Air Conditioner Size Calculator
The right BTU — because oversized AC cools clammy, not comfortable. Free, instant, and private — everything runs on your device.
+600 BTU each over 2
Cooking adds ~4,000 BTU of heat
Recommended AC size
- Room area
- 180 sq ft
- Calculated need
- 3,600 BTU/hr
- Standard unit size
- 5,000 BTU
Bigger is not better: an oversized AC cools fast but short-cycles, leaving the room cold and clammy because it never runs long enough to pull humidity. This uses the DOE 20-BTU-per-sq-ft baseline plus standard adjustments — for whole-home central air, a pro should still run a Manual J load calculation.
The instinct with air conditioning is to buy big — and it's exactly wrong. An oversized unit blasts the room cold in a few minutes, then shuts off before it has run long enough to pull the humidity out of the air. The result is a space that's cold and clammy, a compressor that short-cycles itself toward an early grave, and a higher power bill. This calculator sizes the unit to the room using the U.S. Department of Energy's baseline of 20 BTU per square foot, then adjusts for the things that actually change the load.
Sun exposure, ceiling height, how many people are usually in the room, and whether it's a kitchen all shift the number. Enter them and you'll get the BTU your room needs plus the nearest standard unit size to buy — for a window or portable unit, or as a starting point for a mini-split. For whole-home central air, a contractor should still run a full Manual J load calculation.
How it works
No black box — this is the exact math the calculator runs.
- Base load = room area × 20 BTU per square foot (DOE baseline).
- Sun: a very sunny room adds 10%, a shaded one subtracts 10%.
- Ceiling height scales the load — standard sizing assumes 8-foot ceilings.
- Occupancy adds 600 BTU for each regular occupant beyond two.
- Kitchens add 4,000 BTU for cooking heat; the result rounds up to the nearest standard unit size.
Last reviewed July 24, 2026 · Rooms & Organization
Frequently asked questions
What size air conditioner do I need for a 300 square foot room?
About 6,000 BTU for an average room — the DOE baseline of 20 BTU per square foot. A sunny room, high ceilings, or a kitchen push that up; a shady bedroom pulls it down. The calculator applies those adjustments and rounds to the nearest unit you can actually buy.
Is it bad to buy an oversized air conditioner?
Yes. An oversized AC cools the air fast but short-cycles — it reaches temperature and shuts off before running long enough to dehumidify. You get a cold, damp room, more wear on the compressor, and worse efficiency. Correctly sized beats oversized every time.
How many BTU per square foot for air conditioning?
20 BTU per square foot is the standard baseline. That's before adjustments: add for strong sun, high ceilings, extra people, or kitchen heat; subtract for heavy shade. This calculator starts from 20 and applies the standard modifiers.
How do BTU and tons relate for central air?
One ton of cooling equals 12,000 BTU per hour. So a 2-ton system is 24,000 BTU. Window and portable units are rated directly in BTU; central and mini-split systems are usually described in tons. The calculator shows both when relevant.