Paint Coverage Calculator

How paint coverage is calculated

Paint coverage is the area one gallon covers in a single coat. The number on the label, usually 350 to 400 sq ft per gallon for interior latex, is measured on smooth, sealed surfaces. Real walls often take more paint, so this calculator multiplies label coverage by a surface factor to get effective coverage.

To find gallons, the formula is area × coats ÷ effective coverage. To go the other way, gallons × effective coverage ÷ coats tells you how much area the paint on hand will cover. For example, 400 sq ft with two coats at 350 sq ft per gallon needs about 2.3 gallons, so buy three.

The surface factors are rules of thumb: light texture adds roughly 10 percent more surface and paint, bare drywall and bare wood soak up extra paint on the first coat, heavy textures and rough-sawn wood can take 25 percent more, and porous masonry or stucco often gets only 200 to 250 sq ft per gallon. Manufacturer data sheets for masonry and elastomeric coatings list lower coverage for exactly this reason.

If you are painting bare drywall, masonry or a stained surface, a coat of primer first usually lets the finish coats reach close to their label coverage, which can save paint overall.

Where these numbers come from

The formulas and default values in this calculator are based on:

• Sherwin-Williams and Benjamin Moore product data sheets (typical 350–400 sq ft per gallon on smooth surfaces, lower spread rates listed for rough and porous surfaces)
• surface adjustments are common trade estimates

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.