BTU per square foot for air conditioning
30 to 40 BTU/h per square foot, with caveats that matter.
As a reference, 30 to 40 BTU/h per square foot for a home with a standard ceiling. That is about 320-430 BTU/h per square metre, or 105-128 W/m².
The rule, with its ranges
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| Situation | BTU/h per sq ft | W/m² |
|---|---|---|
| Well insulated, shaded, middle floor | 25-30 | 80-100 |
| Standard home | 30-40 | 105-128 |
| Top floor, west-facing or poorly insulated | 40-50 | 128-163 |
| Space with heavy internal gains | 50-70 | 163-233 |
Calculating by volume is more honest
The per-area rule has an obvious problem: it ignores height. A 200 sq ft room with an 11 ft ceiling has nearly 40 % more air to cool than one with an 8 ft ceiling, and the floor area is identical.
So for tall rooms, work by volume: 30 to 50 W per cubic metre depending on insulation, which is roughly 3 to 5 BTU/h per cubic foot.
What the rule is actually for
For checking, not for deciding. If a contractor proposes 24,000 BTU for a 130 sq ft bedroom, the rule says 185 BTU/h per square foot: five times normal. That is worth questioning, and starting that conversation is exactly what the rule is good for.
What it cannot do is choose the unit, because it ignores precisely the variables that make one room different from another.
Quick equivalences
- 1 BTU/h = 0.293 W = 0.252 frigories/h
- 35 BTU/h per sq ft = 377 BTU/h per m² = 110 W/m²
- A 200 sq ft room at 35 BTU/h per sq ft = 7,000 BTU/h = 0.58 tons
Sources and methodology
Conversions use exact constants: 1 BTU/h = 0.29307107 W and 1 frigorie/h = 1 kcal/h = 1.163 W. The component sizing model (load per volume by insulation level, 120 W per person, solar gain by glazed area) is set out in full on the air conditioner size calculator. It is a guideline estimate: a professional heat-load calculation considers considerably more variables.
Frequently asked questions
How many BTU per square foot for air conditioning?
30 to 40 BTU/h per square foot for a standard home with an 8 ft ceiling. Less with good insulation, more for top floors or heavy sun.
How many BTU for 500 square feet?
15,000 to 20,000 BTU/h at 30-40 per square foot, which is an 18,000 BTU unit in practice.
Does the rule work for high ceilings?
Poorly. Above 9 ft, calculate by volume instead: 3 to 5 BTU/h per cubic foot depending on insulation.
Can I choose a unit with this rule alone?
Better not. It is good for spotting a badly sized quote, but it ignores height, glazing, insulation and occupancy.