Paint Calculator
Enter your room dimensions to get the gallons of paint needed, with doors, windows, and coats accounted for.
Updated October 3, 2026 · Estimates for planning, not a quote
How to estimate paint
Wall area is the room's perimeter times the wall height: 2 × (length + width) × height. Subtract roughly 21 square feet per standard door and 16 per standard window if you want a tighter number. Most painters skip this since the extra paint covers trim touch-ups.
Coverage and coats
A gallon covers about 350 square feet per coat on a primed, smooth wall. Two coats is standard for a color change; drop to one for a same-color touch-up, or plan for three when covering a dark wall with a light color.
Once you know the gallons, the next question is what the job costs. Our cost to paint a room guide prices walls, ceiling, and trim by room size, contractor against DIY, and shows why wall area rather than floor area is the number a quote should be built on.
Method and sources
Where these figures come from: Manufacturer product data sheets, which is where a spread rate is actually published. Sherwin-Williams ProMar 200 Zero V.O.C. Interior Latex Eg-Shel, data sheet 101.83A dated 03/2025, states coverage of 350 to 400 square feet per gallon at 4 mils wet. Benjamin Moore Regal Select Interior Eggshell N549 states a spread rate of 400 to 450 square feet per gallon. This page defaults to 350 because it is the bottom of that spread, and a gallon short is a second trip.
Wall area is twice length plus width, times height, less 21 square feet per door and 16 per window, the deductions HomeGuide's Cost to Paint a Room uses, times the number of coats. A dark colour over a light one, or bare drywall, will take more than the can says.
QuantiYard writes its own formulas and states them in plain language on every page. How these numbers are made covers where the constants come from, how waste factors are chosen, what these calculators deliberately do not do, and how to report an error.
Save the paint chart
Wall area and gallons for five room sizes, with a door and two windows already taken off. Save it and buy the right number of cans.
Save to PinterestOpens Pinterest with the chart and link ready to go.
Paint calculator FAQ
How much wall area does a gallon of paint cover?
Typically 350 to 400 square feet per coat on a smooth wall. Textured or unprimed surfaces may only get 250 to 300 square feet per gallon.
How many coats of paint do I need?
Two coats is standard for a color change. One coat can work for a same-color touch-up; three coats are sometimes needed for light colors over dark walls.
Do I need to subtract doors and windows from wall area?
Yes for accuracy, though many painters skip it since it slightly overestimates paint, which covers trim touch-ups. A standard door is about 21 square feet and a window about 16, the figures HomeGuide uses.
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