Paint Calculator (gallons or liters from room size)
Enter room length, width, height, plus number of coats and openings. Get the paint needed in liters (metric) or gallons (US imperial) using realistic coverage rates.
- Wall area
- 0 ft²
- Total coverage (with coats)
- 0 ft²
How it works
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.
Frequently asked questions
›Is the coverage rate realistic?
Yes — 10 m²/L (350 sqft/gal) is the standard for one coat of interior latex on smooth drywall. Rough surfaces, primers, and glossy paints cover less; check the can.
›Should I include the ceiling?
Not in this calculator. Add length × width × coats / coverage as a separate calculation.
›Why does it subtract 1.7 m² per opening?
That's a typical interior door (~2 m²) or window (~1.5 m²) average. If your room has unusually large openings, multiply the count by an average that fits your room.
›Does this account for trim?
No. Trim work uses separate trim paint. Add roughly 10-20% to your wall paint estimate if you don't want to buy a separate trim can.
›How many coats should I plan?
Two for most projects. One only if you're touching up the same color. Three suggests using a primer first.
›Can I paint metric L over imperial gal cans?
Yes — but cans are sold in fixed sizes (1 gal, 1 quart, 5 gal in US; 1 L, 2.5 L, 5 L in EU). Round up to the nearest can size you can buy.
›Why not include the ceiling automatically?
Many people only paint walls (different paint, different finish for ceiling). The calculator stays focused on walls; do the ceiling separately if needed.
›Is the data sent anywhere?
No. Calculation runs locally.
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