Paint Calculator
How much paint a room needs, from its dimensions: with doors, windows, coats and primer.
The room
8 ft is standard
21 ft² each
15 ft² each
Coats of paint
Coats of primer
for one gallon
This works out how much paint a room needs from its dimensions, allowing for doors, windows and multiple coats.
It gives wall area, ceiling area and the number of gallons, including primer if needed.
How much paint a room needs
Paint coverage is straightforward arithmetic with two steps people skip. Wall area is the perimeter times the height: a 12 × 14 ft room with 9 ft ceilings has 468 sq ft of wall.
The first thing skipped is two coats. A gallon covers about 350-400 sq ft, and calculators that quote one coat give an answer that looks generous and is not. After subtracting a door and window, 433 sq ft of wall needs 866 sq ft of coverage, which is 2.5 gallons.
The second is primer. It is a separate product with its own coverage, and it is genuinely needed over bare drywall, over a big colour change, or over stains. Buying paint without it and discovering the old colour showing through is a common and avoidable annoyance.
Buy three gallons, or two plus a quart. Running out mid-wall means a visible lap mark where the new paint meets the dried edge.
What to enter
- Room dimensions and ceiling height
- Wall area is perimeter × height, so all three matter.
- Doors and windows
- Subtract about 20 sq ft per door and 15 per average window. Skipping this overestimates a typical room by around 7%.
- Number of coats
- Two is standard. Three for a dramatic colour change, or when going light over dark.
- Ceiling
- Length × width if you are painting it, and ceiling paint is a separate flat-finish product.
Coverage and finishes
- Coverage per gallon
- 350-400 sq ft on smooth surfaces. Textured walls absorb more, sometimes 250-300.
- Primer
- Needed over bare drywall, over stains, and for large colour changes. Its own coverage, usually similar per gallon.
- Flat / matte
- Hides wall imperfections best. Hardest to clean, so it suits ceilings and low-traffic rooms.
- Eggshell / satin
- The usual choice for living areas and bedrooms. Wipeable and forgiving.
- Semi-gloss / gloss
- Kitchens, bathrooms, trim and doors. Most durable and shows imperfections most.
What this assumes
Coverage assumes a smooth, primed surface. Textured, porous or unprimed walls take considerably more.
Dark and highly saturated colours often need an extra coat, particularly reds and deep blues.
How to calculate the paint you need
Perimeter times height, subtract openings, multiply by coats, divide by coverage.
- openings
- About 20 sq ft per door, 15 per window
- 350
- Square feet per gallon on a smooth surface
Work out the wall area. Add the four wall lengths and multiply by ceiling height. For 12 × 14 at 9 ft: 2 × 26 × 9 = 468 sq ft.
Subtract doors and windows. One door and one window is about 35 sq ft, leaving 433.
Multiply by the number of coats. Two coats: 433 × 2 = 866 sq ft of coverage needed.
Divide by coverage and round up. 866 ÷ 350 = 2.5 gallons. Buy three, or two plus a quart, and keep the remainder for touch-ups.
See a worked example: a bedroom, walls only
- Room
- 12 × 14 ft, 9 ft ceilings
- Openings
- One door, one window
Wall area: 2 × (12 + 14) × 9 = 468 sq ft.
Less openings: 468 − 35 = 433 sq ft.
Two coats: 866 sq ft. At 350 per gallon that is 2.5 gallons.
Add the ceiling if painting it: 12 × 14 = 168 sq ft, which is another gallon of ceiling paint for two coats.
About 2.5 gallons for the walls
Frequently asked questions
350-400 sq ft on a smooth, primed surface. Textured walls absorb considerably more, sometimes only 250-300 sq ft per gallon.
Manufacturers quote the optimistic end. Using 350 as your planning figure gives a sensible margin.
Almost always, yes. One coat rarely gives even colour and full opacity, particularly over a different existing colour.
Three coats are common when going light over dark, or with saturated reds and deep blues, which are notoriously poor at covering.
Over bare drywall or new plaster, over stains or water marks, when making a large colour change, and when painting over glossy surfaces.
Paint-and-primer-in-one products are fine over an existing similar colour in good condition. They are not a substitute for real primer on bare or stained surfaces.
Yes for anything significant. About 20 sq ft per door and 15 per average window is a reasonable allowance.
Skipping it overestimates the worked example by about 7%, and considerably more in a room with several windows.
Flat for ceilings and low-traffic rooms, since it hides imperfections best. Eggshell or satin for most living areas and bedrooms, as a balance of durability and appearance.
Semi-gloss for kitchens, bathrooms, trim and doors, where moisture and cleaning matter. The glossier the finish, the more every wall imperfection shows.
Yes, and it is worth doing. Sealed properly and stored somewhere that does not freeze, latex paint keeps for several years.
Note the room and the date on the tin. Touching up later with the same tin avoids the batch variation you get from a fresh mix.
Problems people actually run into
Buying for one coat
Coverage figures on a tin describe a single coat on a prepared surface, and calculators often follow that. Real jobs take two.
Doubling the area before dividing is the whole fix. Running out mid-wall leaves a visible lap mark where wet paint meets a dried edge.
Forgetting primer as a separate purchase
Primer has its own coverage and its own cost. A colour change over bare drywall needs primer plus two coats, not two coats total.
Working this out at the shop rather than halfway through the job saves a trip and, on a dark-to-light change, saves a third coat.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026