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BTU Air Conditioner Sizer
Find the right AC size for your room based on area, ceiling, sun exposure and climate.
What this calculator does
Air conditioners are rated in BTU/hr (British Thermal Units per hour). An undersized unit runs flat-out and never cools the room; an oversized unit short-cycles, leaves the air clammy and wears out faster. This sizer applies the standard ASHRAE rule of 20 BTU per square foot, then adjusts for ceiling height, sun, climate and people.
How to use it
Measure the room you want to cool, including any spaces it opens into. Choose the sun setting that matches the worst-case afternoon. Pick a climate zone and tell us how many people are in the room at peak. The result is the cooling capacity your AC needs at the nameplate.
Sizing formula
Base = area × 20 BTU/sq ft. Ceiling > 8 ft adds (ceiling/8 − 1) × base. Sun: shaded −10%, mixed 0%, very sunny +15%. Climate: cold −10%, temperate 0%, hot +20%. Each person beyond two adds 600 BTU. Tons = BTU ÷ 12,000.
Quick sizing reference
| Room area | BTU/hr |
|---|---|
| 100–150 ft² (9–14 m²) | 5,000 BTU/h |
| 150–250 ft² (14–23 m²) | 6,000 BTU/h |
| 250–350 ft² (23–33 m²) | 8,000 BTU/h |
| 350–450 ft² (33–42 m²) | 10,000 BTU/h |
| 450–550 ft² (42–51 m²) | 12,000 BTU/h (1 ton) |
| 550–700 ft² (51–65 m²) | 14,000 BTU/h |
| 700–1,000 ft² (65–93 m²) | 18,000 BTU/h (1.5 ton) |
| 1,000–1,400 ft² (93–130 m²) | 24,000 BTU/h (2 ton) |
| 1,400–1,800 ft² (130–167 m²) | 30,000 BTU/h (2.5 ton) |