How Many BTU Do You Need? Room Size Table

Air conditioner sizing is one honest formula plus a few honest corrections. Get it right and the unit cruises quietly at part load; get it wrong and you either sweat next to an overworked machine or pay for capacity you never use. Here is the table, the formula behind it, and where the usual BTU marketing misleads you.

How many BTU do you need per square metre?

For a typical Swiss room with 2.4 m ceilings and decent insulation, plan 270 to 340 BTU per square metre, which is 80 to 100 W. A 20 m² bedroom needs about 6'000 to 7'000 BTU (1.8 to 2 kW); a 30 m² living room about 9'000 to 10'000 BTU (2.6 to 3 kW). Attic and strongly sun-exposed rooms sit above the range.

Room size	Cooling load	BTU class	Typical fit
up to 15 m²	1.2 to 1.5 kW	~5'000 BTU	small monoblock
15 to 20 m²	1.5 to 2.0 kW	6'000 to 7'000 BTU	bedroom, office
20 to 25 m²	2.0 to 2.5 kW	8'000 to 9'000 BTU	PortaSplit Cool (8'000 BTU)
25 to 30 m²	2.5 to 3.0 kW	9'000 to 10'000 BTU	large bedroom, small living room
30 to 35 m²	3.0 to 3.5 kW	10'000 to 12'000 BTU	PortaSplit / S12 (12'000 BTU)
35 to 45 m²	3.5 to 4.5 kW	12'000 BTU and up	3.5 kW at its limit

What is a BTU, and why does Europe use kilowatts?

A BTU (British Thermal Unit) is an old imperial energy unit; air conditioner ratings mean BTU per hour. The conversion is fixed: 1 kW equals about 3'412 BTU/h. European datasheets rate cooling in kW, marketing still loves BTU. So 8'000 BTU is about 2.35 kW and 12'000 BTU is about 3.5 kW. Same machine, two labels.

What is the formula behind the table?

Start with floor area times 80 to 100 W, then correct for the room's real conditions. These are installer rules of thumb, not lab physics, but they land you in the right BTU class almost every time.

• Base load: area × 80 W for shaded, well-insulated rooms on lower floors; area × 100 W as the standard assumption.
• Top floor or attic: add 20 to 30 percent. The roof stores afternoon heat and releases it into the evening.
• Large south- or west-facing glass without shutters: add 10 to 20 percent.
• People: each person beyond the first adds roughly 100 W.
• Electronics: add their real draw. A 300 W gaming PC is a 300 W heater.
• High ceilings: above about 2.6 m, think in volume instead: roughly 30 to 40 W per cubic metre.

Worked example: a 25 m² top-floor living room with west-facing glass and two people. 25 × 100 W = 2'500 W, plus 25 percent for the attic position = 3'125 W, plus 100 W for the second person: about 3.2 kW. That is squarely a 12'000 BTU room.

Why does undersizing hurt more than oversizing?

An undersized unit runs at 100 percent all afternoon, never reaches the set temperature, and is at its loudest exactly when you want quiet. An oversized inverter unit simply modulates down and runs quieter. So with inverter splits like the PortaSplit or our S12, round up without guilt.

Two caveats. Fixed-speed machines, which includes many cheap monoblocks, do punish oversizing: they cycle on and off, which is loud and dehumidifies poorly. And monoblock BTU numbers flatter themselves: a single-hose unit blows room air outside, which pulls warm air back in through every gap, so the cooling that reaches you is well below the rating. Stiftung Warentest summarised mobile monoblocks as flexible, but not very efficient (test.de). When comparing a monoblock against a split, mentally subtract a BTU class from the monoblock. A portable split avoids the leak entirely because its heat leaves through a closed refrigerant line, not an air hose; how that works through a Swiss window is covered in our venting guide.

Which unit fits which room?

The Midea PortaSplit Cool (2.35 kW, 8'000 BTU, 38 dB(A), CHF 1'090.–) is the bedroom and office unit: right up to about 25 m², cooling only. The Midea PortaSplit and our SplitClima S12 (both 3.5 kW, 12'000 BTU) carry living rooms up to about 35 m², or about 30 m² under a hot roof; both also heat with SCOP 4.0, which we explain in the heating guide.

And the honest limit: none of these will hold 22 °C in a 50 m² open loft under a 35 °C roof. For that you need two units or a fixed installation with the permits that come with it.

Common sizing questions

Is more BTU always better?

No. You pay for capacity in price, weight and power draw. An inverter split sized one class up is fine because it modulates down; buying double the class you need is money spent on nothing.

Should I size for record heat days?

Size for the room, not for the record. On a day like 28 June 2026, when Basel averaged an all-time 10-day high, any correctly sized unit runs near full output, and holding 26 °C instead of 22 °C is the realistic win. The trend data is on our heat days page.

Can one unit cool two rooms?

Not well. Cold air does not turn corners; through an open door you get a weak spillover, several degrees warmer than the room the unit stands in. If you must choose, put the unit where you sleep and let the living room cope. For genuinely cooling two rooms you need two units, or the discipline to roll one 45 kg machine back and forth, which nobody keeps up for long.

Does the same sizing apply to heating mode?

Roughly, yes, and heating is usually easier: a single Swiss room in the shoulder season needs about 50 to 100 W per m², so a 3.5 kW unit has ample reserve. The details, including what SCOP 4.0 means for your bill, are in the heating guide.

Does ceiling height change the answer?

Yes, above about 2.6 m. Use the volume rule (30 to 40 W per m³) instead of the area rule, because you are cooling air, not floor. A 20 m² Altbau room with 3.2 m ceilings holds as much air as a 27 m² room with standard ceilings, and needs the bigger BTU class to match.

If you know your BTU class now, you know your unit: browse the portable split air conditioners and join the free waitlist for the model that matches your room; the next batch arrives in spring 2027 and waitlist members order 48 hours before everyone else.