Results update as you type. Round up when ordering.
Based on about 350 sq ft per gallon, typical for interior latex paint (e.g. Behr, Sherwin-Williams). Check the coverage on your can.
Formula checked against manufacturer specs
Rounded up to whole units
Runs in your browser
How the paint estimate works
Wall area = room perimeter × wall height. Subtract about 20 sq ft per standard door and 15 sq ft per average window. Multiply by the number of coats and divide by the coverage on the can—most interior latex paints state 350–400 sq ft per gallon on smooth surfaces.
Example: a 12 × 12 ft bedroom has a 48 ft perimeter. At 8 ft high: 384 sq ft − 20 (door) − 30 (two windows) = 334 sq ft. Two coats = 668 sq ft ÷ 350 ≈ 1.9 → buy 2 gallons.
When to lower the coverage number
Bare drywall or fresh plaster: use primer first, or assume ~250–300 sq ft/gal.
Textured or rough walls absorb more paint.
Big color changes (dark to light) often need a third coat.
Ceilings and trim
Ceiling area is simply length × width. Trim usually needs well under a quart for a single room.
It uses the standard formula shown on this page and rounds bags, boxes and pieces up to whole units. Product sizes vary by brand, so confirm with the label or your supplier before you buy.
Should I buy extra material?
Yes. A 5–10% allowance covers cuts, spills and uneven surfaces; running short mid-job usually costs more than a few leftovers.
Is this calculator free?
Yes. Every PorchMath calculator is free, needs no account and runs entirely in your browser.