Paint Quantity Calculator
Work out the wall and ceiling area of a room, how much paint the coats require after deducting doors and windows, and the total cost.
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Paint is bought in whole containers and estimated from an area that most people get wrong in two ways: forgetting that each coat needs its own quantity, and forgetting to deduct doors and windows. The first overspends, the second sends you back to the shop mid-job.
Coverage varies by surface as much as by product. A previously painted smooth wall achieves the figure on the tin; bare plaster, textured surfaces and strong colour changes do not.
The calculation
Wall area = 2 × (length + width) × height, less the area of doors and windows. Add the ceiling if you are painting it, multiply by the number of coats, then divide by the coverage per container and round up — you cannot buy a fraction of a tin.
Worked example
A 5 by 4 room with 2.5 high walls has a perimeter of 18, giving 45 of wall area. Deducting 6 for a door and windows leaves 39. Adding a 20 ceiling and applying two coats means covering 118. At 12 per container that is 9.83, so 10 containers, costing 250.
Coverage is optimistic on the tin
The figure printed on a container assumes a smooth, previously painted surface in a similar colour. Bare plaster absorbs far more, textured surfaces increase the real area, and covering a dark colour with a light one often needs three coats. Add a container of margin on anything but a straightforward repaint.
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Create your free property pageFrequently asked questions
How many coats do I need?
Two on a normal repaint. Three when going from dark to light, over a strong colour, or onto bare plaster, which also usually wants a diluted first coat as a primer. One coat is only realistic for a refresh in the same shade.
Should I deduct doors and windows?
Yes, once they add up to a meaningful area. A single door and window in a small room is around six square metres, which on two coats is a full container of paint — enough to change what you buy.
Is more expensive paint worth it?
Often, because better paint covers in fewer coats and lasts longer. A product that covers in two coats rather than three can cost less overall despite a higher price per container, and it saves a day of labour.
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