Paint quantity depends on four separate values: net area, number of coats, product coverage and packaging. The Paint calculator keeps these steps visible.
Calculate wall area
For a rectangular room, wall area is 2 × (length + width) × height. A 4.5 × 3.5 m room with 2.5 m walls has 40 m² of wall surface. Subtract a 1.8 m² door and two 1.5 m² windows and the net area becomes 35.2 m².
Two coats mean 70.4 m² to cover. At 10 m²/L, that is 7.04 L. With an 8% allowance the requirement becomes about 7.6 L.
Use the manufacturer’s coverage
Coverage printed on the can is much more useful than a generic assumption. A porous new wall, repaired surface or major colour change may consume more paint. Primer can also change the amount of finish required.
Do not treat an allowance as an extra coat: they solve different problems.
Convert litres into cans
Paint is sold in fixed sizes. A theoretical 7.6 L may require a combination of 5 L, 2.5 L and 1 L cans. The calculator can use the sizes actually available and show the resulting surplus.
Large windows should usually be deducted. For tiny openings, extreme precision is less useful because roller, tray and touch-up losses can be of the same order.
Common mistakes
Do not confuse floor area with wall area. Do not reuse the coverage value from another paint. And do not assume one coat when the chosen system requires two.
For wallpaper, use the Wallpaper calculator, which works with drops and pattern repeats rather than litres. For a new partition, first estimate boards and framing with the Drywall calculator.
A reliable paint estimate is therefore a purchasing workflow: measure, subtract major openings, multiply by coats, divide by real coverage, add a small allowance and finally convert the result into available can sizes.
