BTU / Air Conditioner Sizing Calculator
Estimate the cooling capacity a room needs — in BTU/hr and kW — to choose an air conditioner that's neither underpowered nor oversized. Adjusts for sun exposure, occupancy and kitchens.
How to use this calculator
- Enter the room's floor area and ceiling height.
- Set the sun exposure, typical occupancy, and whether it's a kitchen.
- Match the recommended capacity to the nearest standard AC unit shown.
Formula used
A common rule is ~130–140 BTU per m² for a standard-ceiling room in a temperate climate. Add ~600 BTU per person beyond two, and ~4,000 BTU for kitchens. Hot climates and poor insulation push these figures up significantly.
Example calculation
A 25 m² living room, 2.7 m ceiling, average sun, 2 people: 25 × 135 × (2.7/2.4) = ~3,800 BTU... adjusted to about 3,800 BTU/hr ≈ 1.1 kW.
Wait — for a warm climate you'd size up: the nearest sensible unit here is a 9,000 BTU (2.6 kW) split, the smallest common size.
Why bigger isn't better for AC
Air conditioner sizing is a Goldilocks problem. Undersized, the unit runs constantly and never quite cools the room on the hottest days. Oversized — the more common and counter-intuitive mistake — it cools the air so fast that it switches off before removing much humidity, leaving the room cold but clammy, and the frequent on-off "short cycling" wears the compressor and wastes energy.
Correct sizing matches capacity to the room's actual heat load: floor area and ceiling height set the baseline, then sun exposure, occupancy, appliances and (in reality) insulation and window area adjust it. This calculator gives a solid ballpark for temperate climates; hot regions, conservatories and poorly-insulated rooms need a professional heat-load calculation, which also weighs orientation and glazing.
Why use this calculator?
- Avoid the oversizing mistake that leaves rooms cold but humid.
- Adjusts for ceiling height, sun, occupancy and kitchens.
- Maps your figure to the nearest standard AC unit size.
Frequently asked questions
What size air conditioner do I need for a room?
Roughly 130–140 BTU per m² for a standard-ceiling room in a temperate climate, adjusted for sun, occupancy and appliances. A 25 m² room typically needs a 9,000–12,000 BTU unit. Enter your details for an estimate, and get a professional survey for a precise figure.
Is it bad to buy an oversized AC?
Yes — an oversized unit cools the air quickly then shuts off before dehumidifying, leaving the room cold and clammy, and the frequent cycling wastes energy and wears the compressor. Right-sizing beats over-sizing for comfort and efficiency.
What's a BTU and how does it relate to kW and tons?
A BTU (British Thermal Unit) measures cooling capacity per hour. 1 kW ≈ 3,412 BTU/hr, and one "ton" of cooling = 12,000 BTU/hr. So a 12,000 BTU unit is 1 ton or about 3.5 kW.