Paint calculator: litres, US gallons and whole cans of paint and primer for a room, from wall area minus doors and windows, coats and coverage.
36.9 m² (397 ft²) to paint × 2 coats needs 7.38 L (1.95 US gal), so buy 2 × 5 L cans.
Rounded up to whole 5 L cans.
Paint needed is the area to cover times the number of coats, divided by the coverage printed on the can. Wall area is the room's perimeter times the wall height, 2(length + width) × height, plus the ceiling if you include it, minus each door and window. The result is rounded up to whole cans of the size you pick.
The default 4.5 × 3.5 m room with 2.6 m walls, one door and two windows leaves 36.9 m² to paint. Two coats at 10 m² per litre need 7.38 L, which is two 5 L cans. For US paint, switch coverage to ft² per gallon; Sherwin-Williams puts a gallon at about 350–400 ft² per coat.
Label coverage assumes a smooth, sealed surface. Bare plaster, rough texture and strong colour changes use more paint, so treat the answer as the minimum to buy.
Checked against: Python decimal: 2(4.5 + 3.5) × 2.6 − 1.9 − 2 × 1.4 = 36.9 m²; × 2 ÷ 10 = 7.38 L; 7.38 ÷ 5 → 2 cans
Checked against: Hand calculation: 4 × 12 × 8 = 384 ft² − 20 − 30 = 334 ft²; 668 ÷ 350 = 1.9086 gal → 2 one-gallon cans
Checked against: Python decimal: 45 m² walls + 20 m² ceiling; 130 ÷ 10 = 13 L → 5.2 → 6 cans; primer 6.5 L → 2.6 → 3 cans
Checked against: Python decimal: 334 ft² ÷ 250 ft²/gal = 1.336 gal × 3.785411784 L = 5.0573 L → 2 one-gallon cans
About 2 US gallons for two coats on the walls. A 12 × 12 ft room with 8 ft walls has 384 ft² of wall; less one 20 ft² door and two 15 ft² windows, 334 ft² remains. Two coats is 668 ft², which at 350 ft² per gallon is 1.91 gallons. Painting the 144 ft² ceiling as well raises the total to 2.73 gallons.
About 350–400 ft² per coat, according to Sherwin-Williams, and a quart covers about 100 ft². In metric, 350–400 ft² per US gallon is 8.6–9.8 m² per litre. Use the figure printed on your own can, because coverage changes with the product, the colour and the surface.
Two for most walls. Dulux's guide gives one coat for its Simply Refresh one-coat range and two for EasyCare Washable & Tough. Every coat adds the full area again: the default 36.9 m² room needs 3.69 L per coat at 10 m² per litre, so a third coat adds another 3.69 L.
Yes. Dulux's method measures each wall as height × width and deducts doors and windows measured the same way. This calculator uses 1.9 m² (20 ft²) per door and 1.4 m² (15 ft²) per window unless you enter your own sizes under More options. In the default room the three openings remove 4.7 m², 11% of the 41.6 m² of wall.
One coat over the whole area, at the primer's own coverage rate. Sherwin-Williams gives about 200–300 ft² per gallon for primer against 350–400 ft² for paint, so 334 ft² of wall takes 1.1–1.7 gallons of primer. Enter the primer label's figure as Primer coverage under More options; left blank, the paint's coverage is used.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “4.5 × 3.5 m room, 1 door, 2 windows, 2 coats” is checked against Python decimal: 2(4.5 + 3.5) × 2.6 − 1.9 − 2 × 1.4 = 36.9 m²; × 2 ÷ 10 = 7.38 L; 7.38 ÷ 5 → 2 cans.
Dulux UK — how much paint do I need (area ÷ coverage × coats); Sherwin-Williams — paint calculator (paint 350–400 ft² per gallon per coat, primer 200–300 ft²).
5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
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