What Size Air Conditioner Do I Need? Room-by-Room Guide

Air conditioner capacity is measured in kilowatts (kW) of cooling, and sometimes in BTU (British Thermal Units per hour). The two simply describe the same thing in different units: 1 kW is roughly 3,412 BTU. A 2.5 kW unit is therefore about 8,500 BTU. UK manufacturers like Daikin and Mitsubishi Electric quote kW; cheaper portables tend to shout about BTU because the numbers look bigger.

The job of sizing is to match the unit's output to the amount of heat a room gains on a hot day. Too little and it never reaches temperature; too much and it short-cycles. Getting it right is what separates a quiet, efficient system from an expensive disappointment.

For a typical, reasonably insulated UK room, start with about 0.1 kW of cooling per square metre of floor area. Measure length × width to get the area, then multiply by 0.1. A 4 m × 4 m bedroom (16 m²) lands around 1.6 kW, comfortably served by a 2.0 kW unit (about 6,800 BTU).

Push that figure up to roughly 0.15 kW per m² for harder-working spaces: kitchens (ovens and fridges throw off heat), conservatories and sun-traps with large south or west-facing glazing, top-floor rooms under a warm roof, and any room packed with computers or people. Surrey and the South East run a little warmer than the UK average, so it pays not to skimp.

The table below is a starting point for common UK room sizes, using 0.1 kW/m² for standard rooms and 0.15 kW/m² for kitchens and sun-traps. Round up to the nearest available unit size, and treat these as a guide to discuss at survey rather than a final spec.

The per-square-metre rule is only the starting point. A proper heat-load assessment adjusts for the things that actually drive heat gain: the area and orientation of glazing (a big south-facing window can add as much load as several square metres of floor), insulation and glazing quality (a draughty Victorian terrace behaves very differently from an airtight new-build), ceiling height (we size on volume, not just floor area), and occupancy and equipment (each person and PC is a small heater).

Open-plan spaces and rooms open to a stairwell need extra capacity because you are effectively cooling more air. This is exactly why we survey: the rule of thumb gets you in the right ballpark, but the right kW for your specific room comes from measuring it.

It is tempting to think bigger is safer. It is not. An undersized unit runs flat out and still never quite cools the room on the hottest days, wearing itself out in the process. But an oversized unit is arguably worse: it cools the air so quickly that it switches off before it has removed the humidity, then switches straight back on. This short-cycling leaves the room cold but clammy, makes the temperature swing up and down, increases wear on the compressor, and pushes up your running costs.

Modern inverter systems tolerate slight oversizing far better than old on/off units because they modulate their output, but the principle holds: a correctly sized inverter unit that runs gently and steadily is quieter, more comfortable and cheaper to run than an oversized one. Right-sizing, not over-sizing, is the goal.

For a single bedroom or study, the rule of thumb will usually get you close enough to choose a unit with confidence. For anything more involved, a proper survey is worth it: multi-room or whole-home systems, open-plan living spaces, conservatories, rooms with large amounts of glass, listed buildings, or any commercial space.

A good survey also covers the practical side: where the outdoor unit can legally and quietly sit (many domestic installs are permitted development where the unit meets MCS 020, keeping it to 42 dB(A) or below at the neighbour's window, though conservation areas, flats and listed buildings usually need consent), and how to route pipework neatly. At Tempest we size every system on a real heat-load calculation, not a guess, so you pay for the capacity you actually need and nothing more.