Paint Calculator
Estimate how much paint you need for a room based on its dimensions, number of coats, and paint coverage.
How this is calculated
Wall area = perimeter of the room (2 × (length + width)) × wall height, minus any door/window area you subtract. Paint needed = wall area × number of coats ÷ your paint's coverage rate (how much area one liter, or one gallon in US units, covers).
Worked example
A 5m × 4m room with 3m walls has a perimeter-based wall area of 2×(5+4)×3 = 54 sq meters. At 2 coats and a coverage of 10 sq meters per liter, that's 54 × 2 ÷ 10 = 10.8 liters of paint.
A room with a door and a window
Take a 4.5 m × 3.5 m room with 2.6 m walls, one door of 0.9 m × 2.1 m (1.89 sq m) and one window of 1.2 m × 1.2 m (1.44 sq m). The wall area is 2 × (4.5 + 3.5) × 2.6 = 41.6 sq m, and subtracting 3.33 sq m of openings leaves 38.27 sq m. Two coats at 12 sq m per liter need 38.27 × 2 ÷ 12 = 6.38 liters. Paint is sold in fixed tin sizes, so a 5 L tin plus a 2.5 L tin (7.5 L) covers it with a small reserve for touch-ups.
Reading the coverage figure
Coverage is the single biggest source of error. The number on the tin assumes a smooth, previously painted, sealed surface. Fresh plaster, bare drywall, brick or textured render soak up paint and can use noticeably more, so a primer or mist coat on new surfaces is worth planning for. Dark-to-light colour changes may need an extra coat that the default of two does not cover.
Choose Metric to work in meters and liters, or US to work in feet and gallons. The room dimensions, the coverage figure and the answer all follow that choice, but the tool does not convert between the two, so enter the coverage in the same system: for reference, 400 sq ft per US gallon is roughly 106 sq ft per liter. If you leave coverage blank, the tool assumes 10 sq m per liter in Metric and 400 sq ft per gallon in US. Tin labels vary, and figures from about 350 to 400 sq ft per gallon are common, so check yours.
What is not included
- Ceilings, skirting boards, door frames, radiators and trim, which are usually painted in different products
- Paint left in the tray, roller and brush, which is commonly a few percent of the total
- Primer or undercoat, which has its own coverage rate and should be calculated separately
- Alcoves, chimney breasts and sloped walls, which need their own measurements added to or removed from the wall area
Frequently asked questions
Where do I find my paint's coverage rate?
It's usually printed on the paint can label, often as 'covers X square meters/feet per liter/gallon' — use that exact figure for the most accurate estimate.
Does this include the ceiling?
No, this calculates wall area only. If you're painting the ceiling too, add its area (length × width) separately using the same coverage-based method.
Should I subtract every door and window?
Large openings are worth subtracting, but small ones barely change the result. If you are unsure, leave openings at 0: you will buy slightly more paint than needed, which is useful for touch-ups later.
Why does my paint tin say a different coverage than I entered?
Manufacturers state coverage for ideal conditions and often for one coat on a sealed surface. Use the tin figure as the starting value, then lower it by 10-20% for rough, porous or newly plastered walls.
Does the result work in gallons?
Yes. Set Units to US, enter the room in feet and the coverage in square feet per gallon, and the result is shown in gallons. With Metric selected it is shown in liters.