Paint Calculator

How to use the paint calculator

1. Choose Metric (m, cm, L) or Imperial (ft, in, US gal). Values you have typed are converted when you switch; untouched defaults are replaced by the usual defaults of the other system.
2. Under Surfaces to paint, tick Walls, Ceiling or both.
3. Enter the Room length, Room width and Wall height. Each dimension has two boxes: metres and centimetres, or feet and inches. You can fill only the first one (3.65 m, or 12 ft) or both (3 m 65 cm, or 12 ft 6 in).
4. Under Doors and windows (not painted), give the number of Doors and Windows and the Area of one door and Area of one window. The defaults (1.8 m² or 21 sq ft per door, 1.5 m² or 15 sq ft per window) are typical sizes: change them if yours differ.
5. Set the Number of coats and the Coverage per coat (spread rate on the can), then pick a Container size.
6. Read the Paint needed, the Containers to buy (always rounded up) and, when it saves product, a Less leftover combination that tops up with 1 L cans or US quarts. The calculation steps are shown under the results, and Reset defaults restores the starting values.

Nothing is sent anywhere: the calculation runs in your browser.

How the amount of paint is calculated

The tool treats the room as a box. The wall area is the perimeter times the height:

Walls = 2 × (length + width) × height − doors − windows

The ceiling is simply length × width. The paint needed is then:

Paint = area to paint × number of coats ÷ coverage per coat

Example: a 4 × 3.5 m room with 2.5 m walls has 2 × 7.5 × 2.5 = 37.5 m² of walls. One door (1.8 m²) and one window (1.5 m²) leave 34.2 m². Two coats at 10 m²/L need 34.2 × 2 ÷ 10 = 6.84 L, so three 2.5 L cans, or two 2.5 L cans plus two 1 L cans (7 L) for less leftover. In imperial units, a 13 × 11 ft room with 8 ft walls, one door and one window gives 348 sq ft; two coats at 350 sq ft/gal need about 1.99 US gallons, so two 1-gallon cans.

Choosing realistic coverage and coats

Coverage (the spread rate) is printed on the can and in the product data sheet, usually as m²/L per coat or square feet per gallon. For interior wall paint on smooth, primed surfaces it is typically about 10 m²/L, or 350 to 400 sq ft per US gallon. Lower it for:

• rough or textured surfaces, fresh plaster or bare drywall, which absorb more paint;
• a roller with a long nap, or spraying (overspray is lost);
• a big colour change, especially light over dark or strong reds and yellows, which may need a third coat.

Two coats is the normal recommendation for an even finish. A primer or mist coat on new plaster or drywall is a separate product with its own spread rate: run the tool a second time for it with one coat.

Buying tips

• Buy all the paint for one room at once and check that the batch numbers match; if they don’t, mix the cans together in a bucket (boxing) before you start.
• Big containers cost less per litre, but leftover paint is waste. The Less leftover line shows when a smaller top-up can is enough.
• Keep a little paint for touch-ups, labelled with the room and the date.
• Trim, doors and skirting boards usually take a different product (satin or gloss): estimate them separately.
• Coverage figures are for the manufacturer’s recommended conditions. Their instructions on surface preparation, drying time and recoating prevail over any estimate.