MathBench

Paint Calculator

Turn room dimensions into gallons of paint: wall area minus doors and windows, multiplied by coats, at the coverage rate your surface will give you.

Ceilings usually use separate ceiling paint.

You need

—gallons of paint
—suggested purchase
—paintable area (ft²)
—total incl. coats (ft²)

How this calculator works

The calculator finds your wall area from the room's footprint and ceiling height: perimeter (twice the length plus twice the width) multiplied by height. It then subtracts standard allowances for each door (21 ft²) and window (15 ft²), multiplies what's left by your number of coats, and divides by the coverage rate to get gallons. The suggested purchase rounds that up into the gallons and quarts you'd put in the cart.

Coverage is the least certain input. The 350–400 ft² printed on the can assumes a smooth, sealed, previously painted wall. Texture, patch-heavy walls, and bare drywall all reduce it, and the surface presets allow for that. Painting just one accent wall? Enter its length as the room length, zero width, and you'll get that single wall's math.

The formula

Wall area   = 2 × (length + width) × height
Paintable   = wall area − (doors × 21) − (windows × 15) [+ length × width if ceiling]
Total area  = paintable × coats
Gallons     = total area ÷ coverage (ft²/gal)

The 21 ft² door deduction is a standard 3′ × 7′ slab. The 15 ft² window deduction is a typical 3′ × 5′ unit. Coverage presets follow manufacturer spread rates (e.g., Sherwin-Williams and Behr both quote 350–400 ft² per gallon on smooth repaints).

Worked example

Say you're painting a 14 ft × 12 ft bedroom with 8 ft ceilings, with two doors, two windows, two coats, typical 350 ft²/gal coverage:

  1. Perimeter: 2 × (14 + 12) = 52 ft
  2. Wall area: 52 × 8 = 416 ft²
  3. Deduct openings: 416 − (2 × 21) − (2 × 15) = 344 ft²
  4. Two coats: 344 × 2 = 688 ft²
  5. Gallons: 688 ÷ 350 = 1.97 gal → buy 2 gallons

That's the classic "two gallons per bedroom" rule of thumb, derived instead of guessed, and now you know it stops holding the moment your ceilings are 9 ft or your walls are textured.

Assumptions & tips

  • Buy all the paint at once. Color is mixed per can, and batch-to-batch variation is visible on a big wall. Buy an extra quart up front so you never need a second batch.
  • Dark-to-light means primer. A tinted primer coat costs less than a third coat of premium paint and covers better. Size it by running the calculator at one coat.
  • Don't deduct tiny windows. The 15 ft² allowance fits an average double-hung. For a small basement hopper window, skip the deduction. The cutting-in around it uses the difference.
  • Trim is a separate bucket. Baseboard, casing, and doors take enamel, not wall paint. A gallon of trim paint typically handles an average room's trim with plenty left over.
  • Keep the leftovers usable. Note the color code on the lid, press it shut, and store it off the garage floor: touch-up paint only helps if it hasn't frozen or skinned over.

Frequently asked questions

How much does a gallon of paint cover?

Most interior paints cover 350 to 400 square feet per gallon on smooth, previously painted walls, the figure printed on the can. Textured, porous, or bare surfaces drink more paint and drop real-world coverage toward 250 to 300 square feet. When in doubt, use the lower number. Leftover paint is touch-up insurance, while running out mid-wall leaves a visible lap line.

How many coats of paint do I need?

Two coats is the standard for a uniform color and sheen. One coat only works when repainting with the same color and finish. Going over a dark color with a light one, or painting bare drywall, usually means a coat of primer first plus two finish coats. Primer is a separate product, so run the calculator at one coat to size the primer purchase.

Do I subtract doors and windows from wall area?

Yes. They add up fast. The standard deductions are about 21 square feet per door and 15 square feet per average window. This calculator subtracts them automatically. If a wall is mostly glass or built-ins, measure the actual paintable area instead.

Sources

  1. ProMar 200 Zero V.O.C. Interior Latex Eg-Shel, B20 Series — Product Data Sheet 101.83A. Sherwin-Williams, March 2025. sherwin.comStates coverage of 350 to 400 sq ft per gallon (the figure the coverage presets are built around) and specifies one coat of primer plus two finish coats over drywall, which is the two-coat default.
  2. Zinsser Bulls Eye 1-2-3 Water-Based Primer — Technical Data BEY123-01. Rust-Oleum. rustoleum.comQuotes 450 sq ft per gallon on non-porous surfaces against 350 on porous ones, the reason this page has surface presets, and recommends tinting the primer to improve the topcoat's hiding, the dark-to-light tip.
  3. Zinsser Ceiling Paint, Paint and Primer In One — Technical Data ZNR-19. Rust-Oleum. rustoleum.comA dedicated ceiling product, quoting practical coverage of roughly 250 to 400 sq ft per gallon after 15 percent material loss: the reason the ceiling checkbox tells you to size ceiling paint as a separate purchase.

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