This finish coverage calculator estimates how much paint, stain, oil, or clear finish you need for a given area, and how the wet film you apply relates to the dry film left behind. It helps you buy the right quantity and apply the right number of coats without guesswork.
Coverage, coats and film thickness
Coverage is the area one unit of finish will cover at the recommended film thickness. The calculator divides the surface area by the product's spread rate and multiplies by the number of coats to give the total volume required. For coatings specified by film build, it relates wet film thickness (WFT) — what you lay down — to dry film thickness (DFT) — what remains after the solvent or water evaporates — using the product's solids content.
Because much of a coating can be solvent that flashes off, the dry film is always thinner than the wet film. Knowing the solids percentage lets you apply the correct wet thickness to hit the specified dry thickness, which matters for durability and warranty on protective coatings.
Real-world coverage
Porous, rough, or thirsty surfaces drink the first coat and reduce coverage, so a primer or a generous first coat is normal. Application method matters too: spraying loses material to overspray, while brushing and rolling are more efficient. Treat the calculated figure as the theoretical coverage and round up for the first coat on bare timber.
You are finishing 20 m² of door fronts with a finish that covers 12 m² per litre, two coats.
- One coat = 20 ÷ 12 = 1.67 litres.
- Two coats = 1.67 × 2 = 3.34 litres.
Buy about 3.5 litres to apply two coats over 20 m² with a little in hand.
