How to use
- 1Choose the paint type (interior emulsion, silicone, ceiling, exterior masonry, primer, wood/metal enamel); a typical coverage is filled in and you can replace it with the figure on your tin.
- 2For each room enter length, width and ceiling height, or the wall area directly; adjust the number and size of doors and windows and tick the ceiling if needed.
- 3Pick the number of coats, a waste allowance and optional primer; add as many rooms as you like and the areas are summed.
- 4Select the can sizes you can buy, optionally enter can prices and labour per area, then copy, share or print the summary. Switch to US units for ft², gallons and pounds.
How the paint calculation works
First the net area is found: wall area = 2 × (length + width) × height. Doors and windows are subtracted, with defaults of 0.8 × 2.0 m per door and 1.2 × 1.2 m per window. If you paint the ceiling, length × width is added. Paint needed = net area × coats ÷ coverage (m² per litre), plus your waste allowance. A 4 × 3 m room with a 2.6 m ceiling, one door and one window has about 33 m² of wall; at 12 m²/L, two coats and 10% waste it needs roughly 6 litres.
Typical coverage by paint type
Manufacturers’ data sheets usually quote, per coat: 10–14 m²/L for interior emulsions, 6–10 m²/L for ceiling paints, 8–14 m²/L for exterior masonry paint on smooth render but only 4–6 m²/L on textured or pebble-dashed walls, 8–12 m²/L for water-based primers and 10–15 m²/L for wood and metal enamels. In US terms, 10 m²/L is roughly 400 ft² per gallon. The presets use the middle of these typical ranges; real coverage depends on porosity, colour change, thinning and the tool you use, so the figure on your product label always wins.
Litres, kilograms and the best can combination
Some paints are sold by volume (for example 1, 2.5, 5 or 10 L tins; quarts, gallons and 5-gallon pails in the US) and others by weight (for example 10 or 20 kg tubs). The calculator converts with an editable density (water-based paints are typically 1.3–1.6 kg/L) and searches every combination of the sizes you tick for the one with the least surplus. If you enter prices, it picks the cheapest combination instead. Prices are only what you type; no brand prices are shown.
When you need primer or an extra coat
New plaster or drywall, a change from a dark to a light colour, stains and very porous surfaces usually call for one coat of primer plus two finish coats. Refreshing a wall in the same colour often needs two coats; high-opacity paints may cover in one. Tick the primer option and the calculator adds one primer coat over the same net area, with its own can suggestion.
Frequently asked questions
How much area does 1 litre of paint cover?
Typically 10–14 m² per coat for interior emulsion, 6–10 m² for ceiling paint and 8–14 m² for exterior paint on a smooth surface. Check the spread rate on your tin for the exact figure.
How much paint do I need for a room?
Enter the room’s length, width and height. A typical 4 × 3 m bedroom with a 2.6 m ceiling needs about 6 litres (around 1.6 US gallons) for two coats on the walls.
Should I subtract doors and windows?
Yes, especially in rooms with large windows. By default one door (0.8 × 2.0 m) and one window (1.2 × 1.2 m) are subtracted; change the quantities and sizes as needed.
How much extra paint should I buy?
A 5–10% allowance is common for brush and roller indoors and 10–20% for textured exteriors or spraying. Keeping a little paint for touch-ups is useful too.
How many square feet does a gallon of paint cover?
Most interior paints cover roughly 350–400 ft² per gallon per coat on smooth walls. Switch the calculator to US units to work in feet, gallons and pounds.
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