Air Conditioner BTU Calculator
Size an AC unit in BTUs from room area and conditions.
Enter your room's square footage and a few conditions to estimate the cooling capacity you need, in BTUs per hour and in tons. This is a quick sizing guide — a full Manual J load calculation accounts for insulation, windows, and climate in detail.
How it works
Start with about 20 BTU per square foot. Add 600 BTU for each occupant beyond two, add about 10% for a very sunny room, and add 4,000 BTU if it's a kitchen. Divide the total by 12,000 to get tons of cooling.
Frequently asked questions
How many BTUs do I need to cool a room?
Start at about 20 BTU per square foot. A 400 sq ft room needs roughly 8,000 BTU, before adjusting for sun, occupants, and kitchens.
How many BTUs is a ton of cooling?
One ton of air conditioning equals 12,000 BTU per hour. An 8,000 BTU unit is about two-thirds of a ton.
Is bigger always better for an AC?
No — an oversized unit short-cycles and leaves humidity behind. Right-sizing, ideally with a Manual J calculation, cools and dehumidifies best.
Is this calculator free and private?
Yes. Every CubicCabin calculator runs entirely in your browser, so the numbers you enter are never uploaded or stored. There's no sign-up, no limits, and it keeps working even if you lose connection after the page loads.