AC BTU Calculator (Air Conditioner Size)
Enter your room's floor area, how much sun it gets, and how many people typically use it to get the right air conditioner size in BTU, tons, and kilowatts — no more guessing between undersized units that never cool the room and oversized ones that waste energy.
How this calculation works
The core rule of thumb is about 25 BTU of cooling capacity per square foot of floor area (roughly 270 BTU per square metre) for a room with standard 8-foot ceilings and normal insulation. Your area in m² is converted to square feet, then multiplied by that base rate.
Sun exposure shifts the baseline: a sun-drenched, west- or south-facing room with lots of glass needs about 10% more capacity, while a shaded, north-facing or heavily curtained room needs about 10% less.
Extra bodies add heat load. The baseline assumes 2 occupants; each additional person beyond that adds roughly 600 BTU, since the human body constantly gives off heat. The final BTU figure is rounded to the nearest 100 and also expressed in tons (1 ton = 12,000 BTU, the standard unit for central AC) and kilowatts (the metric rating used on most non-US aircon nameplates).
Worked example
Room size to BTU, at a glance
For a quick sanity check before adjusting for sun and occupants, here's how common room sizes translate to cooling capacity at the standard 25 BTU per square foot rate:
| Room area | Approx. BTU needed | Approx. tons |
|---|---|---|
| 10 m² (bedroom) | 2,700 | 0.23 |
| 15 m² (small bedroom/office) | 4,000 | 0.33 |
| 20 m² (medium bedroom) | 5,400 | 0.45 |
| 25 m² (large bedroom) | 6,700 | 0.56 |
| 35 m² (living room) | 9,400 | 0.78 |
| 50 m² (open-plan living/dining) | 13,500 | 1.13 |
| 70 m² (large open-plan) | 18,800 | 1.57 |
Why oversizing backfires
It's tempting to size up "just in case," but an air conditioner that's too powerful for the room cools the air quickly and switches off before it has run long enough to remove humidity. The room ends up cold on the thermometer but still feels damp, and the compressor cycles on and off far more often than it should, which increases wear and electricity use from repeated start-up surges.
A correctly sized unit runs longer, steadier cycles that both cool and dehumidify effectively — this is the same reason HVAC professionals size systems to the load rather than simply buying the biggest unit available.
Factors this estimate doesn't fully capture
- Ceiling height above the standard ~8 ft (2.4 m) — add about 10% capacity per extra foot
- Kitchens or rooms with heat-producing appliances — add roughly 4,000 BTU
- Poor insulation, single-pane windows, or a top-floor room under an uninsulated roof
- Large electronics load (multiple computers, a big TV, servers) generating extra heat
- Local climate — hotter, more humid climates may need capacity above the base rule of thumb
BTU, tons, and kilowatts explained
BTU (British Thermal Unit) per hour is the traditional US measure of cooling power. A ton of air conditioning equals exactly 12,000 BTU/h, a unit inherited from the ice industry. Kilowatts (kW) is the metric rating used on most air conditioner nameplates outside the US — this calculator gives you all three so you can shop confidently regardless of how a manufacturer labels its units.