How It Works
This calculator estimates cooling capacity using the industry-standard rule of thumb: 20 BTU per square foot of floor area. We then apply adjustments:
- Ceiling height — 10% more for each foot above 8 ft (more air volume).
- Sun exposure — 10% more for hot afternoon sun, 10% less for shaded rooms.
- Occupants — each person beyond the first two adds 600 BTU.
- Kitchen — adds 4,000 BTU for appliances and cooking heat.
Calculation Explanation
BTU = (Area × 20) × ceiling factor × sun factor + people bonus + kitchen bonus.
Example: a 12 × 12 ft room (144 sq ft) with 8-ft ceilings, normal sun, and 2 people needs 144 × 20 = 2,880 BTU. A window AC rated 5,000 BTU would comfortably cool this room.
For a larger space, 500 sq ft × 20 = 10,000 BTU — the right size for many garage or master-bedroom setups, and close to a 1-ton mini-split (12,000 BTU). Use our AC Tonnage Calculator to convert BTU to tonnage, or the HVAC Load Calculator for a full heating + cooling estimate.
Common BTU Sizes & What They Cover
| BTU Rating | Typical Coverage | Best For |
|---|---|---|
| 5,000 BTU | 150 sq ft | Bedrooms, small offices |
| 8,000 BTU | 350 sq ft | Living rooms, large bedrooms |
| 10,000 BTU | 450 sq ft | Master suites, open rooms |
| 12,000 BTU | 550 sq ft | 1-ton mini-splits, big living areas |
| 18,000 BTU | 800 sq ft | 1.5-ton units, open-plan spaces |
| 24,000 BTU | 1,100 sq ft | 2-ton central units, garages |