A rule-of-thumb estimate for a typical UK room. Server rooms, kitchens, conservatories and high-gain spaces need a full cooling-load calculation — always confirm against a proper heat-gain assessment before quoting equipment.
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This calculator estimates the cooling capacity a room needs. Enter the floor area, ceiling height, how much sun the room gets, the number of people in it and any heat-generating equipment, and it returns the cooling load in kW and BTU/h along with the nearest standard unit size.
It is a quick sizing aid for installers and estimators pricing a single split for a typical room — an office, bedroom, shop floor or small function room. Getting the size roughly right matters: an undersized unit runs flat out and never quite copes, while an oversized one short-cycles, wastes energy and controls humidity poorly.
The maths is a rule-of-thumb heat-gain estimate. It takes a watts-per-square-metre figure based on sun exposure, scales it for ceiling height, then adds a fixed allowance for each occupant and the heat given off by equipment. The total in watts is converted to kW and BTU/h, and the nearest common unit size is suggested.
Load (W) = area × W/m² × (height ÷ 2.4) + (occupants × 100) + equipment (W); kW = W ÷ 1000; BTU/h = W × 3.412
The base load is the floor area multiplied by a watts-per-square-metre figure for the room's sun exposure — 120 W/m² shaded, 150 W/m² average, 180 W/m² sunny — scaled by the ceiling height against a standard 2.4 m. A flat 100 W is added for each occupant, and any heat-generating equipment is added in watts. Dividing the total by 1,000 gives kW; multiplying the watts by 3.412 gives BTU/h.
Type the floor area in square metres and the ceiling height in metres. The height defaults to 2.4 m, a standard UK ceiling — raise it for rooms with high or vaulted ceilings, as the extra air volume adds to the load.
Pick shaded (north-facing, little glazing), average, or sunny (lots of glass or south-facing). This sets the watts-per-square-metre figure — a bright, heavily glazed room gains far more heat than a shaded one of the same size.
Enter how many people are usually in the room; each adds 100 W of body heat. Then add the wattage of heat-generating kit — computers, servers, catering equipment or display lighting — as this all has to be removed by the unit.
The calculator shows the cooling load in kW and BTU/h, and suggests the nearest standard single-split size (2.0, 2.5, 3.5, 5.0, 7.1, 10 or 14 kW) at or above your calculated load.
This is a rule of thumb for a typical room, not a design calculation. It uses broad watts-per-square-metre allowances rather than a full room-by-room heat-gain assessment, so it will not account properly for the specifics of glazing orientation, insulation, shading, external walls or air changes. Treat the result as a starting point that gets you to roughly the right unit class, not a final specification.
Some spaces need a proper cooling-load calculation and should never be sized on a rule of thumb: server and comms rooms, commercial kitchens, conservatories and any room with very high solar gain or dense equipment. These can need several times the capacity a floor-area estimate suggests, and getting them wrong is expensive.
Bigger is not better. Oversizing a unit so it 'definitely copes' leads to short-cycling — the compressor reaches temperature quickly, shuts off, and starts again — which wears the kit, wastes energy and leaves the room clammy because the unit never runs long enough to dehumidify. Aim to match the load, not to exceed it by a wide margin.
A rule-of-thumb estimate for a typical UK room. Server rooms, kitchens, conservatories and high-gain spaces need a full cooling-load calculation — always confirm against a proper heat-gain assessment before quoting equipment.
As a rough guide, allow around 120–180 watts of cooling per square metre of floor depending on how much sun the room gets, then add about 100 W per person and the wattage of any heat-producing equipment. For a shaded 20 m² office with two people that is roughly 2.6 kW, so a 3.5 kW unit. Enter your figures above for a specific estimate.
Multiply the cooling capacity in kW by 3,412 to get BTU/h. A 3.5 kW unit is about 11,942 BTU/h, and a 5.0 kW unit is about 17,060 BTU/h. Air conditioning is quoted in kW in the UK and BTU/h on a lot of imported equipment, so this conversion comes up often.
Neither — you want to match the load. An undersized unit runs continuously and still struggles on hot days. An oversized one short-cycles: it cools the air quickly, switches off, and never runs long enough to pull humidity out, so the room feels cold and damp. Both waste energy and shorten the life of the kit.
No. Server rooms, comms cabinets, commercial kitchens and conservatories have very high, concentrated heat gains that a floor-area rule of thumb badly underestimates. Those spaces need a full cooling-load calculation that accounts for every heat source. Use this tool only for ordinary occupied rooms, and always confirm with a proper heat-gain assessment before quoting.
Because you are cooling a volume of air, not just a floor. A room with a 3 m ceiling holds noticeably more air than the same floor area at 2.4 m, so it needs more capacity. The calculator scales the base load by the height you enter against a standard 2.4 m ceiling.