What this calculator does
Cooling capacity starts from roughly 0.15 kW per square metre of floor and is adjusted for the things that change the heat load. A 20 square metre room with average sun and two occupants needs about 3.0 kW, or 10,236 BTU per hour.
Bigger is not better with air conditioning, which surprises people. An oversized unit cools the air quickly and shuts off before it has removed much humidity, leaving a room that is cold and clammy and cycling constantly. Matching the capacity to the load matters more than having headroom.
The formula
The floor area is multiplied by a base rate per square metre, then adjusted for ceiling height, sun exposure and the number of occupants, each of whom adds roughly 0.15 kW. The electrical input assumes a coefficient of performance of 3.5, meaning the unit moves about three and a half units of heat per unit of electricity consumed.
| Term | Meaning |
|---|---|
| Cooling capacity | Heat the unit can remove, in kW or BTU/hour. 1 kW is about 3,412 BTU/h. |
| COP | Coefficient of performance: heat moved per unit of electricity. Around 3.5 for a decent split system. |
| Sensible and latent load | Cooling the air against removing moisture from it. Oversized units handle the first and not the second. |
| Short cycling | Switching on and off rapidly, which an oversized unit does and which wastes energy and wears components. |
The inputs explained
| Field | What to enter |
|---|---|
| Room length (m) | Room length in metres. |
| Room width (m) | Room width in metres. |
| Ceiling height (m) | Ceiling height. Above the standard 2.4 m, the extra volume adds to the load. |
| Sun exposure | Sun exposure. A west-facing room with afternoon sun needs meaningfully more capacity. |
| People usually in the room | People usually in the room. Each adds roughly 0.15 kW of heat. |
When to use it
Choosing a split system
Units come in fixed capacities, and matching the room load is what determines which to buy.
Understanding a quote
An installer specification can be checked against a rough independent estimate.
Estimating running cost
The electrical input figure multiplied by hours and tariff gives an approximate cost to run.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much do the adjustments matter?
The same room at a range of sun exposures.
| Sun exposure | Cooling capacity needed | In BTU/hour | Electrical input (COP 3.5) |
|---|---|---|---|
| Shaded | 2.55 kW | 8,701 | 0.73 kW |
| Average | 3.00 kW | 10,236 | 0.86 kW |
| Sunny / west facing | 3.60 kW | 12,283 | 1.03 kW |
Questions
What size air conditioner do I need?
Roughly 0.15 kW per square metre as a starting point, so about 3 kW for a 20 m² room, adjusted up for sun exposure, high ceilings, poor insulation and more occupants. Units are sold in fixed steps, so choose the nearest rather than always rounding up.
Is a bigger unit better?
No. An oversized unit reaches the set temperature quickly and shuts off before removing much humidity, leaving the room cold and damp. It also short cycles, which wastes energy and shortens the compressor life. Correct sizing beats spare capacity.
How do I convert kW to BTU?
Multiply by 3,412. A 3 kW unit is about 10,236 BTU per hour. Many markets quote capacity in BTU while others use kW, and older units are sometimes rated in tons of refrigeration, where one ton is 12,000 BTU/h or about 3.5 kW.
Does this include insulation and windows?
Only indirectly, through the sun exposure setting. A room with large single-glazed windows or a poorly insulated roof will need more than this estimate suggests. For anything beyond a straightforward room, a proper heat load calculation is worth having.
For ventilation rates, see the air changes per hour calculator. For heating instead, see the furnace sizing calculator.