Estimate how much paint a room needs from its dimensions, the area of doors and windows, the coverage on the tin and the number of coats.
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The four walls of a 5 × 4 room with 2.5-high walls total 45 square units, and subtracting 4 for a door and window leaves 41 to paint. With coverage of 10 per unit of paint, two coats need 8.2 units and one coat needs 4.1.
Wall area is 2 × (length + width) × height. The area you enter for doors and windows is subtracted, and the paintable area is then multiplied by the number of coats and divided by the coverage figure to give the quantity of paint.
Ceilings and trim are not included, so calculate those separately. Coverage on a label assumes a smooth, sealed surface and the stated application method, and rough or porous walls, bare plaster, strong colour changes, spills and touch-ups all increase real usage, so round up to the tin sizes actually sold.
Enter the room length, width and wall height in one unit, the combined area of doors and windows, the coverage printed on the paint and the number of coats. The four walls of a 5 × 4 room with 2.5-high walls total 2 × (5 + 4) × 2.5 = 45 square units. Subtracting 4 for a door and window leaves 41 to paint. Ceilings and trim are not included, so calculate those separately if you are painting them too.
Quantity is paintable area × coats ÷ coverage. With coverage of 10 square units per unit of paint, two coats on 41 need 8.2 units; one coat needs 4.1. If you leave doors and windows in, the estimate rises to 9 units, which is a reasonable cautious figure. A 4 × 3 m room with 2.4 m walls, 3.5 m² of openings and paint covering 12 m² per liter needs about 5.02 liters for two coats. In feet, a 12 × 10 ft room with 8 ft walls, 40 ft² of openings and 350 ft² per gallon needs about 1.78 gallons.
Round up to the can sizes sold, and allow extra for rough or porous surfaces, bare plaster, strong colour changes and touch-ups; a primer or an extra coat may be needed when covering a dark colour with a light one. Coverage on the label assumes a smooth, sealed surface and the stated application method, so rollers on textured walls often achieve less. Check that the coverage unit matches your measurements, square meters per liter or square feet per gallon, before calculating.
| Input | Result |
|---|---|
5 × 4 × 2.5 − 4 · 10 · ×2 | 8.2 (41 / 45) |
5 × 4 × 2.5 − 0 · 10 · ×2 | 9 (45 / 45) |
5 × 4 × 2.5 − 4 · 10 · ×1 | 4.1 (41 / 45) |
4 × 3 × 2.4 − 3.5 · 12 · ×2 | 5.02 (30.1 / 33.6) |
12 × 10 × 8 − 40 · 350 · ×2 | 1.78 (312 / 352) |
One consistent length unit for the room, and a coverage figure in the matching square unit. A 4 × 3 m room with 2.4 m walls, 3.5 m² of openings and paint covering 12 m² per litre needs about 5.02 litres for two coats. In feet, a 12 × 10 ft room with 8 ft walls, 40 ft² of openings and 350 ft² per gallon needs about 1.78 gallons.
Enter their combined area if you want a close estimate. Leaving them at 0 in the same 5 × 4 room raises the paintable area from 41 to 45 and the two-coat quantity from 8.2 to 9 units, which is a reasonable cautious figure if you would rather buy slightly too much than too little.
Textured walls hold more paint than the label assumes, bare plaster and filler absorb the first coat, and covering a dark colour with a light one often needs a primer or an extra coat. Application matters too: rollers and sprayers waste more than a brush on trim, so keep a small reserve for touch-ups.