How to use the paint calculator
- Pick metric or imperial measurements, and the unit your paint is sold in. Switching keeps your values and converts them.
- Add every wall you are painting. Measure the full width and the height from floor to ceiling.
- Add doors, windows and other areas you won’t paint, and choose which wall each one is on.
- Enter the number of coats and the coverage from the paint tin. The calculator doesn’t assume a coverage for you.
- Optionally enter a can size and price to see how many cans to buy and the estimated cost.
- Tick “Include the ceiling” to get a separate ceiling estimate with its own paint settings.
How the estimate is worked out
- Gross wall area = width × height for each wall, added together.
- Deductions = width × height × quantity for each door or window. Each opening is checked against its wall: it can’t be wider or taller than the wall, and the openings on a wall can’t add up to the whole wall.
- Net area = gross area − deductions.
- Paint = net area × coats ÷ coverage per coat, then increased by the waste allowance.
- Cans = paint ÷ can size, rounded up to a whole can.
Worked example
A room has two walls 4 m wide and two walls 3.5 m wide, all 2.5 m high. There is one door (0.8 × 2 m) and one window (1.2 × 1 m). The paint covers 12 m² per litre per coat, two coats are planned with a 10% waste allowance, and the paint comes in 2.5 litre cans.
- Gross area: 2 × (4 × 2.5) + 2 × (3.5 × 2.5) = 20 + 17.5 = 37.5 m².
- Deductions: 0.8 × 2 + 1.2 × 1 = 1.6 + 1.2 = 2.8 m². Net area: 34.7 m².
- Two coats: 69.4 m². Plus 10%: 76.34 m². Divided by 12 m²/L: 6.36 litres.
- 6.36 ÷ 2.5 = 2.54, so you would buy 3 cans (7.5 litres).
- A 4 × 3.5 m ceiling with the same settings: 14 m² × 2 coats × 1.1 ÷ 12 = 2.57 litres, so 2 cans.
Choose Try an example above (with metric selected) to load these numbers.
Assumptions and limitations
- Coverage varies. Actual coverage depends on the paint, the surface (new plaster, textured or porous walls absorb more), preparation, colour change and how the paint is applied. Treat the result as an estimate and check the product’s own guidance.
- Example coverage values are only examples. Figures such as 12 m² per litre or 350 sq ft per US gallon appear on many wall paints, but your product may differ. Always replace them with the figure on your tin.
- Walls and ceilings are treated as flat rectangles. Sloped ceilings, gable ends and stair walls need to be measured as rectangles or triangles and entered as equivalent areas.
- Trim, skirting/baseboards, doors and window frames are not included.
- Primer, mist coats and stain-blocking undercoats are not included unless you add them as coats.
- The cost is cans × price per can. It doesn’t include tools, tape or primer.
Frequently asked questions
What coverage figure should I use?
Use the spread rate printed on the tin or the manufacturer’s data sheet for the exact product. It is usually given per coat, for example “up to 12 m² per litre” or “350–400 sq ft per gallon”. Rough or porous surfaces, new plaster, textured walls and dark-to-light colour changes all use more paint than the label suggests, so the lower end of the range is safer.
Are US gallons and UK gallons the same?
No. A US gallon is 3.785 litres and an imperial (UK) gallon is 4.546 litres, about 20% larger. Paint in the US is sold in US gallons and quarts. In the UK, Ireland, Canada and Australia paint is normally sold in litres. Choose the unit that matches the coverage figure and can size on your tin.
Should I subtract doors and windows?
Subtract large openings that will not be painted, such as doors, windows and built-in units. Small items like switches and sockets are not worth subtracting. If you are painting the door or window frames, that needs a separate (usually gloss or trim) paint estimate.
How many coats do I need?
Two coats is a common default for walls. A single coat may be enough when refreshing the same colour with a high-opacity paint. New plaster usually needs a mist coat or primer first, and strong colour changes may need three coats. Follow the instructions on the product.
Why is the ceiling calculated separately?
Ceilings are usually painted with a different product, often a flat or matt ceiling paint, and may need a different number of coats. Keeping the totals separate tells you how much of each paint to buy.
What does the waste allowance cover?
Paint left in the tray, roller and brush, spills, and touch-ups. 10% is a common allowance. Set it to 0 if you prefer to see the bare estimate.
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