Paint Coverage Calculator
Calculate how much paint you need based on wall area, number of coats, and the paint's coverage rate.
How this is calculated
Wall area comes from the room's perimeter times its height, minus a standard allowance for doors (1.8m²) and windows (1.4m²). Multiplying by your number of coats and dividing by the standard 12m² per litre coverage rate gives the litres needed, which is then rounded up to a practical combination of standard tin sizes.
The 12m²-per-litre figure used here is a reasonable average for emulsion (matt wall paint), but different paint types spread differently — eggshell and satin finishes typically cover slightly less per litre than flat emulsion, and gloss (often used for trim and doors rather than whole walls) covers less still due to its thicker, more reflective finish. Painting a bare, unprimed surface for the first time also significantly reduces effective coverage, since raw plaster or drywall absorbs much more paint than a primed or previously painted wall — budget for extra paint, or a dedicated primer coat, when working on new or bare surfaces. For other room-renovation estimates, see the Flooring / Tiling Calculator and the Concrete Volume Calculator.
Frequently asked questions
- How is paintable area calculated?
- Wall area is the room perimeter times its height (2 × (length + width) × height), then 1.8m² is subtracted for each door and 1.4m² for each window — typical average sizes for a standard door and window.
- Why 12m² per litre?
- A commonly cited standard coverage rate for emulsion paint on average wall surfaces — actual coverage varies by paint brand, wall texture and colour change, so check your specific tin's label for its exact rate.
- How are tin sizes chosen?
- The calculator buys as many of the largest standard tin (10L) as fully fit, then covers whatever's left with the smallest remaining standard size (2.5L, 5L, or 1L) that's big enough — avoiding an impractical purchase of several small tins when one larger one would do.
- Can I use this in my own app?
- Yes — every calculator on Stupidly Clever has a matching REST API and MCP tool that runs the same underlying logic.
- What is the difference between spread rate and coverage?
- Spread rate (m² per litre) is the manufacturer's stated figure for how far a paint theoretically goes under ideal conditions. Actual coverage is what you achieve in practice, which is usually somewhat lower due to wall texture, absorbency, application method, and colour change — treat the spread rate as a best-case reference, not a guarantee.
- How many coats do I need?
- Two coats is standard for most colour changes and gives even, opaque coverage. A single coat may suffice for touch-ups or when painting a very similar colour over the existing one; three or more coats are often needed for a dramatic colour change (e.g. dark over light) or when using a lower-opacity paint.
- Does surface texture affect coverage?
- Yes — textured or porous surfaces (like unpainted plaster, brick, or heavily textured walls) absorb more paint and reduce effective coverage compared to a smooth, previously painted wall. Expect to use noticeably more paint per square metre on rough or unprimed surfaces.