Portable air conditioner BTU calculator (UK)
Free sizing tool · UK guidance
Not sure what size portable air conditioner you need? Enter your room size and how much sun it gets, and we’ll suggest a sensible BTU size and the kind of unit to look for. It’s a guide, not a precise HVAC calculation — but it will get you to the right ballpark and away from a unit that’s too small (or wastefully too big).
You need roughly 7,000 BTU
We estimate roughly 6,800 BTU/h (about 20 m² × 340 BTU), so look for a 7,000 BTU unit. This is a guide, not a precise HVAC calculation.
Models near this size we track:
- Comfee 7000 BTU — is it in stock?
- EMtronics 7000 BTU Portable Air Conditioner — is it in stock?
- Honeywell 7000 BTU — is it in stock?
- Igenix 7000 BTU — is it in stock?
Browse all portable air conditioners, or read the buying guide.
What is a BTU?
BTU stands for British Thermal Unit. On an air conditioner, the BTU/h rating tells you how much heat the unit can remove from a room in an hour — so a higher BTU number means more cooling power. Portable units sold in the UK typically range from about 5,000 BTU for a small bedroom up to 16,000 BTU and beyond for a large, open-plan space.
What size air conditioner do I need? (UK sizing table)
As a starting point, allow roughly 340 BTU per square metre of floor area, then adjust up for a sunny room, a top-floor room, or lots of people. This table is a quick guide for an average, standard-height room:
| Room | Approx. floor area | Recommended size |
|---|---|---|
| Small bedroom / home office | up to ~14 m² | 5,000 BTU |
| Standard double bedroom | ~15–20 m² | 7,000 BTU |
| Large bedroom / small lounge | ~21–26 m² | 9,000 BTU |
| Living room / open-plan area | ~27–35 m² | 12,000 BTU |
| Large living room | ~36–41 m² | 14,000 BTU |
| Very large / open-plan space | ~42 m²+ | 16,000 BTU or more |
Use the calculator above for a figure tuned to your room’s sun exposure and how many people use it — those factors can move you up a size.
Why size matters: undersizing vs oversizing
Too small and the unit runs flat out and still never gets the room down to a comfortable temperature on the hottest days — you pay for electricity without getting cool. Too big isn’t the safe bet it sounds like either: an oversized unit cools the air quickly but cycles off before it has pulled enough humidity out, leaving the room feeling cold and clammy, and it costs more to buy and run. Sizing to the room, rather than buying the biggest unit you can find, gives the most comfortable result.
Which type of air conditioner?
BTU tells you how powerful a unit to buy; the type tells you how it fits your home. Almost all UK homes use a unit with no outdoor unit:
- All-in-one air conditioners — All-in-one air conditioners — also called monoblock or through-wall units — cool a room without a separate outdoor condenser.
- Portable air conditioners — Portable air conditioners are free-standing units on castors that vent hot air out of a window through a flexible hose.
- Window air conditioners — Window air conditioners fit into a sash or casement window and vent heat outside directly through the unit, so there is no outdoor condenser to install.
Found your size? Get it before it sells out
Once you know the BTU size you want, the hard part in a UK heatwave is actually buying one — the popular models sell out in minutes. BunnyDing watches the UK shops and emails you the moment a matching unit is back in stock, so you don’t have to keep refreshing.
Note: this calculator gives general UK guidance for a standard, well-fitted room. It is not a professional heat-load survey and doesn’t account for glazing, insulation or unusual ceiling heights. For a tricky room, treat the result as a minimum and consider the next size up. New to these units? Read the portable air conditioner buying guide.