Air Conditioner Sizing Calculator

Area, m²

Ceiling height, m

Sun

People in the room

Equipment in the room, W

Result

Matching classa 9 class unit (9,000 BTU)
Cooling needed, kW2.21
In BTU/h7,541
Through walls and windows, kW1.89
From people, kW0.2
From equipment, kW0.12

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.

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

Specific gain: 30 W per cubic metre for a shaded room, 35 for an ordinary one, 40 for a sunny one.
A person adds about 100 W of heat, and equipment about a third of its rated draw.

FAQs

What size air conditioner for 20 m²?

Usually a class 09, about 2.6 kilowatts of cooling. A sunny room full of equipment is better served by 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 it undersizes by nearly a third.

Worked example

An ordinary twenty-metre room

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

Output: 2.21 kW of cooling, a class 9 unit

Note: The familiar '9' for twenty square metres. Face the same room south and put a gaming PC in it and the requirement passes 2.6 kilowatts, which means a 12.

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Air Conditioner Sizing Calculator

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