Paint quantity: subtract openings, then count the coats

Method

Paint quantity starts with the surface being coated, not the room’s floor area. For straight walls of equal height, multiply the room perimeter by that height. Measure other surfaces separately, and subtract doors or windows only if you will not paint them. Use metres throughout so the resulting area is in square metres.

Multiply the net paintable area by the number of coats, then divide by the product’s stated coverage in square metres per litre per coat. Apply any chosen extra allowance once to that volume. A coverage figure for a complete two-coat system is different from a per-coat figure; follow the label rather than multiplying the coats twice.

Worked example

Consider a rectangular room 4 m by 3 m with walls 2.5 m high. Its perimeter is 14 m, so gross wall area is 35 m². Unpainted openings total 3 m², leaving 32 m². Two coats at an illustrative coverage of 8 m²/L need 8 L before allowance. A chosen 12.5% extra gives 9 L. Neither the coverage nor the allowance is a recommendation for a particular paint.

In Paint quantity, enter paintable area 32, coats 2, coverage 8 and extra allowance 12.5. The result is 9 L. Measure a ceiling separately if it needs another product or a different number of coats. Round the final purchase to available container sizes; do not round every wall measurement upward first and then unknowingly add the same reserve again.

⁦2 × (4 + 3) × 2.5 = 35 m²⁩

⁦35 − 3 = 32 m²⁩

⁦32 × 2 ÷ 8 = 8 L⁩

⁦8 × 1.125 = 9 L⁩

Checks and limits

Actual coverage changes with the surface, preparation, application and product. The calculation estimates quantity, not finish quality, drying time or product suitability. Check the chosen product’s instructions and coverage basis. Primer and finishing paint may need separate estimates, and an irregular room needs individual surface measurements rather than this rectangular-room shortcut.

Related calculators

• Paint quantity
• Area converter

Source: Dulux