What Size Air Conditioning Unit Do You Need?
A quick watts-per-square-metre rule gives you a starting number, but the right unit comes from a proper cooling load — glazing, occupancy, gains and orientation all move the figure. Oversize and it short-cycles; undersize and it never holds temperature.
Sizing an air conditioner is the first thing customers ask about and the easiest thing to get wrong. Go on floor area alone and you ignore the things that actually drive cooling demand: big south-facing windows, the number of people in the room, computers and kitchen gains, ceiling height and how well the space is insulated.
This guide gives you a rule-of-thumb capacity by room type to sanity-check a quote, explains how kW and BTU/h relate, and then steers you to the cooling-load method that gets the unit right — because the wrong size costs comfort, efficiency and callbacks.
Key takeaways
- A rough rule is around 120–180 W of cooling per square metre — use it only as a first sanity check.
- Cooling capacity is quoted in kW or BTU/h; 1 kW is roughly 3,412 BTU/h.
- Solar gain, glazing, occupancy and equipment can move the real load well outside the rule of thumb.
- An oversized unit short-cycles, dehumidifies poorly and wastes energy; an undersized one never holds temperature.
- Confirm the size with a proper cooling-load calculation before you quote the equipment.
A rule of thumb by room type
The table below gives a rough cooling capacity for common room types, based on around 120–180 watts per square metre. It is a starting point for a conversation and a way to check whether a quoted unit is in the right ballpark — nothing more.
Read it as an indication, not a specification. A small bedroom with a large west-facing window and blackout demands can need as much cooling as a room twice its size, and a densely occupied office will always sit at the top of the range.
| Room / space | Approx. capacity (kW) | Approx. capacity (BTU/h) |
|---|---|---|
| Small bedroom (~10 m²) | ~1.5 kW | ~5,000 BTU/h |
| Average bedroom (~15 m²) | ~2.0 kW | ~7,000 BTU/h |
| Living room (~25 m²) | ~3.5 kW | ~12,000 BTU/h |
| Large open-plan (~40 m²) | ~5.0 kW | ~17,000 BTU/h |
| Small office (~20 m², occupied) | ~3.5 kW | ~12,000 BTU/h |
kW and BTU/h — the same thing, twice
Cooling capacity is expressed either in kilowatts (kW) or in British Thermal Units per hour (BTU/h). They measure the same rate of heat removal, so you can convert directly: one kilowatt is about 3,412 BTU/h.
Manufacturers' domestic split literature often leads with BTU/h — a '12,000 BTU' unit is roughly 3.5 kW — while cooling-load calculations are done in kW. Keep both in mind so a customer's '9,000 BTU' request and your 2.6 kW load figure aren't treated as two different questions.
Why the rule of thumb isn't enough
Floor area is only one input to the real cooling load. Solar gain through glazing can be the single biggest factor — a room with large unshaded south or west windows needs markedly more cooling than the same room facing north. Occupancy, computers, servers, cooking and lighting all add heat, and ceiling height, insulation and air changes shift the number again.
That is why two rooms of identical size can need very different units. Under-size and the unit runs flat out and still loses the battle on the hottest afternoons; over-size and it cools quickly, cuts out, and short-cycles — poor dehumidification, temperature swings and wasted energy.
The rule of thumb starts the conversation
Use the chart to spot an obviously wrong quote, but never buy the equipment off it. A five-minute cooling-load check protects you from an undersized system that gets blamed on 'the brand'.
From estimate to a sized job
Do a proper cooling-load calculation for the room: dimensions and construction, glazing and orientation, occupancy and equipment gains, then the design conditions. That gives you a defensible kW figure to select the unit from, with a little headroom rather than wholesale oversizing.
Record the load and the selected capacity against the job, so the quote reflects the equipment the room actually needs and you have the numbers if the sizing is ever questioned — the same figures then flow through to your commissioning record.
Keep the sizing on the job
Logging the cooling load, the selected unit and the commissioning readings against the job in TradePlanr keeps the calculation, the quote and the record in one place — ready the next time that customer calls.
Frequently asked questions
How many BTU do I need per square metre?
As a rough rule, allow around 120–180 watts of cooling per square metre — roughly 400–600 BTU/h per square metre. It is only a first check: glazing, occupancy and equipment gains can push the real load well above that, so confirm with a cooling-load calculation.
How do I convert between kW and BTU/h?
One kilowatt of cooling is about 3,412 BTU/h. So a 3.5 kW unit is roughly 12,000 BTU/h, and a 2.6 kW unit is around 9,000 BTU/h.
Is a bigger air conditioner always better?
No. An oversized unit cools the air quickly then cuts out, short-cycling so it dehumidifies poorly and wastes energy, and comfort suffers from temperature swings. Size to the actual cooling load with modest headroom rather than picking the biggest unit.
From guidance to action
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