Paint
Paint quantity and cheapest combination of tins
- where
- paint needed (L)
- area to paint (m²)
- number of coats
- spreading rate (m²/L)
- waste and touch-up allowance (%)
- litres bought (L)
- number of 10, 5, 2.5 and 1 litre tins in the cheapest mix
- cheapest total cost (£)
- price of each tin size (£)
- left over (L)
Explain this equation
Works out how much paint a job needs and finds the cheapest mix of tin sizes that covers it.
Area times coats is the total area painted; dividing by the spreading rate from the data sheet gives litres, and the waste allowance is added on top. The calculator then tries combinations of 10, 5, 2.5 and 1 litre tins (leaving out any priced at 0) and picks the one that holds at least V litres for the least money. V_b and C are that combination's volume and cost; R is what is left over.
Coverage arithmetic: litres equal area times coats divided by spreading rate.
Sense check: 50 m² in two coats at 12 m²/L with 10% waste needs about 9.2 litres, so one 10 litre tin is usually cheaper than two 5s.
Watch out: spreading rates on data sheets are for smooth, sealed surfaces. Rough or porous surfaces and strong colour changes need more.
Works out how much paint a job needs from the area, coats and the data-sheet spreading rate, then finds the cheapest mix of 10, 5, 2.5 and 1 L tins that covers it.
Enter your values to see the result.
Good to know
Trade emulsion data sheets give “up to” rates: Dulux Trade Vinyl Matt and Johnstone’s Covaplus up to 17 m²/L, Leyland Trade Vinyl Matt about 14 m²/L, Farrow & Ball Estate Emulsion 70 m² per 5 L (14 m²/L). Real rates are lower on porous, rough or patched surfaces and with strong colour changes, so many decorators use 10–12 m²/L for estimating. Two full coats is the normal system. Under BS EN 13300 the opacity class is declared at a stated spreading rate, so spreading paint thinner than that loses hiding. Enter 0 for any tin size your merchant does not stock. Ties on price go to the fewest litres, then fewest tins.
Common mistake
Using the “up to” rate from the tin on new or patchy surfaces, then running out on the second coat and getting a colour mismatch from a new batch.
Limits
Assumes paint is used at an even rate. Does not include mist coats, primer or sealer (separate calculators). Prices are yours to enter; check VAT.
Sources
- Dulux Trade Vinyl Matt product data sheet (up to 17 m²/L; 2 full coats; seal new or bare surfaces with up to 1 part water to 5 parts paint; packs 1, 2.5, 5 and 10 L)
- Johnstone’s Trade Covaplus Vinyl Matt (up to 17 m²/L depending on porosity; seal absorbent surfaces with a coat thinned up to 10% with water; 1, 2.5, 5, 10 L)
- Leyland Trade Vinyl Matt technical data (about 14 m²/L; seal absorbent surfaces with up to 1 part water to 9 parts paint; 1, 2.5, 5, 10 L)
- Farrow & Ball, How much paint do I need? (coverage per tin, e.g. Estate Emulsion 70 m² per 5 L; Masonry & Plaster Stabilising Primer 100 m² per 5 L; one primer & undercoat plus two topcoats)
- BS EN 13300 Paints and varnishes. Water-borne coating materials for interior walls and ceilings. Classification (wet scrub resistance, contrast ratio declared at a stated spreading rate)
- BS 6150:2019 Painting of buildings. Code of practice
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.