Film thickness and spreading rate
Theoretical and practical spreading rate from volume solids
- where
- theoretical spreading rate (m²/L)
- volume solids (%)
- dry film thickness per coat (µm)
- practical spreading rate (m²/L)
- loss and wastage allowance (%)
- wet film thickness at this dry film (µm)
Explain this equation
Shows how far a litre of paint should go at a given dry film thickness, first in theory and then after an allowance for real-world losses. Useful for comparing products and checking data sheet coverage claims.
VS is the share of the paint that ends up as film. DFT is how thick that film is. L is the percentage you expect to lose in tins, trays, rollers, uneven application and rough or porous surfaces. TSR is the perfect-world coverage and PSR the realistic one.
A litre spread 1000 µm thick covers exactly 1 m². The film you keep is VS per cent of the wet paint, so a litre covers VS × 10 / DFT square metres. The factor 10 is 1000 µm divided by 100 for the percentage. Losses then reduce that in proportion.
Sense check: 49% solids at 30 µm dry gives about 16 m²/L in theory, which matches the "up to 16 m²/L" printed for many trade matt emulsions. Double the film thickness and coverage halves.
Watch out: theory assumes a smooth, sealed surface. Absorbent new plaster or bare timber takes far more.
Shows how many square metres a litre should cover at a given dry film thickness, before and after an allowance for losses.
Enter your values to see the result.
Good to know
One litre spread 1 mm (1000 µm) thick covers 1 m², so at volume solids VS% and a dry film DFT µm the theoretical rate is 10 × VS / DFT m²/L (Hempel, Promain). Practical coverage is lower: paint left in tins, trays and rollers, uneven film build, rough or porous surfaces and overspray. Promain notes contractors usually allow 10–20%; 15% is used as the default. Manufacturers' printed coverage (for example "up to 16 m²/L") is a practical figure for a stated surface.
Common mistake
Pricing from the theoretical rate. It assumes a perfectly smooth, non-absorbent surface and no waste at all.
Limits
Theoretical rates cannot be given for absorbent surfaces that soak paint in (new plaster, bare timber, masonry). Use the data sheet coverage and a mist coat allowance for those. For quantities across rooms use the materials calculators.
Sources
- Hempel: Explanatory notes to product data sheets (theoretical spreading rate = volume solids % × 10 / DFT µm; WFT = DFT × 100 / volume solids; volume solids to ISO 3233-1; drying and overcoating times depend on temperature)
- Promain: Theoretical coverage rates (volume solids × 10 / DFT; worked example 28% VS at 18 µm = 15.55 m²/L; contractors allow 10–20% wastage)
- Dulux Trade Diamond Matt product data sheet (volume solids 49% white; minimum application temperature 8 °C per BS 6150; recoat 4–6 h; VOC max 1 g/L against 30 g/L limit, cat. A/a; thin up to 1 part water to 10 parts paint)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.