Air Conditioner Sizing Calculator

Written byOleh Soloninupdated 25 September 2026

Area, m²

Ceiling height, m

Sun

People in the room

Equipment in the room, W

Actual consumption, not the power supply's maximum rating: nearly all the electricity equipment draws ends up as heat in the room.

Result

Matching classa 9 class unit (9,000 BTU)
Cooling needed, kW2.49
In BTU/h8,496
Through walls and windows, kW1.89
From people, kW0.2
From equipment, kW0.4

Why units are called 7, 9 and 12

Works out the cooling capacity a room needs from its volume, sun exposure, occupants and running equipment, and converts it into the usual split-system classes.

The number in a split system's name is thousands of BTU per hour. A '7' gives about 2.1 kilowatts of cooling, a '9' about 2.6, a '12' about 3.5. That is where the familiar 'ten square metres per unit' rule comes from.

The rule is crude, because heat comes from three places: through the walls and windows, from the people in the room, and from whatever is switched on. A room with south-facing windows and a working computer needs noticeably more than a north-facing bedroom of the same size.

How to use

Enter the floor area and the ceiling height.
Pick how sunny the room is: north-facing, ordinary, or south- and west-facing.
Add the number of people and the power of the equipment in the room — a computer and a television heat the air too.

What the load is made of

An air conditioner removes heat from three sources: through the walls and windows, from people and from equipment. Walls and windows dominate, but two people and 400 W of equipment add another 0.6 kW between them.

That is why one floor area can need different classes. Twenty square metres facing north and an ordinary room both fit a 9, while a sunny room of the same size already needs a 12.

Every row comes from the calculator on this page. A person gives off about 100 W of heat and equipment releases nearly all the power it draws; the calculator lists those shares on separate lines.
20 m² room, 2.7 m ceiling	Cooling needed, kW	Class
north-facing, 2 people, 400 W of equipment	2.22	09
ordinary, 2 people, 400 W of equipment	2.49	09
sunny, 2 people, 400 W of equipment	2.76	12
sunny, 3 people, 800 W of equipment	3.26	12

Which class for which area

An air conditioner's class is thousands of BTU per hour: a 9 delivers about 2.6 kW of cooling. The steps between classes are large, so you pick the nearest one above rather than cutting it fine.

Ceiling height works here just as it does for heating. A 20 m² room with a 3.5 metre ceiling needs 3.05 kW and no longer fits a 9 — though by floor area it looks entirely ordinary.

Ordinary room, two people, 400 W of equipment and a 2.7 metre ceiling unless stated. Every figure comes from the calculator on this page.
Area	Cooling needed, kW	Class
15 m²	2.02	07
20 m²	2.49	09
20 m², 3.5 m ceiling	3.05	12
25 m²	2.96	12
35 m²	3.91	18

What this calculator shows

The cooling capacity needed, in kilowatts and BTU per hour
The split-system class that matches it
The heat split: envelope, people, equipment

What to keep in mind

For a first estimate the model uses nominal gains of 30 W per cubic metre for a shaded room, 35 for an ordinary one and 40 for a sunny one; they do not replace a heat-gain calculation for the room.
A person adds about 100 W of heat, and equipment nearly all the power it draws: enter actual consumption, not the label maximum.

FAQs

What size air conditioner for 20 m²?

By this page's calculation, a 9: 2.49 kW of cooling for an ordinary room with two people and 400 W of equipment. A sunny room of the same size needs a 12.

What do 07, 09 and 12 mean?

Thousands of BTU per hour. Divide by 3412 for kilowatts: 07 is 2.1 kW, 09 is 2.6, 12 is 3.5.

What happens if the unit is too powerful?

It cycles on and off frequently, dehumidifies poorly and wears its compressor faster. Inverter models tolerate excess capacity far better than fixed-speed ones.

Size by floor area or by volume?

By volume. The "hundred watts per square metre" shorthand quietly assumes a 2.7 metre ceiling; at 3.5 metres the same room needs nearly a quarter more cooling — 3.05 kW rather than 2.49.

How much heat do people and equipment add?

A person at rest gives off about 100 watts, and the calculator counts it that way. Equipment turns nearly everything it draws into heat: a computer actually drawing 300 watts adds about 300 watts of load. So enter actual consumption, not the power supply's rating.

Why do north- and south-facing rooms get different classes?

Sun through the windows heats a room more than any other source. The same 20 m² room needs 2.22 kW facing north and 2.76 facing south — nearly a quarter more, which here is enough to tip it into the next class.

Worked example

An ordinary twenty-metre room

Input: 20 m², 2.7 m ceiling, ordinary room, 2 people, 400 W of equipment

Output: 2.49 kW of cooling, a class 9 unit

Note: The familiar '9' for twenty square metres. Face the same room south and the requirement passes 2.6 kilowatts — 2.76 kW — which means a 12.

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