Paint Calculator
Estimate paint gallons or liters needed for room walls and ceilings. Subtracts doors and windows. Enter your paint's coverage from the label. Coats and primer included. Imperial and metric.
Paint Calculator
How to use the Paint Calculator
Measure the room
You need three numbers for a rectangular room: length, width and wall height, in feet or metres. Measure the wall height from floor to ceiling rather than assuming a standard height.
Count doors and windows
Each door is taken as 21 ft² (7 × 3 ft) or 1.8 m² (2.0 × 0.9 m), and each window as 15 ft² or 1.5 m². These are stated assumptions, not standards; a large sliding door or picture window counts as two or three, so adjust the counts to match.
Enter your paint's coverage and coats
Coverage is the area one gallon (or litre) covers in one coat, and it is printed on the can or the product's data sheet. The default, 350 ft²/gal (about 8.6 m²/L), is a cautious figure for a smooth wall; Nippon Paint's data sheets, for example, give a theoretical 13 m²/L for Matex Emulsion and 11 m²/L for Super Matex at a 30 µm dry film, which real walls do not reach. Rough or porous surfaces take more paint.
Add primer if you need it
Tick primer for new plaster or drywall, stains, or a large colour change. The primer estimate is one coat at the same coverage figure; if your primer's label gives a different coverage, work it out separately.
Paint quantities — area, coats and the coverage on the label
The calculation is simple: wall area is the room's perimeter times its height, less the doors and windows; add the ceiling if you are painting it; multiply by the number of coats; divide by the paint's coverage; add an allowance. On the sample room — 15 × 12 ft, 8 ft walls, one door and two windows — the walls come to 381 ft² and the ceiling to 180 ft². Two coats at 350 ft²/gal with a 10% allowance is 2.4 gallons (about 9.1 litres) for the walls alone and 3.5 gallons (13.3 litres) with the ceiling, which the page includes by default.
Coverage is the number to get right
The coverage figure changes the answer more than anything else, and it varies by product. Manufacturers publish it on the can and in technical data sheets, often as a theoretical figure at a stated dry-film thickness — Nippon Paint's Singapore data sheets give 13 m²/L for Matex Emulsion and 11 m²/L for Super Matex, both at 30 microns. A theoretical figure assumes no losses and a perfectly smooth, sealed surface, so the paint you actually use is more. That is why the coverage here is an input: enter your product's figure and, for a porous or textured surface, a lower one.
Read the coverage on your can before you buy; it moves the answer more than any measurement.
Coats and the allowance
Two coats is the usual plan for a colour change or new paint; a refresh in the same colour over sound paint may need one. The 10% allowance covers what stays in rollers, trays and the can, and touch-ups. Buy enough from one batch, because tinted paint mixed on different days can differ slightly in colour.
Coverage on the label, coats, and the arithmetic behind the gallons
Coverage is the area one gallon or litre covers in one coat; 350 ft²/gal is about 8.6 m²/L.
Nippon Paint's Matex Emulsion data sheet gives a theoretical 13 m²/L at 30 µm dry film; Super Matex 11 m²/L.
Theoretical coverage assumes no losses and a smooth, sealed surface, so real use is higher.
A door is taken as 21 ft² (1.8 m²) and a window as 15 ft² (1.5 m²) — assumptions you can override with the counts.
The sample 15 × 12 ft room has 381 ft² of wall after one door and two windows.
Two coats of the walls at 350 ft²/gal with a 10% allowance is 2.4 gallons; with the 180 ft² ceiling, 3.5.
Doubling the coats doubles the paint; switching to a 300 ft²/gal product adds about 17%.
Primer is estimated as one coat at the same coverage figure; use its own label if it differs.
Ceilings add length × width: 180 ft² for the sample room.
Tinted paint from different batches can vary slightly, so buy enough at once.
Frequently Asked Questions
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Because the rated figure is usually theoretical: it assumes a smooth, sealed surface, a stated film thickness and no losses. Porous plaster and new drywall absorb paint, textured surfaces have more area than they appear to, and some paint stays in rollers and trays. Enter a lower coverage than the label for rough or unsealed surfaces, and keep the allowance.
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For a colour change, new plaster or drywall, or a noticeably uneven finish, two coats is the usual plan. A refresh in the same colour over sound paint may need only one. Check the product's own recommendation.
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Common cases are new plaster or drywall, stains, glossy surfaces and large colour changes; the product's label says when it needs one. The estimate here is one coat of primer at the coverage you entered.
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Compare the prices at your store: larger containers often cost less per gallon, but only if you will use the paint. The page shows the job in gallons and litres so you can match it to the can sizes on sale.
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Split the room into rectangles. For an L-shape, run the calculator twice — once for each rectangle — and sum the gallons. For odd geometries with bay windows or alcoves, treat each alcove as its own mini-room. For tray or coffered ceilings, measure each face and add it to the area. For triangular gable walls, the height × width calculation overcounts — multiply the result by 0.5 since you're painting a triangle, not a rectangle.
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Only if you're painting the ceiling the same color as the walls. Most projects use a different color (typically flat white) for the ceiling vs an eggshell or satin on the walls — these are different paint products, not just different colors. Uncheck "Include ceiling" if you're calculating walls only and run the calc again with just the ceiling area (length × width) for the ceiling paint estimate. Flat ceiling paint costs ~30% less than wall paint and covers slightly less area per gallon (~300 ft²/gal).
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This calculator handles walls and ceilings only. Trim, doors and window frames are usually painted in a different, harder-wearing finish; estimate them separately from the trim paint's own coverage figure and the area of each surface.
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Products differ in coverage, hiding power and durability, and the data sheet is where to compare them. For this calculation, the difference that matters is the coverage figure: enter each product's to see how much of each you would need.
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No. All calculations run entirely in your browser via JavaScript. There's no server roundtrip — open DevTools → Network and confirm zero outbound requests as you change inputs. Room measurements stay on your device. Safe for confidential commercial estimates, multi-property dev calculations, or any room data that shouldn't leave your machine. Close the tab and nothing remains.
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Yes, with three adjustments: (1) Set surface to "Textured" — exterior siding (wood lap, vinyl, brick, stucco) is always textured and absorbs more paint than interior smooth walls. (2) Bump waste buffer to 15–20% for exterior work — wind, overspray, and weather make exterior pours messier. (3) Use exterior-grade paint (Behr Marquee Exterior, SW Duration, Nippon Weatherbond, Dulux Weathershield) — interior paint will not survive outdoor UV / rain cycling. The math is identical; just enter wall dimensions as if measuring exterior siding length × wall height.
Method & sources
How it computes
Wall area = 2(L + W) x H - doors x 21 ft2 (1.8 m2) - windows x 15 ft2 (1.5 m2); + ceiling L x W if chosen. Paint = coats x area / coverage x (1 + allowance); primer = one coat at the same coverage. Coverage is an input from the product label (default 350 ft2/gal = 8.59 m2/L). RecaBuild.paint, tested.
What this tool implements
- Coverage is now an input. The page applied fixed rates it called 'industry standard' (350 smooth, 275 textured, 300 primer ft2/gal) with no source; manufacturers publish coverage per product, often as a theoretical figure - Nippon Paint's Singapore data sheets give 13 m2/L for Matex Emulsion and 11 m2/L for Super Matex at 30 microns dry film - so the page tells readers to use their own label and defaults to a cautious 350 ft2/gal.
- Removed as unsourced or wrong: '15-25% real-world shortfall', '280-320 ft2/gal real coverage', 5-gallon pricing and the '20% saving', ASEAN brand and pricing claims, trim-paint quarts, and trivia including a 40,000-year-old Maltravieso paint date (the hand stencil there has been dated to over 64,000 years).
Sources
- Nippon Paint (Singapore). Matex Emulsion technical data sheet: theoretical coverage 13 m2/litre at 30 microns DFT. https://nipponpaint.com.sg/trade/wp-content/uploads/2017/03/Matex…
- Nippon Paint (Singapore). Super Matex technical data sheet: theoretical coverage 11 m2/litre at 30 microns DFT. https://nipponpaint.com.sg/trade/wp-content/uploads/2017/03/Super…
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