Paint calculator for walls and ceilings

How the calculation works

The calculator works room by room and adds everything up at the end.

1. Walls. For each room: 2 × (length + width) × height. That is the perimeter times the height.
2. Openings. For each door or window: width × height × how many. We take them off the walls only, never off the ceiling. If the openings add up to more than the walls, the wall area stops at zero.
3. Ceiling. If you tick "Paint the ceiling too", we add length × width.
4. Paint. Area to paint × number of coats ÷ coverage.
5. Waste. We add a margin, 10% unless you change it. It covers what stays in the tray, on the roller and in touch-ups.

Nothing is rounded until the end, when we pick the cans.

A worked example

In feet and gallons (US). A 12 × 12 ft bedroom with an 8 ft ceiling, one 3 ft wide door that is 6 ft 8 in tall (type 6.67 ft) and one 3 × 5 ft window. Ceiling included, two coats, 10% waste. The calculator's default coverage is 9 m²/L, which is about 367 sq ft per gallon.

• Walls: 2 × (12 + 12) × 8 = 384 sq ft
• Openings: 3 × 6.67 + 3 × 5 = 20.01 + 15 = 35.01 sq ft
• Ceiling: 12 × 12 = 144 sq ft
• Area to paint: 384 - 35.01 + 144 = 492.99 sq ft
• Paint: 492.99 × 2 ÷ 367 × 1.10 = 2.96 gallons

The calculator suggests three 1-gallon cans. If your label says 350 sq ft per gallon, the need rises to 3.10 gallons and you get three gallons plus one quart.

In metres and litres (UK). A 4.00 × 3.00 m room, 2.70 m high, with a 0.80 × 2.10 m door and a 1.20 × 1.40 m window, ceiling included, two coats, 9 m²/L, 10% waste.

• Walls: 2 × (4 + 3) × 2.70 = 37.80 m²
• Openings: 1.68 + 1.68 = 3.36 m²
• Ceiling: 12.00 m²
• Area to paint: 46.44 m², paint needed 46.44 × 2 ÷ 9 × 1.10 = 11.35 L

With UK tin sizes that is one 10 L tin and one 2.5 L tin.

The second room is the one the calculator opens with. The calculator shows these defaults in metric or imperial depending on the visitor: in the US it starts in imperial, with the same room converted to feet, and everywhere else in metric. You can switch units at any time, and your choice is remembered.

Where to find the coverage, and why it changes

Look on the label or the product data sheet for "coverage" or "spreading rate". US makers give it in square feet per gallon, UK makers in square metres per litre. Most give a range:

• Sherwin-Williams ProMar 200 Zero VOC Flat: 350-400 sq ft per gallon (about 8.6-9.8 m²/L).
• Behr Premium Plus Interior: 250-400 sq ft per gallon (about 6.1-9.8 m²/L), depending on the surface and how you apply it.
• Dulux Trade Vinyl Matt: up to 17 m² per litre. "Up to" means the best case.

Real coverage drops on rough or porous surfaces, on new drywall or fresh plaster, and when you cover a dark colour with a light one. The Sherwin-Williams data sheet lists a primer coat before the two finish coats on new surfaces, and Behr notes that some colours need extra coats.

Coverage sits in the advanced options, set to 9 m²/L (about 367 sq ft per gallon). That is inside the 350-400 sq ft per gallon range on the Sherwin-Williams ProMar 200 data sheet, and well below the best case of Dulux Trade Vinyl Matt. If the label gives a range, use the lower number.

How we pick the cans

In the US, interior wall paint is sold in quarts, gallons and 5-gallon pails. In the UK, emulsion comes mostly in 2.5 L, 5 L and 10 L tins, with 1 L tins for small jobs. The calculator uses quart, gallon and 5 gallon in imperial mode, and 1, 2.5, 5 and 10 L in metric mode.

Among all the combinations that cover what you need, we pick the one with the lowest score: the litres you buy, plus half a litre (about a pint) for each can. So an extra can is only worth it if it saves more than half a litre of leftover paint: we prefer fewer cans even if a little paint is left over. Cans smaller than a litre, like a US quart, count a full litre instead of half, because they cost much more per litre and nobody wants to buy three of them. On a tie, fewer cans win, then the combination with the bigger can, then the smaller total.

In the UK example, 10 + 2.5 L scores 12.5 + 1 = 13.5, and 10 + 1 + 1 L scores 12 + 1.5 = 13.5 too. It is a tie, so the two cans beat the three. Two 5 L tins and a 2.5 L tin score 14.

In the US example, two gallons and four quarts would also hold three gallons, but four of those six cans are quarts, which count as a full litre each: they lose clearly to three gallons. A 5-gallon pail would leave about two gallons unused.

Accessories on the receipt, and our assumptions

Besides the paint, the receipt adds:

• one roller;
• masking tape: one 50 m roll (about 164 ft) for every 25 m (about 82 ft) of room perimeter, since tape goes along the ceiling line and above the skirting board or baseboard;
• dust sheets (drop cloths in the US): one for every 16 m² (about 172 sq ft) of floor, based on a sheet of about 4 × 4 m.

In both examples that is one roll and one sheet.

Common mistakes

• Forgetting the ceiling. In the metric example, leaving it out drops the area to 34.44 m² and the paint to 8.42 L.
• Ignoring the thirsty first coat. New plaster or drywall soaks up the first coat. Prime first or lower the coverage.
• Buying lots of small cans. Small cans cost more per litre and leave more dregs.
• Using the top of the coverage range. That figure assumes smooth, sealed walls.

When to call a professional

Call a decorator or painting contractor if there is damp or mould that keeps coming back, cracks that are growing, old paint that is peeling, or high ceilings and stairwells that need scaffolding. Before sanding paint in an older home, check for lead and follow local rules. For large areas, a pro with a sprayer is faster and gives a more even finish.

These estimates do not replace the manufacturer's instructions. When in doubt, follow the data sheet of the paint you buy.