Paint Calculator - Gallons of Paint and Primer

Coverage per gallon	about 350–400 sq ft, typical
Door deduction	about 20 sq ft each
Window deduction	about 15 sq ft each
Primer	similar coverage per gallon, check the label

What is Paint Calculator?

Paint is sold by the gallon and coverage is quoted per coat, so the estimate has three moving parts: the area to be painted, how many coats it needs, and how much of that area is taken up by doors and windows you are not painting. One gallon covers roughly 350 to 400 square feet on a smooth, previously painted wall. Bare or porous drywall drinks more, and textured surfaces cover less. Primer is estimated the same way. Ceilings add area that most people forget. Buying one extra gallon at the start beats a second trip once the walls are half done.

Common uses for Paint Calculator

Work out gallons for a bedroom in two coats
Add primer for bare drywall or a color change
Estimate paint for a ceiling as well as the walls
Deduct doors and windows from a wall area total
Compare coverage figures between two brands of paint

Area, coats and the coverage figure

Wall area is length times height for each wall, minus large openings. Multiply by the number of coats, then divide by the coverage quoted on the can. Coverage is the number that varies most between products: a thick exterior paint may claim 300 sq ft per gallon while a thin ceiling paint claims 400. Read the can rather than assuming.

Primer is a different decision

Primer is not just cheap paint. It seals porous surfaces so the topcoat does not soak in, blocks stains and odors, and gives a uniform base so colors read as intended. Use a stain-blocking primer over water marks and smoke damage, a bonding primer on glossy or slick surfaces, and a drywall primer on new board. It needs its own gallon estimate.

Buying, storing and touching up

Paint is mixed in batches and color can drift slightly between them, so buy the full quantity in one purchase and keep the label with the room name written on it. Store leftover paint sealed, indoors and away from freezing, and it will be usable for years. Labeled spare cans turn a wall repair into a five-minute job.

Frequently Asked Questions

How much area does a gallon of paint cover?

Most interior paints claim 350 to 400 sq ft per gallon per coat on smooth, sealed surfaces. Bare drywall, rough stucco and unfinished wood absorb far more, sometimes closer to 250 sq ft. Buy one extra gallon, since matching a later batch of color is harder than storing a spare can in the garage.

Do I need one coat or two?

Two coats for almost every job. One coat over a matching color can work, but a color change, a patch repair or a change of sheen needs two for even coverage. Deep base colors, especially reds and yellows, often need three. Primer plus two coats is the standard sequence for new drywall.

How much should I deduct for doors and windows?

About 20 sq ft per door and 15 sq ft per window is a reasonable planning deduction for full-size units. Small windows and narrow doors are not worth subtracting, since the margin protects you against waste. Trim and door faces themselves are usually measured in linear feet and painted separately.

Do I need primer?

Primer is worth it for bare drywall and wood, for repairs patched with joint compound, for glossy surfaces, and whenever you are painting light over dark or dark over light. On a clean, previously painted wall in a similar color, a two-coat paint job is often enough without it.

How much paint for a ceiling?

The same formula as the walls: length times width, times the number of coats. Ceilings are usually painted in one or two coats of flat white, and a single gallon often covers a typical bedroom ceiling twice over. Flat finishes hide roller marks and surface imperfections far better than satin on a ceiling.