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Paint Calculator

Enter room size, doors, windows and coats to get wall area, gallons and cans of paint to buy.

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Box your paint before you start

If you end up with two or more gallons of the same color, pour them together in a clean bucket and mix before painting. Small tint differences between cans show up as patchy walls, especially on flat and eggshell sheens. Painters call this boxing, and it is the reason a five-gallon pail is useful even when you will not use all of it.

How this paint calculator works

The calculator finds the total wall area of a rectangular room with 2 × (length + width) × wall height, then subtracts about 21 sq ft for each door (a standard 3 ft × 7 ft door) and 15 sq ft for each window (a typical 3 ft × 5 ft window). What remains is the paintable wall area.

Gallons needed equal paintable area × coats ÷ coverage per gallon. Major paint makers such as Sherwin-Williams and Benjamin Moore list interior wall paint at roughly 350 to 400 sq ft per gallon per coat on smooth, primed drywall, so 350 is a slightly conservative default. Two coats is the norm for even color and full hide; one coat only works when you are refreshing the same color.

The buying suggestion rounds your total into the cheapest practical mix. If a partial gallon would need three or more quarts, it rounds up to a full gallon, because three quarts usually cost about as much as a gallon. Once you need more than about three and a half gallons of the same color, a five-gallon pail is usually cheaper per gallon and keeps the color consistent across the whole job.

Ceilings are figured separately because most people use a flat ceiling paint rather than the wall paint. Trim, doors and cabinets are not included; a quart of semi-gloss covers the trim in most average rooms. The primer line assumes about 300 sq ft per gallon, which is typical of drywall primers on new, unpainted board.

Where these numbers come from

The formulas and default values in this calculator are based on:

  • coverage rates from Sherwin-Williams and Benjamin Moore interior paint product data (350–400 sq ft per gallon per coat)
  • standard door (3 x 7 ft) and window (3 x 5 ft) deductions used across the painting trade

These are planning estimates. Check product labels, your local building code, and a licensed contractor before you buy materials or build.

Frequently asked questions

Should I subtract doors and windows from the wall area?

Yes for normal rooms, which is what this calculator does. In a room with only one or two openings the difference is small, but in a room with a slider or several large windows it can save you a gallon. If the room has an unusually large opening, measure it and increase the window count to match its area.

Why does the can say 400 sq ft per gallon but I ran out sooner?

Label coverage assumes smooth, primed, non-porous walls and an even film. Textured walls, bare drywall, dark-to-light color changes and heavy rolling all use more paint. If your walls are textured, lower coverage to about 300 sq ft per gallon, or use the Paint Coverage Calculator to adjust by surface type.

Do I need two coats if the paint says paint-and-primer in one?

Usually yes. Paint-and-primer products help with stain blocking and adhesion, but most manufacturers still recommend two coats for uniform color and sheen, especially on new drywall or when changing colors. Plan on two coats and treat any leftover paint as touch-up stock.

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