Paint Calculator
Coats and openings included, at your product's real coverage.
Work out Paint. Coats and openings included, at your product's real coverage. Waste is a separate input rather than a hidden assumption.
The tin assumes smooth primed walls. Bare drywall or texture takes far more.
Gallons to buy
2
1.72 needed · 6.5 litres
Coats are doing most of the work here: 2 coats over 301 square feet is 602 square feet to cover. The 350 sq ft per gallon figure assumes a smooth, primed, similarly coloured wall — bare drywall, masonry or a dark-to-light change can cut it by a third, and that is the input to revisit if the job runs short.
How the Paint Calculator works
Two things decide a paint order and neither is the room size. The first is coats — most jobs need two, which doubles everything. The second is coverage, and the figure on the tin assumes a smooth, primed, similarly coloured surface. Bare drywall, textured render or a dark colour going light will all take substantially more, so coverage is an input here rather than a constant.
Also known as: how much paint do I need · paint coverage calculator · wall paint calculator · paint calculator gallons
The calculation itself
Wall area is twice the length plus width, times the ceiling height — the perimeter times the height. Subtract doors and windows, multiply by the number of coats, divide by the coverage rate.
Coverage is quoted per gallon and is the input that decides the answer. Everything else in this calculation is measurement; coverage is a property of the product and the surface.
In practice
A 12 by 10 room with 8-foot ceilings has 352 square feet of wall. One door and two windows take off 51, leaving 301.
Two coats makes 602 square feet to cover. At 350 square feet per gallon that is 1.72 gallons — so two gallons, since you cannot buy the fraction.
Notice how close that sits to the boundary. Drop the coverage to 250, which is realistic on bare drywall, and it becomes 2.41 gallons — three tins. The room did not change; the surface did.
Why the number on the tin is optimistic
Manufacturers quote coverage for a smooth, primed, similarly coloured surface, applied by someone who is not in a hurry. That is the best case and it is what goes on the label.
Bare drywall drinks paint. So do masonry, render and any texture, because the actual surface area is larger than the measured rectangle. A significant colour change adds a coat on top of that.
Which is why coverage is the field to revisit if a job runs short — not the measurements. The measurements were almost certainly right.
Acting on it
Buy for two coats unless you are repainting the same colour. One coat rarely gives even coverage and discovering that mid-job risks a batch mismatch on the second tin.
Deduct large openings and leave small ones in. A single window in a big room is inside the rounding, and over-ordering slightly is the cheaper error.
Work out ceilings separately — the area is the room's footprint, and ceiling paint is usually a different product.
Frequently asked questions
How much paint do I need?
Wall area less doors and windows, multiplied by the number of coats, divided by the coverage rate. A 12 by 10 room with 8-foot ceilings is 352 square feet of wall; less one door and two windows, two coats comes to about two gallons at 350 square feet per gallon.
How many square feet does a gallon cover?
Manufacturers usually quote 350 to 400, and that assumes a smooth primed surface. Bare drywall, masonry or texture can cut it by a third or more, which is why it is adjustable here.
Do I need two coats?
Usually. One coat rarely gives even colour, and a significant colour change may need a primer plus two. Buying for one coat and discovering otherwise mid-job means a second trip and a possible batch mismatch.
Should I subtract doors and windows?
For a large opening, yes. For one small window in a big room the difference is inside the rounding, and over-ordering slightly is the cheaper error — touch-ups need the same batch.
How much for ceilings?
Ceiling area is the room's footprint, calculated separately, and ceiling paint is usually a different product. Add it as its own job rather than folding it into the wall figure.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/paint-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Paint Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.