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How the estimate works
The calculator uses the room perimeter for wall area, subtracts openings, adds the ceiling when selected, then applies coats, coverage and waste.
Formula
Paint volume = paintable area × coats ÷ product coverage × (1 + waste %). Containers are always rounded up.
Worked example
A 4 × 3 × 2.7 m room with one door, one window and its ceiling has 46.68 m² to paint. Two coats at 10 m²/L plus 10% waste need 10.27 L, so you would buy three 5 L containers.
Common mistakes
- Using floor area as wall area.
- Ignoring a second coat or porous new plaster.
- Using a generic coverage instead of the product label.
- Buying the exact decimal result without rounding to full containers.
FAQ
Paint calculator FAQ
Should I subtract every door and window?
Usually yes. For very small openings, painters sometimes leave them in as a practical allowance for touch-ups.
How much waste should I add?
Ten percent is a useful starting point. Textured walls, strong color changes and difficult rooms can need more.
Is primer always required?
No. It is commonly useful on new, repaired, stained or very porous surfaces. Follow the coating manufacturer’s system.
This is a planning estimate. Surface condition, application method and the actual product can change consumption. Check the product label before buying.
About this material
Interior paintFinishes, coverage rates and the two numbers on the tin that decide how much you buy.