What size air conditioner do I need? (kW by room)

Getting the size right matters more than the brand you choose. A unit that is too small struggles on a hot day, and one that is too big can cool a room too fast without properly drying the air, which leaves it feeling cold but still a bit damp. Here is how we size a room, and how to read a BTU figure if that is what you have seen elsewhere.

This is the same size guide behind our full price list, where you can pick a room type and see the price for all six packages at once.

Some manufacturers and older guides still quote size in BTU (British Thermal Units per hour) rather than kW. The two are just different units for the same thing, and the conversion is fixed: 1 kW is about 3,412 BTU/h.

• 2.5kW is about 8,530 BTU/h
• 3.5kW is about 11,940 BTU/h
• 5kW is about 17,060 BTU/h

If a unit is advertised in BTU, dividing by roughly 3,412 gives you the kW figure to compare against our size guide above.

Square metres are a useful starting point, but they are not the only thing that decides the right size. A few things push a room towards the next size up:

• A high ceiling, such as in a loft conversion with exposed rafters, means more air to cool for the same floor area.
• A lot of glass, like a conservatory, garden room or a living room with bi-fold doors, lets in more heat from the sun.
• Which way the room faces. A south-facing room usually needs more cooling capacity than a similarly sized room that stays shaded.
• How many people regularly use the room, since each person adds a small amount of heat.

This is why we always check the size at a free survey rather than from a floor plan alone. It costs nothing and means the system is sized for how the room actually behaves, not just how big it is on paper.

A price tool or a simple room guide is a good starting point. We still carry out a free survey to check the size, because a few things are hard to judge from a floor plan alone:

• Room shape and ceiling height. A long, narrow room cools differently from a square one of the same floor area, and a higher ceiling means more air to treat.
• Windows and orientation. A room with large windows facing the afternoon sun needs more cooling capacity than a similarly sized room that stays shaded.
• What else is in the room. Electronics, a kitchen area in an open-plan room, or several regular occupants all add a small amount of heat that a floor-area figure alone does not capture.

Choosing a bigger unit "to be safe." A system sized well above what the room needs can cool the air quickly and switch off before it has properly removed the humidity, leaving the room feeling cold but still a little damp. The correct size, running steadily, usually gives a more comfortable result than an oversized one cycling on and off.

Sizing from floor area in a room with a lot of glass. A conservatory or a garden room with floor-to-ceiling windows behaves differently from a similarly sized bedroom, which is why these rooms often need the next size up.

Guessing for a multi-room job. Because several indoor units share one outdoor unit in a multi-room system, the right outdoor unit size depends on the whole layout, not just one room added up. This is always checked at survey rather than estimated room by room.

If a room sits between two sizes, for example a mid-sized bedroom that could suit either 2.5kW or 3.5kW, it is rarely a big problem either way; an inverter system adjusts its output rather than just switching on and off, so it is not working flat out all the time even at the larger size. Where it matters more is a genuinely oversized room for the size chosen, which can leave corners of the room barely cooled on the hottest days.

Once you know roughly which size suits your room, the price tool on our prices page shows all six packages side by side for that size, so you can compare Haier, Samsung, Fujitsu, Mitsubishi Heavy Industries and Daikin options at a glance. We also quote for all major brands, so see the full price list for other options. Every air conditioning installation starts with a free survey to check the size before any work is booked.

If you are sizing a conservatory specifically, with its large areas of glass and strong summer sun, our conservatory air conditioning guide goes into more detail on why these rooms often need the next size up from a similarly sized room elsewhere in the house.