How many litres of paint to cover a surface.
Tip: round up by 10–15 % to absorb mixing losses, touch-ups and uneven absorption on bare plaster.
Buying too little paint means a return trip to the store mid-project, often to find the original batch is gone and the new tin's pigment is subtly off — visible against the half-dried first coat. Buying too much wastes money and leaves you with half-full cans that will dry out in the garage. The paint coverage calculation answers exactly the right question: how many litres (or gallons) of a specific paint do I need to cover this specific surface in this specific number of coats? Manufacturers print a coverage rate on every can, expressed as square metres per litre (in Europe) or square feet per gallon (in the United States). With that rate and your wall area, the rest is one division. Add the number of coats — almost always two for a quality finish — and you have your answer. Painters, builders, decorators, and DIY enthusiasts all need this number before they shop, and getting it right within 10 % is a question of arithmetic, not skill.
The base relationship is:
paint volume = (surface area × number of coats) / coverage rate
In SI units this is litres, square metres, and square metres per litre. In imperial units it is gallons, square feet, and square feet per gallon. The two systems convert with: 1 sq ft = 0.0929 m², 1 gallon (US) = 3.785 litres. A typical interior wall paint covers roughly 10 to 14 m²/L (350 to 500 sq ft per US gallon) at one coat on a smooth, primed surface. Textured or porous surfaces (fresh plaster, old wallpaper paste, raw drywall) absorb more paint and the coverage rate drops by 30 to 50 %. The number of coats depends on opacity: most modern interior emulsions are formulated for two coats over a primer; high-opacity "one coat" paints exist but cost 30-50 % more per litre and rarely hit their stated coverage on dark base colours.
Three inputs: the surface area with a metric/imperial unit toggle (m² or sq ft), the coverage rate specified by the paint manufacturer (in m²/L or sq ft per gallon, again toggleable), and the number of coats (1, 2, or 3). The result is the volume of paint to buy, in litres or gallons depending on the coverage unit you picked. The panel also shows the total covered area (surface × coats), the coats count, and a small advisory about adding 10-15 % overhead for losses. Default values are 40 m² at 10 m²/L over two coats, which yields exactly 8 L — the typical bedroom paint job.
You are painting a 25 m² living-room wall in a satin emulsion that the manufacturer rates at 12 m²/L on a primed surface. You plan two coats. The calculator computes (25 × 2) / 12 = 4.17 L. Round up to a 5 L tin and you have a small buffer for touch-ups. A second case: an outdoor 200 sq ft fence in a low-coverage exterior stain at 200 sq ft per gallon, three coats. The result is (200 × 3) / 200 = 3 gallons. A third case: a freshly plastered 30 m² ceiling in a single primer coat at 8 m²/L (lower because plaster is thirsty) gives 30 / 8 = 3.75 L — buy 4 L. In all three, the final shopping decision should add 10 to 15 % beyond the calculated number to absorb mixing losses, brushes-loaded-too-heavily, and unexpected touch-ups two weeks after the project supposedly ended.
First, using the manufacturer's optimistic coverage rate on a porous or unprimed surface. Read the can carefully: the headline number usually applies "to a smooth, sealed surface", and the small print specifies a 30 % derate for raw plaster. Second, forgetting subtractions. A 4 m by 3 m wall is 12 m² before you remove the door (about 1.8 m²) and the window (1.5 m²); you actually need to paint about 8.7 m². For most rooms, doors and windows take up 10 to 20 % of nominal wall area. Third, painting dark over light without a primer. Dark blues, deep reds and pure blacks need three or even four coats over white; the calculator's "coats" input handles this, but you must remember to set it to 3. Fourth, ignoring the trim, ceiling, and skirting boards which usually take a different paint than the walls and have their own coverage rates. Fifth, mixing partial cans of two different batches: minor pigment differences become very visible on a single wall, especially under raking light.
European cans are typically 1, 2.5, 5, or 10 litres; American cans are 1, 5 or 32 fl oz, 1 quart (0.95 L), or 1 gallon (3.79 L). Trim paint, ceiling paint, and floor paint each have distinct coverage rates because the resin systems differ — alkyd trim coverage is often 8-9 m²/L versus 12 m²/L for wall emulsions. Specialty surfaces have very different math: a roofing paint may cover only 3-4 m²/L on a textured tile, while a clear floor sealer can cover 18-20 m²/L. For commercial estimating, painters add a wastage factor of 5 % for spray application and 10-15 % for brush-and-roller work. The paint industry uses a standard called "spreading rate" that bakes in a single coat at 75 % opacity; multiplying by your coat count yields the buyer's number. Stores frequently sell the result of this calculation pre-tinted, and most paint counters will accept a no-questions return on unopened tins within 30 days, which converts the 10-15 % overhead from a sunk cost into a refund.