU per additional occupant beyond two, add capacity for a kitchen or a room with heavy direct sun, and add roughly 1,000 BTU per foot of ceiling height above 8 feet. Oversizing wastes money and can leave a room clammy (the unit short-cycles before it dehumidifies); undersizing means the unit runs constantly and never catches up — match BTU to square footage first, then adjust for the specific room, not the other way around.
BTU-per-room-size table
| Room size | Baseline BTU |
|---|---|
| 100–150 sq ft | 5,000–6,000 |
| 150–250 sq ft | 6,000–7,000 |
| 250–350 sq ft | 7,000–8,500 |
| 350–450 sq ft | 8,500–10,000 |
| 450–550 sq ft | 10,000–12,000 |
| 550–700 sq ft | 12,000–14,000 |
| 700–1,000 sq ft | 14,000–18,000 |
Adjust the baseline for your actual room
- Kitchen adjoins the room: add ~4,000 BTU for appliance heat load.
- Heavy direct sun most of the day: add ~10% to the baseline.
- Heavily shaded all day: subtract ~10% from the baseline.
- Each occupant beyond two: add ~600 BTU per person.
- Ceiling height above 8 feet: add ~1,000 BTU per additional foot.
Match BTU to the room's actual conditions, not the biggest unit that fits the window or floor space. An oversized AC short-cycles — cooling air fast, shutting off, and repeating — without running long enough to pull out humidity properly, which can leave a "cold but clammy" room. An undersized unit runs constantly and never quite catches up on a hot afternoon.
For the full ratings context — SACC, CEER, hose loss, fan CFM, evaporative humidity limits — see The Cooling Numbers.
This page is general sizing guidance, not an engineering load calculation for your specific home.
Frequently Asked Questions
A 12x12 room is 144 sq ft, which falls in the 100–150 sq ft band — roughly 5,000–6,000 BTU baseline, adjusted up if the room gets heavy sun, has more than two occupants, or has ceilings above 8 feet.
Yes — oversizing wastes money on a pricier unit and can leave the room feeling clammy, because an oversized AC cools the air fast enough to shut off before running long enough to properly dehumidify. Size to the room, not to 'just in case.'
Use the SACC rating, not the ASHRAE BTU printed larger on the box — a portable marketed as '14,000 BTU' may only be a genuine 8,000–9,500 SACC-rated machine. Size a portable off SACC using this same square-footage table.
Yes — add roughly 4,000 BTU if a kitchen adjoins or is part of the room you're cooling, since appliance heat adds real thermal load beyond square footage alone.
Yes — the baseline assumes 8-foot ceilings; add roughly 1,000 BTU for every additional foot of ceiling height, since you're cooling more actual air volume than the floor-area number implies.