Tool

Room cooling calculator

Most cooling advice starts with the machine. This starts with the room: how much heat it actually gains, where that heat comes from, and whether buying anything is the right move.

The room

Length

Width

Ceiling height

The building

Where the room sits

Roof insulation

Only counts when the room is directly under the roof.

Walls

Glazing

Glazing area

Glass only, not the frame.

Which way it faces

Shading already there

What is in the room

People

Equipment left running

A television and a games console is roughly 200 watts.

This calculator needs JavaScript. Nothing is sent anywhere: the whole calculation runs in your browser.

The answer

peak heat gain, which is in a room of and

Working it out

The working

Where the heat comes from, before any uplift for the roof
Source	Watts	Share
Walls, floor and ceiling
Sun through the glazing
People in the room
Equipment left running
Subtotal		100%
Roof uplift
Total

Look for a unit rated at or more. That is the calculated load with headroom added, because a machine sized exactly to the peak never gets ahead of the room.

Questions

Why does it ask which way the window faces?: Because it changes the answer more than almost anything else. A west facing room takes low afternoon sun almost horizontally, at the hottest part of the day. That is why west facing bedrooms are the worst rooms in a British house, and why two rooms of identical size can need entirely different answers.
Why is my loft conversion so much worse than the room below?: A room directly under the roof gets an uplift applied to its whole load, because the roof is absorbing sun all day and the room is sitting under it. Insulation between the rafters is doing far less than most people assume once the roof surface itself is hot.
Is BTU or watts the number I should be shopping on?: BTU per hour, because that is what units are advertised in. The watt figure is the same quantity in sensible units, and it is shown so you can compare the load against the equipment already heating the room.
Why does it sometimes tell me not to buy anything?: Because that is sometimes the correct answer. Below a certain load a fan and better shading do the job for a fraction of the cost and noise. Above a certain load no consumer portable will ever get ahead of the room, and buying a bigger one does not change that.
Does anything I type get sent anywhere?: No. The whole calculation runs in your browser. There is no request to a server, no analytics on the inputs and nothing stored.