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HeizerDesk BTU Calculator
Enter the room's dimensions and how it's used. This returns the cooling load in BTU per hour and the nearest standard unit size, using the adjustments in the ASHRAE-derived rule of thumb most contractors size small rooms with. This is a free BTU Calculator — no sign-up, and the arithmetic is written out below so you can check it.
Your numbers
Result
Cooling load needed
6,000
BTU/hr
- Nearest unit size up
- 6,000BTU/hr
- In tons12,000 BTU/hr = 1 ton
- 0.5tons
- Room area
- 300sq ft
- Base load before adjustments
- 6,000BTU/hr
How this is calculated
- Base load = floor area × 20 BTU per square foot.
- Ceiling adjustment = base × ((height − 8) ÷ 8) × 0.5, applied only above 8 ft.
- Sun adjustment = −10% heavily shaded, +10% very sunny.
- Occupancy = 600 BTU for each person beyond the first two.
- Kitchens add a flat 4,000 BTU for appliance heat.
- Tons = total BTU ÷ 12,000.
Before you order
- This is the room-by-room rule of thumb, not a Manual J. For a whole-house system, a full load calculation accounts for insulation, window type, infiltration and duct losses, and will give a different — usually smaller — answer.
- Oversizing is a real problem, not a safety margin: an oversized unit short-cycles, never runs long enough to pull humidity out, and leaves the room cold and clammy.
- For heating, the same envelope needs a different calculation — heating load depends on your design temperature difference, not on floor area alone.