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Sherwin-Williams: a gallon covers about 350–400 sq ft
2.09 gal
Buy 3 × 1 gal — 0.91 gal spare
Coverage is the whole answer, and it is not a constant: the same room needs 1.06 gal at the 17 m²/L a trade datasheet quotes for a smooth sealed wall, and 2.09 gal at the 350 sq ft per gallon Sherwin-Williams publishes. Use the figure on your own tin, and expect the low end on bare plaster, new filler or anything textured.
Not included: primer or undercoat, which is a separate calculation at its own coverage rate; woodwork, skirting, doors and frames, which are painted with a different product; and the paint left in the tray, the roller and the tin. Nothing here allows for a colour change that needs a third coat.
The first is arithmetic: walls are the perimeter times their height, a ceiling is length by width, and doors and windows come off. The second is how far a litre or a gallon goes, and that is not arithmetic at all — it is a property of the paint, the roller and the wall. This calculator does the first exactly and refuses to guess at the second, which is why coverage is a figure you choose rather than a constant hidden in the code.
Walls 8 ft high give a perimeter of 52 ft and 416 sq ft of wall. Take off one doorway at 20 sq ft and two windows at 15 sq ft each, and 366 sq ft is left to paint. Two coats makes 732 sq ft of covering to do, and at 350 sq ft per US gallon that is 2.09 gallons — three one-gallon tins, with 0.91 gallons over.
Adding the ceiling moves it more than people expect: 168 sq ft of ceiling takes the job to 3.05 gallons and a fourth tin. The same room in metric, 4.2 m by 3.6 m with 2.4 m walls, is 37.44 m² of wall; less 4.7 m² of openings and doubled for two coats it is 65.48 m², which at 12 m²/L is 5.46 litres, or three 2.5 litre tins.
Sherwin-Williams publishes about 350 to 400 square feet per gallon for paint and 200 to 300 for primer. Dulux Trade and Johnstone’s Trade datasheets quote up to 17 m² per litre for vinyl matt emulsion, depending on the porosity of the surface. That is no small difference: 17 m²/L is 693 sq ft per US gallon, nearly double the American figure.
Neither is wrong. The 17 is a spreading rate — what the paint does on a smooth, sealed wall with nothing absorbed. The 350 is a practical rate that already allows for what soaks in and what stays in the tray. The bedroom above needs 2.09 gallons at one figure and 1.06 at the other, so buying for the optimistic one sends you back to the shop. Each option here names its publisher, and Dulux’s own advice is to divide by the number printed on your tin.
Paper is cut into full-height strips, and whatever is left at the bottom of a roll is waste unless another whole drop fits — which is why an area calculation misleads. The bedroom has a 15.85 m run of wall; at 0.53 m wide, the European standard roll, that is 30 drops. Each drop is the 2.44 m height plus 0.1 m for trimming plus the pattern repeat, so with a 32 cm repeat it is 2.86 m: only three drops come off a 10.05 m roll and 1.47 m is offcut. Thirty drops at three per roll is ten rolls. The repeat is added to every drop because the match must be found again on each one, and American and wide-width rolls are different sizes, so both roll dimensions are inputs.
Excluded: primer, which has its own coverage rate and deserves its own run through the calculator; skirting, architrave, doors and frames, which take a different product; and the paint that never reaches the wall, in the tray, the sleeve and the tin. Nothing here knows that a dark colour going over white may want a third coat, or that fresh plaster drinks a mist coat first.
Two is the usual answer on a similar colour over a sound surface. Going lighter over a dark wall, or using a strong colour with weak pigment such as some reds and yellows, often takes three. One coat is realistic only when refreshing the same shade.
Yes for doorways and large windows — Dulux’s own instructions say to deduct them. Ignoring one doorway and two windows in the example above would add 50 sq ft, which is a fifth of a gallon you do not need to buy.
Almost always the coverage figure. A tin quoting 17 m²/L will not achieve it on bare plaster, filler, textured paper or a porous old wall, where the first coat is partly absorbed. Buy against the pessimistic end of the range for anything unpainted.
Because rolls are consumed in whole drops. If a 10.05 m roll yields three drops at your wall height, a few centimetres more takes it to two and ten rolls become fifteen. Measure the tallest point of the wall, and buy every roll in one batch — dye lots differ between print runs.