What Size Air Conditioner Do I Need?

Getting the size right is the most important part of choosing air conditioning. Too small, and the unit runs flat out on hot days without reaching the temperature you want. Too large, and it costs more to buy, switches on and off in mild weather, and can leave the room feeling clammy. This guide gives you a starting point. Our survey then checks it properly.

Quick Size Guide

As a rough starting point for a room with a standard ceiling height, allow about 0.1 kW of cooling for every square metre of floor space, and more for rooms that get a lot of sun. These are typical unit sizes:

Up to 15 m² (small bedroom, study): 1.5 to 2.0 kW, about 5,000 to 7,000 BTU/h.
15 to 25 m² (double bedroom, home office): 2.5 kW, about 8,500 BTU/h.
25 to 35 m² (living room, kitchen-diner): 3.5 kW, about 12,000 BTU/h.
35 to 50 m² (large lounge, open-plan room): 5.0 kW, about 17,000 BTU/h.
50 to 70 m² (large open-plan space): 7.0 kW, about 24,000 BTU/h.

Rooms with large south or west-facing windows, roof windows or high ceilings, and kitchens, often need the next size up.

How to Work It Out Yourself

Measure the length and width of the room in metres and multiply them to get the floor area. A room 4 m by 5 m is 20 m².
Multiply the floor area by 0.1 to get a starting figure in kW. For example, 20 m² × 0.1 = 2.0 kW.
Adjust for the room. For a sunny room, a loft room or a kitchen, multiply by up to 0.15 instead.
Round up to the nearest unit size. Common sizes are 2.0, 2.5, 3.5, 5.0 and 7.0 kW.

Worked example: a 20 m² south-facing bedroom gives 20 × 0.1 = 2.0 kW as a starting figure. It gets strong afternoon sun, so we allow a little more and choose a 2.5 kW unit.

Converting kW to BTU

Some brands and online calculators use BTU per hour (BTU/h) rather than kilowatts. One kilowatt is about 3,412 BTU/h, so multiply kW by 3,412 to get BTU/h, or divide BTU/h by 3,412 to get kW.

2.0 kW is about 6,800 BTU/h.
2.5 kW is about 8,500 BTU/h, often sold as a 9,000 BTU unit.
3.5 kW is about 12,000 BTU/h.
5.0 kW is about 17,000 BTU/h, often sold as an 18,000 BTU unit.
7.0 kW is about 24,000 BTU/h.

Model names often give the size away. Mitsubishi Electric and Panasonic use kW, so an MSZ-LN25 is a 2.5 kW unit. Brands that use BTU label their units 09, 12, 18 and 24, for 9,000, 12,000, 18,000 and 24,000 BTU/h.

What Else Affects the Size?

Sun and orientation. South and west-facing rooms with large windows pick up the most heat.
Insulation and glazing. Homes with older glazing or little insulation gain more heat in summer and lose more in winter.
Ceiling height. Rooms with high ceilings have more air to cool. A rough rule is to add about 10% for each extra metre of height.
People and equipment. Each person adds heat, and so do computers, televisions and kitchen appliances.
Heating in winter. If you want the unit to heat the room on cold days, it needs enough heating output for the coldest weather, which can mean a larger unit.
Open-plan spaces. One large unit or two smaller ones may work better, depending on the shape of the room.

Why Oversizing Is a Problem

Modern inverter units slow down to match the room, which helps, but every unit has a minimum output. If the unit is much too big, it still ends up switching on and off in mild weather, removes less moisture from the air and costs more to buy. An undersized unit has the opposite problem: it runs flat out on hot days, is louder and may never reach the temperature you set.

Why a Survey Matters

A rule of thumb gets you close. A proper survey measures each room and looks at the windows, walls, insulation, orientation and how you use the space. We calculate the right size for each room at the survey, and the survey is free. See how our home air conditioning installation works.

Frequently Asked Questions