Paint Coverage Surface
Calculator

It divides the total surface area to be painted by the paint's coverage rate, typically stated in square feet per gallon (commonly 350-400 sq ft/gallon for one coat), to determine how many gallons are needed for the specified number of coats.

Coverage depends on the paint's solid content, viscosity, and the surface's porosity and texture — a smooth, previously painted wall needs less paint per square foot than raw drywall, brick, or a heavily textured surface, which absorbs more paint before achieving full coverage.

Roughly double is a reasonable estimate, but the second coat often goes slightly further than the first since it's covering a surface that's already partially sealed and colored by the first coat — so two coats typically need somewhat less than exactly twice the single-coat gallon amount, though budgeting for a full double is the safer approach.

For a precise calculation, yes — subtract the area of large openings like windows, doors, and any surfaces that won't be painted, though many painters skip this for small openings since the time saved isn't worth the complexity, effectively building in a small buffer.

Real-world coverage is affected by application method (rolling and brushing typically use more paint than spraying), surface texture, primer quality, and color changes requiring extra coats for full opacity (especially light-over-dark or dark-over-light transitions) — the stated coverage rate on a paint can assumes ideal, smooth-surface conditions.