How much paint do I need to paint a room?

Published July 21, 2026.

You need enough paint to cover your total wall area times the number of coats, divided by how much a gallon covers. In plain numbers: measure the walls, subtract about 20 square feet for each door and 10 for each window, plan on two coats for walls, and figure a gallon covers roughly 350 to 400 square feet. For a standard 10 by 12 bedroom that lands at two gallons for the walls. When the math falls between sizes, round up, because running short mid-wall is worse than a little left over.

The formula, step by step

Here is the same method the pros use, written out so you can do it on paper.

Add up the wall length. Measure the length of every wall you plan to paint and add them together. That total is the room’s perimeter.
Multiply by wall height. Multiply the perimeter by the height of the walls, floor to ceiling. This is your raw wall area in square feet. This Old House describes the same perimeter times height step.
Subtract the openings. Take off about 10 square feet for every window and 20 square feet for every door, following This Old House’s rule of thumb.
Multiply by coats. Most walls get two coats, so multiply the adjusted area by two.
Divide by coverage. Divide by 350 to 400 square feet per gallon, then round up to the next whole gallon.

A worked example: a 10 by 12 room with 8 foot ceilings has a perimeter of 44 feet. That times 8 is 352 square feet. Subtract one door and one window (about 30 square feet) and you are at 322. Two coats is 644 square feet. Divided by 350, that is about 1.8 gallons, so you buy two.

How much a gallon actually covers

Coverage is the number that drives the whole estimate, so it helps to know the real range.

Container	Typical coverage (one coat)	Good for
Quart	About 100 sq ft	An accent wall, a door, touch-ups
Gallon	About 350 to 400 sq ft	A small to average room, one coat
Gallon of primer	About 200 to 300 sq ft	Sealing new or patched drywall

Those figures come from Sherwin-Williams and This Old House. Our interior paint calculator uses 350 square feet per gallon as its default, which is the conservative end of that range and leaves you a small cushion. Always check the can too, since a specific product may state its own coverage.

How many coats, and when you need primer

Plan on two coats for walls and one for the ceiling. That is the standard rule of thumb, and it is what most color changes require to look even. You can sometimes get away with one coat when you are repainting close to the existing color over a clean, previously painted wall.

Primer is a separate step, not a coat of your finish color. Reach for it on bare or patched drywall, over stains, and when you are covering a much darker color. A gallon of primer covers less than paint, about 200 to 300 square feet, so measure for it the same way. Many modern paints are labeled paint and primer in one, which can save a step on sound, similar-color walls, but new drywall still benefits from a dedicated primer.

Ceilings and trim

Ceilings and trim are measured on their own, not lumped in with the walls.

For a ceiling, multiply its length by its width to get the square footage, then divide by 400 for a one-coat estimate. A 10 by 12 ceiling is 120 square feet, well under a gallon.

Trim, doors, and window casings use far less than you would guess. You will likely need less than a gallon for a standard room, so buying trim paint by the quart is usually the smart move. Trim paint is also a different sheen, commonly semi-gloss, so keep it as a separate purchase from your wall paint.

Common mistakes that throw off the estimate

Forgetting the second coat. The single biggest under-buy. Coverage numbers are per coat, so a two-coat wall needs double.
Ignoring surface texture. Porous or textured surfaces like stucco and unpainted drywall absorb more than smooth walls, so add a margin there.
Rounding down. A dead-even estimate leaves no room for touch-ups. Round up to the next gallon and keep the leftover for future dings.
Skipping primer over big color changes. Trying to cover a deep color with two coats of a light one often turns into three. Prime first.

For the exact amount for your specific room, including coats, doors, windows, and primer, run the numbers through our interior paint calculator. You can also browse the full set of paint and stain calculators for ceilings, whole houses, decks, and fences.

Sources

This Old House · Paint Calculator: How Much Paint Do I Need? thisoldhouse.com/painting/how-much-paint-do-i-need
Sherwin-Williams · How to Use the Paint Calculator (FAQs). sherwin-williams.com/en-us/color/color-tools/paint-calculator

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Frequently asked questions

How many gallons of paint do I need for a 12x12 room?

For a 12 by 12 room with 8 foot walls, the walls total about 384 square feet before openings. Subtract roughly one door (20 sq ft) and one window (10 sq ft) and you are near 354 square feet. At two coats that is about 708 square feet of coverage, so two gallons is the safe buy. If the room has only one coat over a similar color, one gallon may be enough.

Do I really need two coats of paint?

Most walls look best with two coats. This Old House gives two coats on walls and one on the ceiling as the rule of thumb. One coat can work when you are repainting a similar color over a sound, primed surface, but big color changes, new drywall, and light-over-dark almost always need two.

How much does a gallon of paint cover?

A gallon of interior paint usually covers about 350 to 400 square feet in one coat, per Sherwin-Williams and This Old House. Rough, porous, or textured surfaces like stucco and unpainted drywall soak up more, so plan for extra there.