What size air conditioner do I need?

Floor area is the starting point, not the answer. Here's what actually determines the right unit size for a room in a UK home or office.

It's not just square metres

Room size gets you in the right ballpark, but several other factors push the correct unit size up or down from a simple floor-area guess:

• Ceiling height — a room with a higher-than-standard ceiling has more air volume to cool than its floor area suggests.
• Insulation quality — older, poorly insulated rooms lose the fight against outside heat faster, needing more cooling capacity to compensate.
• Windows and orientation — a south-facing room with large glazing gains significantly more heat through the day than a shaded, north-facing one.
• Occupancy and usage — more people in a room means more body heat to remove; a room used all day needs different sizing to one used occasionally.
• Heat-generating equipment — TVs, computers, servers, kitchen appliances and lighting all add to the cooling load beyond the room's basic footprint.

A rough starting guide

As a general indication of cooling capacity by room size:

Treat this as a starting point, not a spec. The factors above can shift a room up or down a band — which is exactly why a proper sizing conversation (or survey) matters more than a chart alone.

Why bigger isn't automatically better

It's tempting to assume a larger unit is a safer bet, but oversizing causes its own problems: an oversized unit cools the air quickly and then switches off before it's properly removed humidity from the room, leaving the space feeling cool but clammy — and cycling on and off more than it should, which adds wear.

Undersized units have the opposite problem — they run constantly trying to reach temperature, struggle on the hottest days of the year, and often end up costing more to run than a correctly sized system despite being the "cheaper" unit upfront.

Multi-room and commercial spaces

The same logic applies per room or per zone in a multi-split or commercial installation — a meeting room and an open-plan office of the same floor area can need noticeably different capacity depending on glazing, headcount and equipment. That's why our instant quote asks about each room or area individually rather than sizing an entire property off one guess.