An air conditioner that is too small runs all day and never catches up; too big, and it short-cycles and leaves the room clammy. Enter your room details below to get the right size in BTU and tons.
BTU Calculator — What Size AC Do I Need?
Enter your room size and conditions to get the right air conditioner size — in BTU and tons.
Your estimate
Sized for a single room or zone. An oversized AC is not better — it short-cycles, cools unevenly and removes less humidity. For whole-house central AC, a professional Manual J calculation is recommended; as a rough guide, homes typically need about 1 ton per 400–600 sq ft in hot climates (smaller homes need relatively more per sq ft).
How to use this BTU calculator
- Measure the room's length and width — one room or open zone per calculation.
- Enter the ceiling height; taller ceilings mean more air to cool.
- Be honest about insulation and sun exposure — they swing the answer by 15% or more.
- Buy the unit size shown; do not round up to the next size 'just in case'.
How we calculate BTU
We start with 25 BTU per square foot for a standard 8 ft ceiling, scale for taller ceilings, then adjust for insulation quality and sun exposure. Each regular occupant beyond two adds 600 BTU of body heat, and a kitchen adds 4,000 BTU for cooking heat. The result is matched to the nearest standard air conditioner size.
AC size chart (by room size)
| 100–300 sq ft | 5,000–8,000 BTU |
| 300–500 sq ft | 8,000–12,000 BTU |
| 500–700 sq ft | 12,000–18,000 BTU |
| 700–1,000 sq ft | 18,000–24,000 BTU |
BTU calculator FAQs
How many BTU do I need for a 12×12 room?
A 12×12 room (144 sq ft) with standard ceilings needs about a 5,000 BTU unit (144 sq ft × 25 BTU = 3,600 BTU, rounded up to the nearest standard size). Taller ceilings, very sunny rooms or poor insulation push it toward 6,000 BTU — the calculator above adjusts for both.
How many BTU per square foot do I need?
We use 25 BTU per square foot as a conservative warm-climate figure for rooms with 8 ft ceilings; common rules of thumb range from 20–25 BTU per square foot, before adjustments for sun exposure, insulation, occupants and climate.
How many tons is 12,000 BTU?
Exactly 1 ton. AC tonnage is simply BTU ÷ 12,000 — so a 24,000 BTU unit is 2 tons, and a small 6,000 BTU window unit is half a ton.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly but shuts off before removing humidity, so the room feels cold and clammy — and constant start-stop cycling wears the compressor and wastes energy. Right-sizing beats oversizing.