BTU per square foot is an American shortcut that treats floor area as the load. British rooms are drawn in metres, built in brick or block, and glazed very differently from one wall to the next. Two rooms with the same floor area can need different kW. Convert any BTU figure you already have, then drop the area rule and survey glass, orientation, insulation and how the room is used.
Twenty BTU per square foot on a room drawn in metres
A common US chart uses about 20 BTU/h for each square foot of floor. People then multiply that by a room they measured in metres, or they convert metres to feet badly and buy the result. A 16 m2 room is about 172 square feet. At 20 BTU per square foot that sum is only about 3,400 BTU/h, near 1 kW, which is a feeble cooler for many real UK rooms of that size. Other people apply 20 BTU to each square metre and get a different wrong answer. The method is unstable before the building is even considered. There is no reliable single BTU-per-square-foot factor for a UK room, and multiplying square metres by a feet chart produces nonsense even before the building is considered.
UK practice states cooling in kW after a heat-gain assessment. Floor area is one input. It is not the design. Drawings in metres must stay in metres until a proper conversion. The same floor area can hide a bay window or a solid wall. Loft rooms and lantern roofs add fabric the floor area never shows. Occupancy and appliances add heat the chart ignores.
Take two 16 m2 rooms. A north-facing bedroom in a brick house, one small window, door shut, two people asleep, might sit happily on a 2.0 or 2.5 kW split. A south-facing kitchen diner of 16 m2 with bi-fold doors, a roof lantern and the oven on will not. The square-foot rule gives both rooms one number. Thermal mass matters as well. A heavy masonry room can ride through a short hot spell that would cook a lightweight conservatory of the same area. Insulation, trickle vents and whether you open the doors to a stairwell all move the result. A single watt-per-square-metre factor has the same flaw. It can be a rough check after a survey, not a way to order equipment.
Glass, use and a kW you can question
Measure length, width and height. Note glass area and which way it faces, roof windows, and whether a kitchen shares the air. Photos of the elevations let an installer see shading from neighbouring houses. That is a survey. A phone calculator that only asks for square footage is the same failed rule in an app. If you already know a BTU total from a listing, divide by about 3,412 to see the kW, then throw the area method away. The 3.5 kW and 5 kW classes exist so the survey can land on a real model, not so you can back-calculate from feet.
Do not tell an installer the unit must be 20 BTU per square foot. Tell them the room and the hot hours. They should show a kW selection you can question. F-gas install, outdoor siting and condensate are unchanged by whichever chart you saw first. In England the outdoor unit still has to satisfy planning rules for air source heat pumps, or a planning application. An oversized unit bought from a floor chart is harder to site and noisier for no comfort gain.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.