How much paint do I need for a room?

Add up the wall area (the room's perimeter times its height), take off the doors and windows, multiply by the number of coats and divide by the coverage on the can. A 12 × 12 ft room with 8 ft walls, one 32 × 80 in door and one 3 × 4 ft window has about 354 sq ft of wall; two coats at 400 sq ft per gallon is 1.77 gallons, so two gallons. The calculator does this for every room at once and adds the ceiling if you tick it.

Do I need one coat or two?

Plan for two. Benjamin Moore recommends 1 or 2 coats of Regal Select, and Dulux Trade says Vinyl Matt typically takes 2 full coats, with one coat sometimes enough on previously painted walls in good condition and a similar colour. If one coat covers, the calculator's two-coat figure leaves you paint for touch-ups.

Should I subtract doors and windows?

Yes, for walls. A 32 × 80 in interior door is about 18 sq ft, and a 762 × 1981 mm one, the most common internal door in England and Wales, about 1.5 m², and every window you leave out saves paint too. Each room starts with one door and one window: set the counts and sizes to yours, or add more sizes.

How much primer do I need?

Tick Primer first: it adds one coat over the same area at the primer's own coverage. Zinsser gives Bulls Eye 1-2-3 a practical coverage of 350 sq ft per gallon (8.6 m² per litre) on porous surfaces and 450 on non-porous ones, so the calculator starts at 350.

Which cans should I buy?

The calculator finds the combination of the can sizes you tick that leaves the least paint over, and, when fewer cans would do with a little more left over, shows that too. Untick sizes your shop doesn't sell. Compare the prices of both: the fewer-cans option can cost less.

Can I use the room from the Room planner?

Yes. If you built a room in the Room planner in this browser, the calculator offers to add it, with its size, height, L-shape and every door and window. It only reads the planner's room; nothing in the planner changes.

How do I work out paint for the outside of a house?

Switch to House exterior and enter the footprint's length and width, the wall height to the eaves and the gable ends. Exterior paint covers less: Sherwin-Williams gives 250–300 sq ft per gallon for Duration Exterior, and Dulux Trade 4–6 m² per litre for Weathershield Smooth Masonry on pebbledash, so the calculator starts lower.

Paint Calculator: How Much Paint Do I Need?

How to use the paint calculator

Enter each room's length, width and ceiling height. Measure along the floor, wall to wall. Type feet and inches as 12'6" or metres as 3.8; other units work too (380 cm, 150 in). Feet · litres is for paint sold in litres with its coverage in square feet per litre, as in India. For an L-shaped room, tick L-shaped and give the size of the corner that is cut out.
Choose what to paint: walls, ceiling or both, per room.
Set the doors and windows you won't paint. Each room starts with one door and one window: change the counts with − and +, the sizes to yours, or add another size (a patio door, a small window).
Check the paint settings. Set Paints sold in to your country (the US, Canada, the UK, the UAE, Australia, New Zealand, South Africa, India, Malaysia or the Philippines) and pick your paint in the Paint menu if it's there: that country's range is listed, and the paint's published coverage and its own can sizes load in. The same brand can differ from one country to the next, so Dulux in Australia and Dulux in the UK are separate entries. Otherwise pick the number of coats and type the coverage from your can or its data sheet. Tick Ceiling in a different paint if the ceiling gets its own paint, and Primer first for a coat of primer.
Read what to buy. The big figure is the paint you need. Under it, each paint shows its area, the sum behind it and the cans to buy.

Add a room for every room you're painting and the totals add up. Everything stays in your browser: nothing you type is sent anywhere, and your rooms are still here when you come back. Share result makes a link that opens your rooms and the paint list for someone else, and Copy shopping list puts the list on your clipboard.

How the paint is worked out

Wall area is the room's perimeter times its height, minus the doors and windows: 2 × (length + width) × height − openings. An L-shaped room has the same perimeter as the rectangle around it, so only its ceiling is smaller. The paint is area × coats ÷ coverage. A 12 × 12 ft room with 8 ft walls has 384 sq ft of wall; take off a 32 × 80 in door and a 3 × 4 ft window and 354 sq ft is left. Two coats at 400 sq ft per gallon is 1.77 gallons, so you buy two.

Coverage is the one number worth checking. Makers print it on the can and in the product's data sheet, as a range or an "up to" figure for smooth, sealed walls. Benjamin Moore gives 400–450 sq ft per gallon for Regal Select and for its Waterborne Ceiling Paint; Dulux Trade gives up to 17 m² per litre for Vinyl Matt and up to 18 m² for Supermatt. The calculator starts at 400 sq ft per gallon (about 10 m² per litre), the low end of a common range, because textured, porous or patched walls take more paint than a smooth test panel.

Choosing the cans

Paint comes in fixed sizes: Benjamin Moore sells Regal Select by the quart and gallon, Sherwin-Williams sells Duration Exterior in 1 and 5 gallons, and Dulux Trade sells Vinyl Matt in 1, 2.5, 5, 7.5, 10 and 12 litres. The calculator starts with quarts, gallons and 5-gallon pails, or 1, 2.5, 5 and 10 litre tins, ticked. For each paint the calculator works out every combination of the sizes you tick and shows the one with the least left over; if fewer, larger cans would do with a little more left over, it shows that as well. Compare the prices of both. Untick the sizes your shop doesn't stock and the advice follows.

Primer, ceilings and the outside of the house

Primer is one coat at its own coverage over everything you paint. Zinsser gives Bulls Eye 1-2-3 a practical 350 sq ft per gallon (8.6 m² per litre) on porous surfaces and 450 on non-porous ones, so the calculator starts at 350. If only a patch needs priming, leave it off and spot-prime from a quart.

Ceilings are the floor area of the room. If the ceiling gets its own paint, such as a ceiling white, tick Ceiling in a different paint to get it as its own line. The ceiling paint calculator opens with the walls switched off.

Painting a single feature wall? One wall takes just its width and height, and can split it into two colours with a separate amount for each: see the accent wall calculator. For typical rooms worked out in advance, open the paint by room size chart, in feet and in metres.

For the outside of a house, switch to House exterior: walls are the footprint's perimeter times the height to the eaves, plus the triangles of the gable ends. The exterior paint calculator explains the lower coverage of masonry and siding paint.

Tips

Planning the room first? Build it in the 3D room planner: the calculator offers to add that room with all its doors and windows.
New floor too? The flooring calculator works out boxes of laminate, tiles and grout, or carpet off the roll, for the same rooms.
Keep a little of each colour, labelled with the room, for touch-ups.

Sources, checked September 2026: Benjamin Moore, Regal Select Interior Paint and Waterborne Ceiling Paint 508 data sheet; Dulux Trade, Vinyl Matt and Supermatt; Rust-Oleum, Zinsser Bulls Eye 1-2-3 data sheet; JB Kind, standard door sizes; NIST, Handbook 44, Appendix C (1 US gallon = 3.785411784 L).

How to use the paint calculator

How the paint is worked out

Choosing the cans

Primer, ceilings and the outside of the house

Tips

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