Paint Coverage Calculator
Calculate how many litres of paint you need for a job, given the area to cover, the number of coats, and the spread rate of your specific paint. Lets you order the right amount without overspending on excess or running short mid-coat.
Last updated: May 2026
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About this calculator
The formula is V = (A × N) / C, where V is paint volume in litres, A is the total surface area to cover in m², N is the number of coats, and C is the manufacturer's coverage rate in m²/L (sometimes labelled 'spreading rate'). Coverage rates vary significantly by paint type: typical interior emulsion paints cover 12–14 m²/L on smooth previously-painted walls, 8–10 m²/L on bare plaster (first coat absorbs more); exterior masonry paints cover 6–10 m²/L due to surface texture; oil-based gloss paints cover 12–15 m²/L; primers cover 8–12 m²/L. Variables: A is the total area to paint (not the floor area; for a room it's the sum of wall areas plus ceiling minus openings); N is typically 2 for a colour change, 1 for a touch-up; C is the manufacturer's stated coverage — read the tin label. Edge cases: rough or porous surfaces absorb significantly more paint than the manufacturer's rate (which is usually quoted for smooth, primed surfaces); add 20–30% for textured walls, raw plaster, or exterior masonry. Dark-to-light colour changes typically need 3 coats, not 2. Spray application is more wasteful than roller or brush — coverage rates drop 20–40% with airless spray due to overspray. The calculator assumes coverage rate is per coat; the × N coats multiplier handles multiple applications correctly. Always round up to the next available paint tin size (1 L, 2.5 L, 5 L, 10 L); buying a fraction of a tin is impossible.
How to use
Example 1 — Painting a small room, two coats. The room has 35 m² of wall and 12 m² of ceiling to paint (47 m² total). You're applying 2 coats of emulsion that covers 13 m²/L on smooth walls. V = (47 × 2) / 13 = 94 / 13 ≈ 7.23 L. ✓ Round up to the next available tin: a 10 L tin (about £35–50) covers it with margin. A single 5 L tin (~ 65 m² per coat × 2 = 32.5 m² max) is too small. Example 2 — Exterior masonry, one heavy coat on rough render. 80 m² of textured render with a thick textured paint rated 7 m²/L. V = (80 × 1) / 7 ≈ 11.4 L. ✓ Add ~20% for the porosity of fresh render → 14 L. Round up to two 10 L tins (excess goes to touch-ups). Some painters apply a thinned 'mist coat' first on bare render (water added 5–10%), which extends coverage of the first coat but doesn't replace the second; budget 20% extra paint for the mist coat regardless of the formula.
Frequently asked questions
Why does my paint cover less area than the tin says?
Manufacturer coverage rates are stated for smooth, well-primed, non-absorbent surfaces under ideal conditions — fresh paint on a previously painted wall in moderate temperature and humidity. Real-world conditions usually reduce actual coverage: rough surfaces (textured walls, masonry, bare plaster) absorb 20–40% more; porous surfaces (raw timber, fresh plaster, MDF without primer) can absorb 50%+ on the first coat; spray application loses 20–40% to overspray; brush vs roller affects film thickness (rollers tend to spread more); and dark-to-light colour changes need 3 coats instead of 2. Always treat the manufacturer's rate as an upper bound and add 20–30% for typical jobs. For accurate estimation on unusual surfaces, paint a test square (1 m² is standard) and measure how much paint you used.
How many coats do I really need?
Two coats is standard for most jobs and gives the most uniform finish. One coat is acceptable only for touch-ups in matching colour and for primers on bare surfaces (the primer then has a topcoat over it, which is the second coat). Three coats are needed when going from dark to light (the underlying dark colour shows through a single layer of light paint), when changing from one paint type to another (oil to water-based or vice versa), or for high-traffic surfaces that need extra durability. Some 'one-coat' paints claim coverage in a single application; in reality they are highly pigmented two-coat paints applied very thickly, and a true uniform result still usually requires two thinner coats applied evenly.
What's the difference between coverage rate and spreading rate?
They mean the same thing — both express how many square metres one litre of paint covers. UK and European paint cans typically quote both metric (m²/L) and imperial (sq ft/US gal) figures; conversion is 1 m²/L ≈ 40.7 sq ft/US gal. Spreading rate, coverage, and 'practical coverage' are all interchangeable labels; some manufacturers distinguish 'theoretical coverage' (from film thickness) from 'practical coverage' (real-world average), with theoretical being the larger and unrealistic figure. Always use the practical coverage from the tin label or manufacturer's TDS (technical data sheet); the headline marketing number on the can is sometimes the theoretical figure dressed up to sound generous.
What are the most common mistakes people make estimating paint?
The first is computing floor area rather than wall area for room jobs — paint covers vertical walls, not floors, and a 4×5 m room has ~36 m² of walls (assuming 2.4 m ceiling) plus 20 m² of ceiling, not just 20 m² of floor. The second is forgetting to subtract windows and doors; typical residential openings reduce wall area by 10–20%. The third is using the manufacturer's coverage rate without adjustment for the actual surface; smooth previously painted walls cover at quoted rates, but rough or porous surfaces don't. The fourth is forgetting that primers, undercoats, and topcoats are separate purchases — a job requiring primer + two topcoats needs paint for three applications. The fifth is mixing coverage and area units (m² vs sq ft, L vs gallons) without converting. The sixth is buying an exact volume with no margin — paint goes bad after opening (typically 1–2 years even when resealed), and touch-ups years later are easier from the same batch than buying new and matching.
When should I not use this calculator?
Skip it for spray-applied finishes where overspray and atomisation losses make the simple area × coats / coverage formula systematically optimistic — get a coverage figure from the manufacturer specifically for spray application. Avoid it for textured coatings (Artex, decorative renders, masonry paints) where the texture's surface area exceeds the geometric area substantially; consult the manufacturer for textured-surface coverage. It is the wrong tool for industrial coatings (epoxies, polyurethanes, automotive paints) where film thickness is specified in microns and computed differently. Do not use it for special-effect finishes (metallic, pearlescent, sand-textured) that often require specific application techniques and dedicated coverage calculations. And for any commercial or high-spec residential job, get a professional decorator's takeoff with surface preparation, primer, and any specialty paints all priced together.