Film thickness and spreading rate
Dry film thickness from wet film thickness and volume solids
- where
- dry film thickness (µm)
- wet film thickness (µm)
- volume solids (%)
- film lost as water or solvent (µm)
Explain this equation
Tells you what dry film a coat will leave from a wet film gauge reading. Use it on site to check you are laying paint on thick enough to meet a specification or the data sheet coverage.
WFT is the thickness of the paint as it goes on. VS is the volume solids: the share of the paint, by volume, that stays behind as the film once the water or solvent has gone. DFT is what is left. Lf is the part that evaporates.
Dividing VS by 100 turns the percentage into a fraction, so the dry film is simply that fraction of the wet film. Everything that is not solids leaves the film as it dries.
Sense check: a 49% volume solids emulsion applied at 100 µm wet dries to about 49 µm. Halve the volume solids and you need twice the wet film for the same dry film.
Watch out: use volume solids, not weight solids, and the value after thinning. On porous surfaces some paint soaks in, so less film stays on top.
Turns a wet film gauge reading into the dry film thickness you will be left with once the water or solvent has gone.
Enter your values to see the result.
Good to know
Volume solids is the share of the paint's volume left behind as dry film, measured to ISO 3233-1 and printed on the data sheet (for example 49% for Dulux Trade Diamond Matt white, 38% for Weathershield Smooth Masonry white). Take the wet film reading immediately after application, with the comb square to the surface. If you thinned the paint, use the volume solids after thinning (see the thinning calculator).
Common mistake
Using the solids by weight figure instead of volume solids. Weight solids are much higher because pigments are dense, and would overstate the dry film.
Limits
Assumes all the volatile part evaporates and the film does not shrink otherwise. On porous plaster, timber or masonry some paint soaks in, so the dry film left on the surface is less than this.
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.