Paint Calculator — How Much Paint Do I Need?
Estimate how many gallons of paint your room needs in seconds. Enter the dimensions, the openings, and your coverage rate, and we’ll show you the exact gallons to buy plus a 10% waste buffer.
Industry standard is 10%. Increase for complex cuts, patterns, or rough surfaces.
Results are estimates based on the dimensions and specifications you provide. Actual material needs may vary based on project conditions, material waste, and installation method. Always consult with a professional for complex projects.
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How this calculator works
We compute the paintable wall area as 2 × (length + width) × height, subtract 21 sq ft per door and 15 sq ft per window, multiply by the number of coats, and divide by your paint’s coverage rate (350 sq ft per gallon for most modern interior latex paints). A configurable waste factor accounts for spills, edging, and second-coat touch-ups.
More about this calculator
Buying the right amount of paint matters: too little means a second trip and the risk of mismatched dye lots, while too much means leftover cans you’ll never finish. Most interior latex paints cover roughly 350 sq ft per gallon on smooth, primed drywall — but rough or porous surfaces can drop coverage to 250 sq ft, and dark-to-light color changes typically need three coats instead of two.
For accurate results, measure each wall independently if your room isn’t a simple rectangle, and don’t forget the ceiling if you’re painting it (a flat ceiling area equals length × width). Subtract any large openings — standard 30×80 inch doors are about 21 sq ft and standard double-hung windows about 15 sq ft. Add a 10% waste buffer for cut-in, drips, and the can you keep around for touch-ups.
Frequently asked questions
An average 12×12 ft bedroom with 8-foot ceilings, one door, and one window needs about 1.5 gallons for two coats — buy two gallons to allow for waste and touch-ups.
Yes if you’re going from dark to light, painting bare drywall, or covering stains. Self-priming paint can skip a separate primer coat on previously painted walls in good condition.
10% is the standard waste factor for paint, accounting for cut-in, brush loading, and a small reserve for touch-ups. Increase to 15% for textured or rough walls.
Yes. Add the ceiling area (length × width) to your wall area before entering, and skip the door and window deductions for the ceiling portion.