How many BTU do I need per square metre?

Roughly 200 watts (about 680 BTU) per square metre as a starting point.

What size AC for a 30 m² room?

Around 6,000 W or about 20,000 BTU, before adjusting for sun and use.

Is a bigger air conditioner better?

No — an oversized unit short-cycles, wastes energy and leaves the room humid.

What Size Air Conditioner Do I Need?

Published 10/28/2025 · 3 min read · Everyday calculators

Marco Bianchi — Home, DIY & motoring writer at Allin

Renovation · Materials

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In short

A rough rule is about 20 BTU of cooling per square foot of room — roughly 200 watts per square metre. A 30 m² living room therefore needs around 6,000 W, or about 20,000 BTU. Add capacity for sunny rooms, high ceilings, kitchens or many people, and subtract a little for well-shaded, well-insulated spaces.

A bedroom with a wall-mounted air-conditioning unit above the door. — Max Vakhtbovych · Pexels · Pexels

Around 20 BTU per square foot is the starting rule. Here's how to size a unit from your room's area, adjust for sun and use, and why bigger isn't better.

The area rule

Cooling need scales with floor area, and the common starting figure is about 20 BTU per square foot, or roughly 200 watts per square metre. Multiply your room's area by that number and you have a first estimate. A 30 m² living room comes out at around 6,000 W, which is about 20,000 BTU, since one kilowatt is roughly 3,400 BTU per hour. That estimate is the base you then adjust up or down.

Adjust for sun, ceiling and use

The area rule assumes an average room, so tune it to reality. A very sunny, west-facing room needs roughly 10% more; a heavily shaded one about 10% less. High ceilings mean more air to cool, so scale up with the volume. Kitchens add a lot of heat — add around 4,000 BTU. And people are little heaters: add about 600 BTU for each regular occupant beyond the first two.

Why the right size matters

It is tempting to buy big for safety, but an oversized air conditioner is a real problem. It cools the room so fast that it switches off before it has pulled moisture from the air, then switches straight back on — 'short-cycling' that wastes energy, wears the unit and leaves the room cold but clammy. An undersized one, meanwhile, runs flat out and never quite catches up on a hot day. Matching the size to the room gives steady, efficient, comfortable cooling.

BTU and watts

Air-conditioner capacity is quoted in BTU per hour in some markets and in watts (or kilowatts) in others, which causes confusion. The link is simple: one kilowatt is about 3,412 BTU per hour, so a 3.5 kW unit is roughly 12,000 BTU — often called 'one ton' of cooling. When you read a label or compare models, convert both to the same unit first so you are genuinely comparing like with like.

Worked with our own calculator

BTU calculator (air conditioning)

Given

Room area (m²): 28

Result

Power (BTU/h): 9,520
Power (kW): 2.79

These figures are produced by the calculator below, not typed in by hand — they are recomputed whenever the tool changes.

Run it on your own figures →

Frequently asked questions

How many BTU do I need per square metre?: Roughly 200 watts (about 680 BTU) per square metre as a starting point.
What size AC for a 30 m² room?: Around 6,000 W or about 20,000 BTU, before adjusting for sun and use.
Is a bigger air conditioner better?: No — an oversized unit short-cycles, wastes energy and leaves the room humid.

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