Paint Calculator (gallons or liters from room size)

How the math works

Wall area = perimeter × ceiling height. Perimeter for a rectangular room is 2 × (length + width). For a 4 m × 3 m × 2.5 m room: perimeter is 14 m, wall area is 35 m². Subtract approximately 1.7 m² per opening (a typical door or window). Multiply by the number of coats. Divide by 10 m² per liter (typical interior latex one-coat coverage) to get liters needed.

US figures use 350 sq ft per gallon at one coat, which is the industry standard for flat to eggshell interior latex paint. Glossier finishes and primers cover slightly less; high-coverage one-coat formulas cover slightly more. Manufacturers print coverage on the can — adjust if your paint differs significantly.

Why two coats are usually correct

Two coats give consistent color and uniform sheen, especially when going from a darker to lighter color or repainting after years. Single-coat 'one and done' products exist but require careful preparation and the right surface; for typical drywall in a typical home, two coats is the safe default.

Three coats is rarely needed and usually means a primer would have been better. If you're going over deep red, navy, or stained surfaces, use a primer first instead of stacking color coats.

What this estimate does not include

Ceiling: this calculator covers walls only. To paint the ceiling, calculate length × width and add it as additional area at the same coverage rate.

Trim and detail work: doors, baseboards, and window frames need separate trim paint. Add 10-20% for trim depending on detail.

Texture and porosity: rough textured walls or unprimed drywall absorb more paint. Add 15-25% to be safe.

Always buy slightly more than you need — running out mid-job means a second can-mix that may show as a stripe.