Paint coverage calculator

Working out paint is one multiplication and one division, and the division is where every estimate goes wrong. Wall area times coats, divided by how far a litre goes — but how far a litre goes is not a property of paint, it is a claim a particular manufacturer makes about a particular product on a particular surface. Dulux prints "up to 17m² per litre" for Trade Vinyl Matt and "up to 12m² per litre" for Diamond Satinwood. Sherwin-Williams prints 350 to 400 square feet per gallon for ProMar 200 Eg-Shel, which is between 8.6 and 9.8 m² per litre — half the Dulux figure, on a product doing much the same job. Neither is wrong; they are measuring different paints, on different substrates, in different trade cultures. So the spreading rate here is an input with the published figures attached to the names that published them, and the number on your own tin beats all of them.

How it is calculated

litres = ((wall width × height × walls) − openings) × coats ÷ spreading rate

The area of the walls comes off the tape; the doors and windows come back off it because nobody paints a window. Multiply by coats — Dulux calls two full coats "the normal finishing process", and one coat only where you are repainting a similar colour over sound work. Then divide by the spreading rate in square metres per litre. If your tin is American and quotes square feet per gallon, this tool converts it for you: a square foot is exactly 0.09290304 m² and a US gallon is 3.785412 L, so 350 sq ft/gal is 8.59 m²/L. Round the answer up to whole tins, and buy the whole job at once — a second batch mixed later will not match.

Source: A wall is a rectangle, so its area is width times height; paint needed is that area times the number of coats divided by the spreading rate the tin publishes

Questions people ask

Why do the published coverage figures differ by so much?

Because they describe different things and are measured differently. A UK trade emulsion is quoted as "practical coverage which can be achieved under normal conditions", on a sound, previously painted plaster wall, and the phrase "up to" is doing real work. A US data sheet quotes a spreading rate at a stated wet film thickness — Sherwin-Williams ties its 350 to 400 sq ft per gallon to "4 mils wet; 1.7 mils dry" — which is a specification for how thickly you are meant to apply it, not a promise about how far the tin goes. A bare, porous or heavily textured surface will take noticeably more than either.

Which figure should I actually use?

The one on the tin you are buying, which is why that is an option here. Failing that, take the lower end of any published range rather than the upper: the difference between 350 and 400 square feet per gallon is fourteen percent, and running out three quarters of the way along a wall costs more than a spare litre does. The presets in this tool keep the manufacturers apart on purpose instead of averaging them into one confident-looking number that belongs to nobody.

Should I really subtract the doors and windows?

For anything sizeable, yes — a pair of French doors is several square metres and paint is priced by the litre. For a single internal door it barely moves the answer, and the offcut is useful slack against the cutting-in you will do around it. This tool asks for the total opening area rather than a count of doors because there is no standard door: sizes vary by country, by age of building and by whether it is a bathroom or a garage, and a number invented here would be worse than the two minutes it takes you to measure.

How many coats?

Two, unless you know otherwise. Dulux Trade Vinyl Matt puts it plainly: "The normal finishing process is 2 full coats of Dulux Trade Vinyl Matt, but on previously painted surfaces in good condition where similar colours are used, 1 coat may be sufficient." New or bare plaster wants a thinned mist coat first, which the same sheet describes as up to one part water to five parts paint, and that coat soaks in — count it as a coat here and expect it to go further than the figure suggests.

Does it cover ceilings and woodwork?

Only if you enter them as walls. A ceiling is a rectangle like any other, so add it as one more "wall" of the right size and it goes into the same sum. Woodwork is a different paint at a different rate — the satinwood preset here is 12 m² per litre against 17 for the wall emulsion — so run skirting, architrave and doors as a separate job with that rate rather than folding them into the wall total.

Where do the spreading rates on this page come from?

Four manufacturer data sheets, all readable in full. Dulux Trade Vinyl Matt, sheet 411 issued 08/25, is at https://msp.images.akzonobel.com/prd/dh/eukddc/documents/dt_411__vinyl_matt.pdf; Diamond Eggshell, sheet 408 issued 04/21, at https://msp.images.akzonobel.com/prd/dh/eiedlx/documents/dt_408__diamond_eggshell.pdf; Diamond Satinwood, sheet 1006 issued 08/25, at https://msp.images.akzonobel.com/prd/dh/eukddc/documents/dt_1006__diamond_satinwood.pdf. The Sherwin-Williams figure is from product data sheet 101.83A for ProMar 200 Zero V.O.C. Interior Latex Eg-Shel, at https://www.paintdocs.com/docs/webPDF.jsp?SITEID=ECOMEDES&lang=2&doctype=PDS&prodno=B20W12651. The square foot and gallon conversions are NIST SP 811 Appendix B.8. No figure here is an average of the four, because an average would describe a paint nobody sells.

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