Paint Spread Rate Calculator

How it works

A gallon of liquid is 231 cubic inches. Spread as a film one mil (0.001 in) thick, it covers 231,000 square inches, or 1,604 sq ft. Only the solids stay on the wall once the water or solvent evaporates, so a coating with 35% volume solids leaves a dry film 35% as thick as the wet film it went on as.

The theoretical spread rate is therefore 1,604 × volume solids ÷ dry film thickness. Thicker films or lower solids mean less area per gallon. The wet film thickness, which is what a wet-film gauge reads right after application, is the dry film thickness divided by the volume solids.

Theoretical coverage assumes every drop lands on the surface as a perfectly even film. In practice some paint stays in the roller, tray and bucket, overspray drifts away, and rough or porous surfaces take more. The material loss percentage reduces the theoretical figure to a practical one, and the gallons for your area and coats come from that.

Volume solids and the recommended dry and wet film thickness are printed on a coating's technical data sheet. For example, Benjamin Moore's Advance Waterborne Interior Alkyd Semi-Gloss 793 sheet (793 US 041023, 2023) lists 35.2% volume solids, a dry film of 1.1–1.4 mils, a wet film of 3.2–4.0 mils and a spread rate of 400–500 sq ft per gallon, which is what this formula gives.

Formula

1,604 = 231 cu in per gal ÷ 0.001 in ÷ 144 sq in per sq ft   (1,604.17 to two decimals)
theoretical (sq ft/gal) = 1,604.17 × volume solids ÷ DFT (mils)
theoretical (m²/L)      = theoretical sq ft/gal converted to m² per litre
WFT (mils)              = DFT ÷ volume solids
practical (sq ft/gal)   = theoretical × (1 − loss)
gallons                 = area × coats ÷ practical

Example

Benjamin Moore Advance 793 has 35.2% volume solids. At its 1.4-mil dry film the theoretical spread rate is 1,604.17 × 0.352 ÷ 1.4 = 403.3 sq ft per gallon, and the wet film is 1.4 ÷ 0.352 = 3.98 mils. The data sheet's figures are 400 sq ft and 4.0 mils.

With 15% material loss, practical coverage is 403.3 × 0.85 = 342.8 sq ft per gallon. Two coats on 400 sq ft is 800 sq ft, or 2.33 gallons: 3 whole gallons.

Assumptions and limitations

• The film is taken as perfectly uniform. Real application varies in thickness, and data sheets give a range of film thickness for that reason.
• Material loss is an editable estimate, not a measured value. Rust-Oleum's data sheets state that their practical coverage assumes 15% loss; spraying, wind, rough surfaces and open shapes such as railings lose more.
• Volume solids is the figure from the data sheet for the product as supplied. Thinning lowers the solids of what goes on the surface and raises the wet film needed for the same dry film.
• Porous surfaces absorb part of the first coat, so the first coat on bare material covers less than the formula shows.
• The result is an estimate. It is not a substitute for the coating manufacturer's data sheet or application instructions, which set the film thickness a coating system needs.

Frequently asked questions