How to Measure Room
Home Calculators

Paint Calculator

Estimate how many gallons of paint you need for a room.

Paint Calculator

Room dimensions in, gallons of paint needed out.

Coats

Paint needed

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How the estimate is calculated

The calculator above takes your room's perimeter (2 x (length + width)) times the wall height to get total wall area, then subtracts standard allowances for doors (21 sq ft each) and windows (15 sq ft each) to get the actual paintable area. That number is multiplied by your chosen number of coats, then divided by a standard coverage rate of 350 sq ft per gallon — rounded up to the nearest quarter gallon, since paint is sold in fixed increments.

This gives a solid estimate for budgeting and shopping, but always check the specific paint product's stated coverage rate before buying, since it can vary from the 350 sq ft/gallon standard used here — some premium or textured paints cover less per gallon.

A few things the estimate doesn't account for

  • Unusually large openings. If a room has an oversized window, patio door, or built-in that takes up significant wall space, measure it directly and subtract its actual area rather than relying on the standard door/window allowances.
  • Textured or porous surfaces. Stucco, brick, or unprimed drywall can soak up noticeably more paint than the standard coverage rate assumes.
  • Color changes. Going from a dark wall to a light color (or vice versa) sometimes needs a third coat or a tinted primer first, which this estimate doesn't add automatically.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

About 350 square feet per gallon is the standard coverage most paint manufacturers list, though it varies some by brand and paint type (350-400 sq ft is a typical range). Check the specific can's label, since a textured wall or a very porous unpainted surface can lower actual coverage.

Do you need two coats of paint?

Almost always, yes, unless you're using a high-build paint-and-primer product on a wall that's already a similar color. Two coats gives more even coverage and color depth, and is the standard assumption for a paint estimate — a single coat often shows roller marks or patchy spots, especially with lighter colors over a darker existing wall.

Do you need to subtract doors and windows from wall area?

Yes, for an accurate estimate. A standard interior door is about 21 sq ft, and a standard window is about 15 sq ft — subtracting these from your total wall area avoids over-ordering. For a room with unusually large openings (patio doors, picture windows), measure them directly instead of using the standard estimate.

Should you round up when buying paint?

Yes — paint is sold in fixed can sizes (typically quarts and gallons), and running short mid-project means a possible color-match issue with a second batch. It's standard practice to round up to the next quart or gallon rather than trying to buy the exact calculated amount.

Does ceiling paint use the same coverage rate as wall paint?

Roughly yes, per square foot of coverage, but ceilings are usually calculated separately from walls (ceiling area = room length x width, not part of the wall-perimeter calculation above) and often use a flat ceiling-specific paint rather than the same product used on the walls.

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