Paint guide

How to measure walls for paint

Reviewed October 6, 2026 · By the Room Calculators team · How we check facts

Wall paint starts with a simple measurement: add the wall lengths, then multiply by the height you will actually coat. The details that follow decide whether the estimate is useful.

Measure the perimeter

Measure each wall at floor level and add the lengths. For a rectangular room, double the length plus width. Record alcoves, bump-outs, and walls that will not receive paint separately instead of hiding them in waste.

Measure openings instead of guessing

Measure each door and window width and height, then add their areas. Large openings can change the result materially; a generic allowance is only a quick planning shortcut. Keep trim and the ceiling as separate surfaces when they use different products.

Handle the awkward walls

• Gable walls: add the triangle on top, half of the width × the extra height at the peak.
• Stairwells: split the wall into a rectangle and a triangle along the stair slope.
• Different heights: calculate each height group separately and add them.

Cross-check before buying

• Confirm the can’s coverage is stated per coat.
• Use a separate ceiling or trim calculation when those surfaces differ.
• Compare your assumption with the manufacturer guidance; Sherwin-Williams describes typical interior paint coverage as about 350–400 sq ft per gallon, with surface and application conditions affecting the result.

Reference: Sherwin-Williams paint calculator guidance