Paint Calculator (Gallons or Liters)
Calculate how much paint you need for a room. Accounts for wall area, coats, doors, windows, and typical 350 sq ft/gallon coverage.
What this calculates
Paint waste is expensive — running short halfway through is worse. This calculator estimates the gallons (or liters) you need based on wall area, number of coats, and any doors and windows you'll cut around. Defaults assume standard 350 sq ft per gallon coverage; rough surfaces like brick or stucco can drop that to 200.
Formula & how it works
Wall area = perimeter × ceiling height. Subtract roughly 21 sq ft per door and 15 sq ft per window. Multiply by number of coats. Gallons = total paintable area ÷ coverage per gallon (default 350 sq ft/gal). Round up to the next whole quart or gallon — paint cans only come in fixed sizes and a fresh quart is cheap insurance against running short.
Worked example
12 × 14 ft room with 9 ft ceilings, 2 doors and 3 windows, painting 2 coats. Perimeter = 2 × (12 + 14) = 52 ft. Wall area = 52 × 9 = 468 sq ft. Subtract 2 × 21 + 3 × 15 = 87 sq ft → 381 sq ft. Times 2 coats = 762 sq ft. Gallons = 762 ÷ 350 ≈ 2.18 → round up to 2.5 gallons (or buy 3 single-gallon cans for color consistency).
Frequently asked questions
Do I need a primer coat?
Yes if you're covering bright color with light, painting over stains, or painting fresh drywall. A primer counts as one coat — you'll typically still want 2 finish coats on top. Skip primer only if you're refreshing the same color over an already-good surface.
How much does ceiling, trim, and door paint add?
Ceiling paint covers more thinly. For a typical 12×14 room ceiling that's about ½ gallon for one coat. Trim paint is separate — usually a quart covers a typical room's trim. Door/window paint: a quart of trim paint covers ~3 doors.
What's the difference between flat, eggshell, and semi-gloss?
Flat hides imperfections best but stains easily — good for ceilings. Eggshell or matte is the modern living-room standard. Satin/semi-gloss is washable, used for kitchens, bathrooms, trim, and doors. All cover at similar square-feet-per-gallon.
Should I buy more paint than the calculator says?
Add 10 % for touch-ups over the years. Paint stored in a sealed can in temperate conditions lasts 5–10 years. Color-matching later is unreliable because formulas shift slightly — having the original can is the only way to be sure.