Measure the room, account for glass, insulation, occupants and heat-producing equipment, then get an instant planning range—without giving us your details.
Free calculator
Measure one room and adjust for the heat that a simple floor-area chart misses. The calculator returns a planning range in kW and BTU/h plus the nearest common unit sizes.
Instant result · no email requiredPlanning estimate only. Conservatories, server rooms, commercial spaces, open-plan zones and rooms with unusual heat gains require a room-by-room design calculation by the installer.
Floor area is only the start. A 16m² shaded bedroom inside a well-insulated house and a 16m² west-facing loft room under a dark roof do not have the same cooling load. The calculator begins with room volume, applies factors for insulation, glazing and floor position, then adds heat from occupants and equipment. The result is shown as a range because weather, construction and use are not exact online inputs.
Cooling capacity is expressed in kilowatts and BTU per hour. One kilowatt of cooling is about 3,412 BTU/h. UK product ranges commonly step through nominal sizes such as 2.0kW, 2.5kW, 3.5kW, 5.0kW and 7.0kW, but a nominal badge is not proof that the model delivers that output under the conditions that matter to your room.
Use internal room dimensions and include connected space that normally remains open while cooling. A hallway or stairwell can turn one apparent room into a much larger zone. Measure average ceiling height where roofs slope, and treat large roof lights as glazing. If two bedrooms keep their doors shut, calculate them separately rather than dividing one outdoor unit’s headline capacity between them.
For equipment and people, think about peak use: gaming PCs, ovens, salon dryers, server racks and a full meeting room release heat when cooling is most valuable. Enter the realistic simultaneous load, not the quietest hour of the week. The final survey should replace these categories with measured watts and construction data where they materially affect selection.
An oversized fixed-speed unit can cool the air quickly and stop before it has removed enough moisture. Frequent starts also add noise and wear. Modern inverter equipment can modulate, but it still has a minimum output and still needs a sensible design range. Undersizing creates the opposite problem: long full-power operation that cannot recover the room after afternoon solar gain.
Treat this result as a way to interrogate a proposal. Ask the installer for the room-by-room peak cooling load, the outdoor and indoor design temperatures, the allowances used for glass and occupancy, and the model’s delivered capacity at those conditions. “The old one was 3.5kW” and “this many square metres always needs 12,000 BTU” are not calculations.
| Nominal unit size | Approximate BTU/h | Common application—not a specification |
|---|---|---|
| 2.0kW | 6,800 | Small, low-gain bedroom or office |
| 2.5kW | 8,500 | Typical bedroom or modest living room |
| 3.5kW | 11,900 | Larger room or higher solar gain |
| 5.0kW | 17,100 | Large open room, subject to design |
| 7.0kW | 23,900 | Very large or equipment-heavy zone |
Conservatories can gain several times more heat through glass than a normal room, and shading can change the answer dramatically. Use the conservatory guide before relying on a generic size. Lofts need roof construction, knee walls, air leakage and top-floor temperatures considered; see loft air conditioning.
Server rooms are sized from electrical and IT loads plus resilience, not floor area. Kitchens, gyms, salons, restaurants and shops have people, lighting, appliances, doors and ventilation loads that a home calculator cannot represent honestly. A commercial air conditioning designer should document those assumptions.
Give each bidder the same rooms and comfort brief. Request exact indoor and outdoor model numbers, sound data, pipe and condensate routes, electrical scope, controls, warranties and commissioning. Then compare the result with our installation cost calculator and run the written proposal through the UK quote checker.
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Free planning tools
Use a cooling or heating estimate to prepare for a properly surveyed installer quote.