BTU / AC Sizing Calculator
BTUs of cooling a room needs to size an AC unit.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Sizing an air conditioner is a Goldilocks problem — too small and it never cools, too large and it short-cycles without dehumidifying. This calculator estimates the BTUs a room needs so you buy a unit that's actually right.
How it works
About 20 BTU per square foot, adjusted for sun exposure. 12,000 BTU equals one ton of cooling.
The baseline is about 20 BTU of cooling per square foot of floor area, adjusted for sun exposure — a sunny room runs 10% higher, a shaded one 10% lower. The result is rounded to a clean hundred and also shown in tons, where 12,000 BTU equals one ton of cooling.
Square footage is the dominant factor, but occupants and heat sources matter too. The calculator notes that you should add roughly 600 BTU per regular occupant and about 4,000 BTU for a kitchen, where appliances pump out heat.
BTU = room area × 20 × sun factor, rounded to the nearest 100; tons = BTU ÷ 12,000.
Worked examples
A 15 ft × 12 ft bedroom with average sun. → 3,600 BTU (0.3 tons).
15 × 12 = 180 sq ft × 20 = 3,600 BTU. A small window unit handles this room.
A 20 ft × 18 ft sunny living room (+10%). → About 7,900 BTU.
20 × 18 = 360 sq ft × 20 × 1.1 = 7,920, rounded to 7,900 BTU — solidly into mid-size window-unit territory.
Tips & gotchas
- Add about 600 BTU for each person who regularly occupies the room, and roughly 4,000 BTU if it's a kitchen with heat-producing appliances.
- Oversizing is a real mistake — an AC that's too big cools fast then shuts off before pulling humidity out, leaving the room cold and clammy.
- Sunny, west-facing, or top-floor rooms run hot; the sun adjustment exists precisely for those.
- For whole-house central air, this room-by-room estimate is a starting point — a professional Manual J load calculation is worth it for the full system.
FAQ
What happens if my AC is too big?
It short-cycles — cooling the air quickly then shutting off before it removes humidity. The room feels cold and damp, the compressor wears faster, and efficiency drops. Right-sizing beats oversizing.
How many BTU equal a ton of cooling?
12,000 BTU per hour equals one ton. Central systems are usually rated in tons, so the calculator shows both numbers to bridge window-unit and whole-house sizing.
Does ceiling height affect the BTU need?
Yes — the 20-BTU-per-square-foot rule assumes standard 8-foot ceilings. Tall or vaulted ceilings add volume to cool, so size up if your room has them.
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