Size an air conditioner for a room

The job

Air conditioner sizing starts with a rough cooling load. The classic quick rule is about 20 BTU per square foot, adjusted for ceiling height, sun exposure, occupancy, and kitchens. This calculator does that arithmetic and suggests the common unit size to look at.

Estimating a room

Enter the floor area and ceiling height in metres or feet, then set the modifiers: sun exposure, occupant count beyond two, and whether it is a kitchen.

• Sunny rooms add ~10%, heavily shaded rooms subtract ~10%.
• Each occupant beyond two adds ~600 BTU — bodies are heaters.
• A kitchen adds ~4,000 BTU for the cooking load.
• Ceilings scale the estimate against a 2.7 m (9 ft) baseline — taller rooms need more, lower ceilings a bit less.

Worked example

A 20 m² (≈215 ft²) sunny living room for three people, 2.5 m ceilings: base ≈4,300 BTU, ×0.93 for height, +10% sun, +600 for the third person ≈ 5,000 BTU — a 5,000–6,000 BTU unit is the honest range to shop.

Where it stops

This is a first estimate, not a Manual J load calculation. Poor insulation, big windows, high climates, and open floor plans move the real load. When the number lands near a boundary, the honest advice is to size up slightly or get a proper assessment. Results are in BTU and watts equivalent.