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Paint Calculator

Work out how much paint you need for your walls.

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How to use

  1. 1.Pick your units, then enter the total wall area — the length of every wall times the ceiling height, added together.
  2. 2.Subtract the doors and windows you are not painting, then set the number of coats (two is typical).
  3. 3.Enter the coverage printed on the tin to see how much paint to buy, rounded up to whole gallons or litres.

About Paint Calculator

To work out how much paint you need, take your total wall area, subtract the doors and windows you are not painting, multiply what is left by the number of coats, then divide by the coverage printed on the tin and round up to whole tins. This calculator does all of that live as you type, in square feet with gallons or square metres with litres.

Start with the wall area. Measure the length of each wall and multiply by the ceiling height, then add the walls together — a room with 60 linear feet of wall and an 8 ft ceiling is 480 square feet. Keep every measurement in the same unit and the coverage and paint come out in the matching pair: square feet with gallons, or square metres with litres.

Next, subtract the openings. You do not paint the glass in windows or the face of doors, so knock their area off the total. A standard interior door is roughly 20 square feet and an average window around 12 to 15 square feet; add up the openings on your walls and enter the total. Leave this blank if you would rather ignore it — the small over-estimate simply becomes spare paint. The calculator will not let the openings equal or exceed the wall area, because then there is nothing left to paint.

Then choose the number of coats. Most repaints need two coats for an even, durable finish, which is why the calculator defaults to two. A fresh, well-matched colour over primer might manage one, while a light colour going over a dark wall — or bare, patchy or porous drywall — can need three. The tool multiplies your net wall area by the number of coats to get the true area the paint has to cover.

Finally, divide by coverage. Coverage is the area one gallon or one litre covers in a single coat. As a starting point, one US gallon covers about 350 to 400 square feet per coat and one litre of emulsion covers about 10 to 12 square metres per coat, on smooth primed walls. Those are ideal-condition figures from the tin: rough, textured or unprimed surfaces drink more paint, and thinner semi-gloss and high-gloss finishes cover less, so real-world coverage runs 10 to 20 percent lower. The calculator divides the total area by your coverage and rounds up, because you cannot buy a fraction of a tin.

A few things to keep in mind. This is a straightforward wall estimate, not a full paint schedule — ceilings, trim, doors and a separate primer coat are their own calculations. Paint type matters: premium paints with more titanium dioxide hide better in fewer coats. And it is always worth buying a little extra and keeping one spare tin from the same batch for touch-ups, as colours drift slightly between production runs. Everything here is an estimate to help you buy with confidence; measure twice and check the exact coverage on the tin you are buying. All calculations run entirely in your browser.

Frequently asked questions

How much paint do I need?
Take the total wall area, subtract the doors and windows, multiply by the number of coats, then divide by the coverage on the tin and round up. For example, 480 sq ft of wall minus 40 sq ft of openings is 440 sq ft; at two coats that is 880 sq ft, and at 350 sq ft per gallon you buy 3 gallons.
How much does a gallon or litre of paint cover?
As a rule of thumb, one US gallon covers about 350 to 400 square feet per coat and one litre of emulsion covers about 10 to 12 square metres per coat, on smooth, primed walls. These are ideal-condition figures — rough or porous surfaces and glossy paints cover less, so check the exact rate printed on your tin.
Should I subtract doors and windows?
Yes, if you want a tighter estimate. You do not paint glass or door faces, so subtracting them avoids over-buying. A standard door is roughly 20 sq ft and a window around 12 to 15 sq ft. Leaving the field blank just rounds your estimate up slightly, which becomes handy spare paint.
How many coats of paint do I need?
Two coats is the standard for an even, durable finish, so this calculator defaults to two. One coat can work for a fresh colour over matching primer, while a light colour over a dark wall, or bare and porous drywall, often needs three. More coats means more paint, so set the number to match your job.
How much extra paint should I buy?
The calculator already rounds up to whole tins, but it is still wise to buy a little extra. Real-world coverage runs 10 to 20 percent below the tin's figure on textured or thirsty walls, and colours drift slightly between batches — so keep one spare tin from the same batch for future touch-ups.

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