Ask five people what size AC a room needs and you'll get five answers, most of them too big. The honest method is one baseline number and five adjustments — and it fits on a napkin.
The baseline: 20 BTU per square foot
Decades of DOE guidance and HVAC practice converge on roughly 20 BTU/hour per square foot for typical US construction and climates: 250 sq ft → 5,000 BTU; 500 sq ft → 10,000; a small 1,000 sq ft apartment → 20,000. Everything after this is adjusting for your room's reality.
The five adjustments
- Ceilings: the baseline assumes 8 feet. Scale linearly — a 10-foot ceiling adds ~25% volume to condition.
- Sun: west- and south-facing rooms with real window area add ~10%; deeply shaded rooms save ~10%.
- People: each body beyond two adds ~600 BTU — kitchens, offices, and gaming dens notice.
- Kitchens: add ~4,000 BTU for cooking heat wherever the stove lives.
- Climate: hot-humid regions (Southeast, Southwest) add ~10%; mild-summer regions can trim a little.
The BTU calculator applies all five for you and lands on a single number you can take shopping.
Why oversizing is the industry's favorite mistake
An oversized AC nails the temperature fast, then shuts off — before removing the humidity. The result is the signature oversize complaint: cold but clammy, with short cycles that waste energy, wear compressors, and swing the room between cool and warm. Right-sized units run longer, gentler cycles that dehumidify as they cool — which is why "smaller than you think" is the professional's reflex.
Shopping in the real world
Window units come in 5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 15,000 BTU steps — buy the nearest size at or slightly below your calculation. Mini-splits come in 9,000 / 12,000 / 15,000 / 18,000 BTU "heads" (12,000 BTU = 1 ton). For whole-house central air, stop calculating and request a Manual J load calculation — duct losses, insulation levels, and air sealing push whole-house sizing beyond rules of thumb, and any installer who sizes central AC "by square footage" without one is guessing with your money.