UK Paint Coverage Calculator

Buying too little paint means a frustrating extra trip to the shop partway through a job, possibly with a slightly different batch colour. Buying too much wastes money on tins you did not need. This calculator works out roughly how much you actually need, based on your room size.

Enter your room dimensions once and get a clear answer on litres needed and how many tins to buy, rather than guessing. It works whether you are planning a single feature wall or repainting an entire house room by room, since you can simply re-run it for each space.

How to use the paint coverage calculator

Enter your room's length, width and height, in metres or feet. Add how many coats you plan to apply, and how many doors and windows are in the room, since these reduce the area you actually need to paint. Choose your paint's coverage rate, or use the common default of 12 square metres per litre, and pick your preferred tin size to see how many you need to buy.

How the calculation works

The calculator first works out the total wall area using the room's length, width and height, then subtracts a standard allowance for each door and window you specify, since these areas do not need paint. That net wall area is then multiplied by your number of coats, since each extra coat needs the same area covered again. Finally, dividing by your paint's coverage rate gives the litres needed, which is rounded up to a whole number of tins based on your chosen tin size.

Rounding up to a whole tin, rather than the exact litres calculated, is deliberate. Paint is sold in fixed sizes, and buying a fraction of a tin is not usually possible, so the tins figure always rounds up to make sure you have enough to finish the job in one go.

Worked example

Take a room 4 metres long, 3.5 metres wide and 2.4 metres tall, with 2 coats, 1 door and 1 window.

The gross wall area comes to 36.0 square metres. After subtracting a standard door and window allowance, the net paintable area is around 32.9 square metres. With 2 coats, that is 65.8 square metres of total paint coverage needed. At the default 12 square metres per litre, that works out to around 5.5 litres, meaning you would need 3 tins if buying 2.5 litre tins, since 2 tins would only cover 5 litres.

Common mistakes to avoid

A common mistake is forgetting to account for a second coat, which roughly doubles the paint needed compared with a single coat calculation. Another is using a generic coverage rate rather than checking the actual tin of the specific paint being used, since premium and budget paints can have noticeably different coverage per litre. It is also worth adding a small buffer for wastage, spillage, and future touch ups, rather than buying the exact calculated amount with nothing spare.

It is also easy to underestimate how much door and window area actually reduces the paintable surface, particularly in a room with several windows or a set of double doors. Measuring your own doors and windows directly, rather than relying on the standard allowance this calculator assumes, will give a more accurate result if your room has unusually large or unusually small openings.

Textured walls, very absorbent new plaster, and strong colour changes can all reduce real world coverage below the figure on the tin, so treat this calculator's result as a solid starting estimate rather than an exact figure, particularly for a tricky wall or a dramatic colour change.

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