Primer Coverage Calculator — How Much Primer Do I Need?

Our primer coverage calculator divides your total area by the coverage rate for your chosen surface type, then multiplies by the number of coats. Coverage rates vary significantly between surface types because porous surfaces like new plaster absorb far more primer than smooth, sealed surfaces.

The formula

Litres needed = (Total area × Number of coats) ÷ Coverage rate per litre × Wastage factor

For example, priming 40 m² of new plaster (8 m²/L) with 1 coat: 40 ÷ 8 = 5.0 litres, plus 10% wastage = 5.5 litres. You'd need 3 × 2.5L tins.

Coverage rates by surface type

• New Plaster: 8 m²/L — high absorption, the plaster soaks up primer rapidly
• Bare Wood: 10 m²/L — varies by wood type, softwood absorbs more than hardwood
• Previously Painted: 12 m²/L — sealed surface requires less primer
• Metal: 14 m²/L — smooth, non-porous surface; metal primers vary from roughly 10 to 18 m²/L by product

These are typical, conservative UK trade coverage figures, consistent with the datasheet ranges published by Dulux, Zinsser and Crown rather than a single named product. Actual coverage depends on surface condition, porosity, dilution and application method, so always check the figure on your primer's tin and use the Advanced Options above to enter the exact coverage if you know it. We recommend 10% wastage for brush/roller application.