Paint calculator: square feet to gallons

Most quality interior latex paints rate at 350 to 400 sq ft per gallon for one coat on smooth, sealed drywall. That number drops on porous, textured, or unprimed surfaces.

For most rooms you’ll do two coats. Plan for around 175 sq ft of finished wall per gallon. A 12×14 room with 8 ft ceilings (about 416 sq ft of wall) needs around 2.5 gallons for two coats — round up to 3.

Gallons = area to be painted x number of coats / 350. That is the whole calculation, and 350 square feet per gallon per coat is the working figure for interior latex on smooth, primed drywall.

For 1,500 square feet of wall, that is 4.3 gallons in one coat and 8.6 in two — so nine gallons, or eight plus a quart if you want to cut it fine. Buy the whole quantity in one order, because paint is tinted in batches and a top-up can gone visibly off-shade.

There is an ambiguity worth resolving before you use any of these numbers. When people say “paint for 1,500 sq ft” they sometimes mean 1,500 square feet of wall and sometimes a 1,500 square foot house. Those are very different orders.

There is no reliable multiplier between the two. Wall area divided by floor area is set by room shape, and it runs from about 2.1 for a large open room up to 3.2 for a small square one — a 12 by 12 ft room with a 9 ft ceiling has 432 square feet of wall against 144 of floor, a ratio of 3.0, while a 15 by 20 ft room at the same height gives 630 against 300, a ratio of 2.1. Measure the walls rather than scaling the floor area.

Primer is a separate order and covers less: 200 to 300 square feet per gallon. Ceilings are usually figured separately too, since ceiling paint is a different product and one coat is often enough over an existing painted ceiling.

New drywall, dramatic color changes (dark to light), stain coverage, and high-gloss surfaces all benefit from primer. One gallon of primer typically covers 300 sq ft.

Skip primer only when you’re repainting a similar color over an existing finish in good condition.

The standard answer — a gallon covers 350 sq ft per coat — works for smooth walls and average paint quality. Real-world coverage varies more than people expect.

Premium paints (Sherwin-Williams Emerald, Benjamin Moore Aura) cover 400-450 sq ft per gallon and often need only 2 coats over a similar color. Budget paints (contractor-grade) cover 250-300 sq ft per gallon and need 3 coats for full hide. The premium paint is usually cheaper per coverage-foot despite the higher sticker price.

Coverage drops for: textured surfaces (15-25% less), color changes (3 coats minimum), bare drywall (primer required first), and ceiling paint (typically 250-300 sq ft per gallon because of the flat formula).

Worked numbers: a 12 × 14 ft bedroom (374 sq ft net wall area) needs 374 × 2 coats = 748 sq ft = 2.14 gallons of standard paint. Round up to 3 gallons or accept the risk that you might need to buy 1 more quart later — at which point the dye lot won't match perfectly.

The table below runs that arithmetic for common bedroom and living-room sizes at an 8 ft ceiling. Gross wall area is the room perimeter times ceiling height. Openings are deducted at 21 sq ft per door and 15 sq ft per window — the standard allowances a painter uses when quoting. Gallons are rounded up to whole cans, because that is how paint is sold.

Two habits keep these numbers honest. Get all your paint mixed at once, in one order, so every can shares a dye lot. And buy for two coats from the start: a second trip mid-job costs more in time than the extra gallon costs in money. For the full coverage-rate breakdown by surface and sheen, see the paint coverage guide linked below.

Primer is required: on bare drywall (new construction), bare wood, dramatic color changes (dark to light, white to red), water-stained or repaired walls, and any surface with mildew or grease.

Primer is optional: when repainting the same color family on existing finished walls, when using paint-and-primer-in-one over a similar color, and on glossy surfaces if the topcoat is also glossy.

Primer coverage rate is 300 sq ft per gallon, lower than topcoat. For 374 sq ft of wall: 1.25 gallons of primer = 2 gallons (round up). Primer is usually less expensive per gallon than topcoat, and one primer coat is sufficient even when topcoat needs two.

Tinted primer for dark colors: have your primer tinted toward the topcoat color. A pink primer under red dramatically reduces the number of red coats needed. Most paint stores tint primer at no extra cost.

Exterior paint covers less square footage per gallon than interior paint because exterior surfaces are typically rougher and more porous.

Exterior paint coverage: 250-350 sq ft per gallon on smooth siding, 200-275 on rough/textured siding, 150-200 on brick or stucco, 200-250 on bare wood.

A 2,000 sq ft house typically has 1,500-1,800 sq ft of exterior wall after subtracting windows and doors. Two coats: 3,000-3,600 sq ft ÷ 275 = 11-13 gallons of paint. Buy a 5-gallon bucket plus extra individual gallons.

Trim paint is bought separately, typically in quart cans. One quart covers about 90 sq ft of trim. A 2,000 sq ft house has 100-150 linear feet of exterior trim — about 75-100 sq ft of trim area — so 1-2 quarts of trim paint suffices.

Everything above uses 350 sq ft per gallon for one coat, which is the figure the industry quotes and a reasonable planning default. It is not what your particular tin will say, and the gap between the default and the label is frequently worth a whole gallon.

Manufacturers print a spread rate, and it moves with the product. A thin, high-hiding wall paint may genuinely reach 400 sq ft; a deep-base colour carrying a lot of tint, a low-sheen product, or anything designed for a textured surface can come in nearer 250. On 400 sq ft of wall in two coats — 800 coverage-square-feet — 350 gives 2.29 gallons and you buy 3. At 250 it is 3.2 gallons and you buy 4. Same wall, same two coats, one more tin, decided entirely by a number printed on the back of the can.

The coat count moves it again. A third coat is not a sign of poor work; it is what a strong colour change actually needs. Going dark over light, light over dark, or any saturated red, orange or yellow over a pale ground will usually take three, because those pigments have the least hiding power. On the same 400 sq ft wall at 350 sq ft per gallon, three coats is 3.43 gallons — a fourth tin against the two-coat estimate.

The way to avoid buying three times is to settle both numbers before the first tin: read the spread rate off the product you have actually chosen, and decide honestly whether the colour change is a two-coat or a three-coat job. Then order the whole quantity at once, because paint is tinted in batches and a top-up tin mixed later can differ visibly on a long wall.

Primer is what usually converts a three-coat job back into two. A tinted primer close to the finish colour gives the topcoat a uniform, absorbent ground to sit on, which is the entire reason bare drywall, patched plaster and any drastic colour change ask for one.