It sounds intuitive: bigger air conditioner, colder room, faster. In practice, an oversized unit cools the air quickly, shuts itself off, and never runs long enough to pull the humidity out — leaving the room cold but clammy instead of actually comfortable.
What size air conditioner do I need
Enter your room's square footage and this calculator looks up the base cooling capacity from the Energy Star room air conditioner sizing guide's published table — not a straight-line formula, since real BTU needs per square foot actually shrink a bit as rooms get larger.
BTU per square foot isn't a fixed number
A 150 sq ft room needs about 33 BTU per square foot; a 2,000 sq ft space needs closer to 15 BTU per square foot. That's why a simple "multiply by a constant" approach gets less accurate the bigger the room gets — the table-based approach here doesn't have that problem.
Air conditioner sizing calculator, with real adjustments
Beyond square footage, this factors in the Energy Star guide's standard corrections: reduce by 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU for each person beyond two regular occupants, and add 4,000 BTU if it's a kitchen.
