Air Conditioner Size Calculator
Calculate the right BTU air conditioner for your room.
Enter room dimensions and conditions to get an accurate AC size recommendation.
Air conditioner sizing uses BTU (British Thermal Units) per hour as the measure of cooling capacity. An undersized AC runs constantly and can’t reach temperature; an oversized AC cools too fast without dehumidifying properly, leaving the room feeling clammy.
The Formula:
Required BTU = Room area (sq ft) × 25
Then apply adjustment factors:
| Condition | Adjustment |
|---|---|
| Sunny room, south/west facing | Add 10% |
| Heavily shaded room | Subtract 10% |
| Kitchen / cooking area | Add 4,000 BTU |
| Ceiling height > 9 ft (2.7 m) | × (actual height / 8) |
| Occupancy above 2 people | Add 600 BTU per extra person |
BTU to Tonnage Conversion:
1 ton of cooling = 12,000 BTU/hour
Worked Example:
Living room: 20 ft × 15 ft = 300 sq ft. South-facing windows. 3 regular occupants.
Base BTU: 300 × 25 = 7,500 BTU Sun adjustment: +750 BTU (+10%) Extra person: +600 BTU Total: 8,850 BTU → choose a 9,000 BTU or 10,000 BTU unit
Common AC Unit Sizes:
| Room Size | Recommended BTU |
|---|---|
| Small bedroom (up to 150 sq ft) | 5,000–6,000 BTU |
| Medium bedroom (150–250 sq ft) | 6,000–8,000 BTU |
| Large room (250–400 sq ft) | 8,000–12,000 BTU |
| Open plan (400–600 sq ft) | 12,000–14,000 BTU |
| Whole floor (600–1,500 sq ft) | 18,000–24,000 BTU |
Practical Tips:
- SEER (Seasonal Energy Efficiency Ratio) matters: SEER 16–20 costs significantly less to run than SEER 10
- Mini-split systems offer zoned control with no ductwork — ideal for room additions
- Insulate and air-seal before upsizing AC — sealing leaks can reduce the required capacity by 20–30%