Paint Calculator

Paint Calculator: How Much Paint Do I Need for This Room?

Guessing paint quantities is how a simple weekend job turns into a second trip to the shop, a slightly different shade, or two spare tins gathering dust in a cupboard. This page works out how much paint a room actually needs from its real wall area, subtracts the doors and windows you will not paint, and then allows for the number of coats, the finish you have chosen, and a sensible amount of spare. Use the quick table for a fast answer, or enter your own measurements in the calculator below for a figure based on your room, not on an average one.

Quick answer by room size

The table assumes walls only, 8 ft ceilings, one standard door and one standard window already deducted, two coats, a coverage of 350 sq ft per gallon, and 10% extra included as spare. Those are the same assumptions the calculator below uses as its starting point, and you can change every one of them.

Room size	Paint needed (2 coats)
10 × 10 ft	1.79 gallons
12 × 12 ft	2.19 gallons
12 × 15 ft	2.49 gallons
16 × 20 ft	3.39 gallons

As a rough check, a small bedroom usually lands close to two gallons and a larger living space closer to three and a half. If your ceilings are higher than 8 ft, if the walls are rough, or if you are covering a strong colour, expect to land above the table figure rather than below it, and use the calculator with your own ceiling height and coverage.

The formula

The calculator does nothing mysterious. It works out the area of the four walls, takes off the parts you will not paint, and then turns that area into a volume of paint using the coverage, the number of coats and your spare allowance.

• Wall area = 2 × (length × height) + 2 × (width × height). A room has two pairs of walls. One pair is as wide as the room is long, the other pair is as wide as the room is wide, and both pairs are as tall as the ceiling.
• Deductions = doors × 21 sq ft + windows × 15 sq ft. Those are standard door and window sizes. In Metric the calculator deducts 1.95 m² per door and 1.4 m² per window instead.
• Net wall area = wall area − deductions. This is the surface that will actually receive paint, and it is the figure every later step uses.
• Wall paint = net wall area × coats ÷ coverage × (1 + waste %). More coats and a bigger spare allowance increase the amount; a paint that covers more per gallon reduces it.
• Ceiling paint, separately = ceiling area (length × width) × 1 coat ÷ coverage × (1 + waste %). The ceiling is worked out on its own because it is normally a different, flatter paint, bought in its own tin.

If you already know your wall area, choose “I know my wall area” in the calculator. It then skips the wall-area step, still deducts your doors and windows, and applies the same coats, coverage and spare steps.

Worked example

Take a 12 × 10 ft room with 8 ft ceilings, one door and one window, painted in an eggshell finish at 350 sq ft per gallon, with two coats and 10% spare.

• Step 1 — wall area: 2 × (12 × 8) + 2 × (10 × 8) = 192 + 160 = 352 sq ft.
• Step 2 — deductions: one door at 21 sq ft plus one window at 15 sq ft = 36 sq ft.
• Step 3 — net wall area: 352 − 36 = 316 sq ft.
• Step 4 — paint: 316 × 2 coats ÷ 350 × 1.1 spare = 1.99 gallons.
• Step 5 — what to buy: 1.99 gallons is, in practice, 2 gallons. Buying exactly the calculated figure is rarely possible, so round up to the tin sizes on sale.

How much does a gallon cover?

Coverage is the single biggest reason two paint calculators disagree. The same room can need noticeably more or less paint depending on the finish, because flatter paints generally spread further and hide better in one pass, while glossier paints are thinner and show the surface beneath them more.

• Flat / matte: up to about 400 sq ft per gallon on smooth walls.
• Eggshell and satin: about 350 sq ft per gallon, which is why this page uses it as the default.
• Semi-gloss: about 300 sq ft per gallon.
• Rough or porous surfaces: often only 250–300 sq ft per gallon. Bare plaster, brick, heavy texture and unsealed repairs all drink paint, especially on the first coat.

Treat all of these as starting points only. The tin's label always overrides these defaults, so find the coverage printed on the paint you are actually buying and type it into the coverage field. If the label gives a range, using the lower end is the safer choice.

Do you need primer?

Primer is not simply watered-down paint, and it is not always necessary. Its job is to seal a surface, help the topcoat grip, or stop something underneath showing through. You are likely to need it in these cases:

• New drywall or plaster, which is porous and will absorb topcoat unevenly without a sealing coat.
• Bare wood, where primer seals the grain and stops knots and tannins bleeding through.
• Stain blocking, for water marks, smoke, marker or other stains that would otherwise reappear through ordinary paint.
• Big colour changes. Going from a dark colour to a light one is much easier over a tinted primer, and even then you may need primer plus an extra coat of colour.

On the other hand, sound walls that are already painted, in a similar colour to the new one, usually do not need primer at all — a wash, light sanding and two topcoats are enough. If you do need it, turn on the primer option in the calculator. Primer is calculated separately, at one coat and 300 sq ft per gallon (8.6 m² per litre in Metric), because you prime once even when you apply two topcoats.

US gallons vs UK litres

The maths is identical on both sides of the Atlantic; only the units and the words change. In the US, paint is sold in gallons and quarts and coverage is quoted in sq ft per gallon. In the UK and Australia, paint is sold in litre tins — commonly 1, 2.5, 5 and 10 L — and coverage is quoted in m² per litre, typically around 10–13 m² per litre for emulsion. Use the Metric toggle and the calculator will suggest a combination of those tin sizes instead of gallons and quarts. Some vocabulary differs too: what the UK calls emulsion and plaster, the US usually calls paint or primer and drywall, so check which terms your tin uses before comparing coverage figures.

Common mistakes

• Assuming one coat is enough. One coat rarely gives an even colour or sheen, especially over repairs or a different colour underneath. Plan on two coats and treat a good one-coat result as a bonus.
• Not mixing (“boxing”) multiple tins. Colour can vary slightly between tins and batches. Pouring them into one large container and mixing gives a consistent colour across the whole room.
• Ignoring wall texture. Rough, stippled or porous walls can use a quarter more paint than smooth ones. Lower the coverage figure in the calculator instead of discovering the shortfall halfway round the room.
• Forgetting the ceiling and trim are separate paint. Ceilings normally take a flat ceiling paint and trim takes a tougher gloss or satin, in different quantities. Neither is included in the wall figure unless you switch the ceiling option on, and trim is best estimated separately.

Paint guides

If you would rather work through the thinking behind the numbers, these three guides cover the same ground in more depth — how much to buy room by room, the metric version in litres and square metres, and why real coverage rarely matches the tin.

• How Much Paint Do I Need?
• How Many Litres of Paint per Square Metre?
• Paint Coverage per Litre

Frequently asked questions