The quick answer: figure about 20 BTU per hour per square foot of floor area, then adjust up or down for the things that make a room harder or easier to cool. A cooling capacity of 12,000 BTU/hr is 1 ton of air conditioning, which is the unit you will see on larger equipment.
BTU by room size
| Room area | Cooling (BTU/hr) | Rough size |
|---|---|---|
| 150 sq ft (14 m²) | ~5,000 | small bedroom |
| 250 sq ft (23 m²) | ~6,000 | bedroom / office |
| 350 sq ft (33 m²) | ~8,000 | large bedroom |
| 500 sq ft (46 m²) | ~10,000 | studio / living room |
| 700 sq ft (65 m²) | ~14,000 (1 ton) | open living area |
| 1,000 sq ft (93 m²) | ~18,000 (1.5 ton) | large living area |
What changes the number
- Sun — a sunny, west-facing or top-floor room adds roughly 10%; a shaded room saves about 10%.
- Ceiling height — the rule assumes ~8 ft (2.4 m). Taller ceilings hold more air, so scale up with the height.
- People — each person beyond two adds about 600 BTU/hr of body heat.
- Kitchen — cooking appliances add roughly 4,000 BTU/hr.
- Equipment — TVs, computers and servers turn nearly all their watts into heat (about 3.4 BTU/hr per watt).
HVAC Load & AC Tonnage Calculator
Enter the room, sun, insulation, people and equipment for the tons, BTU load and unit size to buy.
Bigger is not better
It is tempting to round way up, but an oversized air conditioner is worse than a right-sized one. It cools the air so fast that it short-cycles — switching off before it has pulled moisture out — leaving the room cold and clammy, and wearing the compressor with frequent starts. Match the load, round up to the nearest standard size (9,000 / 12,000 / 18,000 / 24,000 BTU), and no further.
A guide, not a Manual J
The square-foot rule gets a normal room close, but a formal load calculation (a “Manual J” in the US) accounts for window sizes and orientation, wall and roof construction, air leakage and your local climate. Get one done for anything unusual — a glass-walled room, a sunroom, a server room, or a whole-house system.
Working in metric?
The same rule is about 600 BTU/hr per m², and cooling capacity is often quoted in kilowatts — 12,000 BTU/hr is roughly 3.5 kW. The calculator shows BTU, tons and kW together.