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COOLING LOADBTU/hTONNAGE
Estimate room air-conditioner capacity from floor area, ceiling height, climate, insulation, sun exposure, occupants and kitchen heat.
BTU symbols and meaning
Heat removed per hour, expressed in BTU/h.
Conditioned space in square feet.
Room height compared with an 8-foot baseline.
Climate, insulation and solar multipliers.
1 refrigeration ton = 12,000 BTU/h.
Estimator formula
Qbase = A × 20 × (H ÷ 8)
Qadjusted = Qbase × Fclimate × Finsulation × Fsun
Q = Qadjusted + 600 × max(P − 2, 0) + Qkitchen
Cooling tons = Q ÷ 12,000
This transparent rule-of-thumb starts near 20 BTU/h per ft², then adjusts the baseline instead of hiding the assumptions. It is useful for preliminary comparisons—not final system design.
Where room heat comes from
Walls, windows, roof exposure, outdoor temperature, people, lighting and equipment all add heat. A professional load calculation also accounts for orientation, glazing, air leakage, humidity and ventilation.
Worked example
For a 500 ft² room with an 8 ft ceiling, typical climate, average insulation, mixed sun and two occupants:
Q = 500 × 20 × (8 ÷ 8) = 10,000 BTU/h
The next common room-unit size is approximately 10,000 BTU/h. This is about 0.83 ton. If the same room has poor insulation and strong afternoon sun, the estimate increases because both factors multiply the base load.
Common cooling capacities
| Nominal capacity | Cooling tons |
|---|---|
| 5,000 BTU/h | 0.42 ton |
| 9,000 BTU/h | 0.75 ton |
| 12,000 BTU/h | 1.00 ton |
| 18,000 BTU/h | 1.50 tons |
| 24,000 BTU/h | 2.00 tons |
| 36,000 BTU/h | 3.00 tons |
Available sizes differ between window units, mini-splits and central systems. Do not automatically choose a much larger unit “for safety.”
Measurement checklist
- Use conditioned floor area, not the entire building if only one room is cooled.
- Measure ceiling height rather than assuming 8 feet.
- Note west-facing glass and strong afternoon sun.
- Assess insulation and air sealing honestly.
- Include people normally present, not occasional visitors.
- Identify kitchens and heat-producing equipment.
- For whole-home systems, obtain a room-by-room Manual J or local equivalent.
Frequently asked questions
What does BTU/h mean?
It is the number of British thermal units of heat the equipment can remove in one hour. Product labels often shorten BTU/h to BTU.
Why can an oversized AC be uncomfortable?
It may cool the thermostat area quickly, stop before removing enough moisture and create temperature swings or short cycling.
Does this calculator include humidity?
Only indirectly through the climate choice. High-humidity sizing and latent loads require a detailed HVAC load calculation.
Can I size a central system with floor area alone?
No. Whole-building systems need envelope, windows, orientation, leakage, ventilation and local design temperatures.
Common mistakes
- Ignoring ceiling height.
- Using total-home area for one room.
- Adding large arbitrary safety margins.
- Assuming bigger equipment always performs better.
- Ignoring solar gain, insulation and air leakage.