Estimate a room’s approximate cooling capacity in kW and BTU/h from its size, occupancy, sunlight and insulation.
This is a preliminary room estimate, not a professional cooling-load calculation. Equipment selection should be confirmed by a competent HVAC designer or installer.
How to use the Air Conditioning Size Calculator
Measure the room length, width and ceiling height in metres. Add the regular number of occupants, sunlight level, insulation level, glazed area and heat from equipment. Enter zero where the glazing or equipment allowance does not apply.
How this estimate is calculated
| Part | Method |
|---|---|
| Base room load | Room volume × 40 W/m³ |
| Sun and insulation | Base load × selected adjustment factors |
| Extra occupants | 100 W for each person after the first |
| Glazing allowance | 100 W per m² of entered glazed area |
| Final estimate | Adjusted room load + people + glazing + equipment |
Why room size is not enough
Two rooms with the same floor area can have different cooling loads. Ceiling height, glazing, solar gain, insulation, occupants, computers and other equipment all affect the result. Open-plan spaces and rooms connected by large openings may need to be assessed together.
kW and BTU per hour
Air-conditioning products may state capacity in kilowatts or BTU per hour. This calculator shows both, using 1 kW as approximately 3,412 BTU/h. Capacity is not the same as the electrical power drawn by the unit.
Professional sizing and inspections
Oversizing can cause short cycling and poor humidity control, while undersizing may not maintain comfort. For larger systems, review the UK government’s air-conditioning inspection guidance and obtain a room-by-room cooling-load assessment.
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Frequently asked questions
Is the result an exact AC size?
No. It is an initial estimate and should be checked with a professional cooling-load calculation.
What is the difference between kW and BTU/h?
They are different units of cooling capacity. One kilowatt is approximately 3,412 BTU per hour.
Why does sunlight increase the estimate?
Solar energy entering through glazing and heating external surfaces adds to the room’s cooling load.
Should connected rooms be combined?
Large permanently open connections may make spaces behave as one zone, so a professional should assess them together.
Does the calculator estimate running cost?
No. Running cost depends on efficiency, electricity price, controls, weather and operating time.