The calculator
Runs in your browser. µm, m² and litres, or mils, ft² and US gallons.
How the numbers follow
Volume solids is the percentage of the wet paint's volume that stays on the part as film. At 50 % volume solids, half of what you spray evaporates. So DFT = WFT × VS / 100, and the wet film to aim for is WFT = DFT × 100 / VS.
Coverage is the same volume spread thin. One litre at 100 % solids makes a film 1 µm thick over 1,000 m², so theoretical coverage is 10 × VS / DFT m²/L, DFT in µm. In US units one gallon is 231 in³, which over 1 mil (0.001 in) covers 1,604 ft²: coverage is 1604 × VS / 100 / DFT ft²/gal, DFT in mils.
Neither figure counts losses. Overspray, the profile of the surface and paint left in the cup can take a large share. Enter your shop's allowance as losses.
Worked example
A primer with 50 % volume solids, specified at 25 µm dry, on 20 m² of panels, 30 % losses.
- WFT = 25 × 100 / 50 = 50 µm wet.
- Coverage = 10 × 50 / 25 = 20 m²/L.
- Paint = 20 m² / 20 m²/L / (1 − 0.30) = 1.43 L mixed.
Spray to 50 µm wet on the notch gauge, and mix about 1.4 L.
Limits — when NOT to use this
- The data sheet governs the film thickness, the number of coats and the volume solids. Use the figure for the product as mixed and reduced.
- Volume solids, not weight solids. Weight solids is a different, usually higher, figure. It gives the wrong film thickness here.
- Not a measurement. Dry film thickness on aircraft parts is measured with the gauge the finish specification calls for. This predicts it.
Questions people actually ask
Does adding thinner change the dry film?
It lowers the volume solids of the mix, so the same wet film dries thinner. Recalculate with the reduced product's volume solids: a 10 % reduction of a 50 % product takes it to about 45 %.
What is a mil?
One thousandth of an inch, 25.4 µm. Not a millimetre.
Sources
- Definition of volume solids, and the geometry above — DFT = WFT × VS; coverage from the volume of one litre (1,000 cm³) or one US gallon (231 in³, exact) spread at the dry film thickness. No figure on this page comes from elsewhere: volume solids, film thickness and losses are inputs from your product's data sheet and your shop.