Paint Calculator
Calculate how many gallons of paint you need from wall size, doors, windows, coats, and coverage. Shows paintable area, liters, cans to buy, and optional cost.
How to use this paint calculator
A paint calculator answers “how many gallons of paint do I need?” before you stand in the paint aisle. Measure each wall’s length and height, count full-size doors and windows, and pick one or two coats. Most interior latex covers about 350–400 sq ft per gallon; primer and dark colors cover less.
US gallons are what big-box shelves sell. This tool also shows liters so a metric can or a Canadian label still makes sense. Buy the extra gallon if you are cutting in a bold color or the walls are textured — leftover paint matches touch-ups years later. Fractions such as 10 1/2 are allowed.
Formula
Worked example
Four 12×8 ft walls, two doors, two windows, two coats at 350 sq ft/gal: 384 − 70 = 314 sq ft × 2 ÷ 350 ≈ 1.79 gallons. Buy 2 gallons (~7.6 L). At $42/gal that is $84.
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Frequently asked questions
How much paint do I need for a 12×12 room?
Four 12×8 walls are 384 sq ft. Subtract two doors and two windows (~70 sq ft) and two coats at 350 sq ft/gal is about 1.8 gallons — buy 2.
How many square feet does a gallon of paint cover?
A typical interior gallon is labeled 350–400 sq ft. Primer, masonry, and deep colors often cover 200–300.
Do I count doors and windows?
Yes if they are not being painted the same color. This calculator deducts 20 sq ft per door and 15 sq ft per average window.
One coat or two?
Two coats is the usual finish coat on new drywall or a color change. One coat can work over a similar color with good primer.
Gallons or liters?
US shoppers buy gallons. 1 gallon ≈ 3.785 liters. Switch Units to metric to enter meters and coverage in m²/L.
Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-19.