Mis à jour July 2026

Paint Square Footage Calculator

Measure your walls, subtract the doors and windows, and see the paintable square footage and the number of gallons to buy.

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Utilisée par les propriétaires et les artisans

Largeur du mur

Hauteur du mur

Portes

−21 sq ft chacune

Fenêtres

−15 sq ft chacune

Surface de mur à peindre

60sq ft

Surface brute

96 sq ft

Ouvertures

36 sq ft

La surface nette soustrait les ouvertures standard de porte (21 sq ft) et de fenêtre (15 sq ft).

Ce que cela signifie

To find paint square footage, multiply each wall's width by its height and add the walls together, then subtract about 21 sq ft per door and 15 sq ft per window. One US gallon covers roughly 350–400 sq ft in a single coat, so multiply the paintable area by your number of coats and divide by 350 to get the gallons you need.

Formule et guide

Comment le calcul est effectué

Paint is estimated from wall area, not floor area. You measure the surface you'll actually cover, take out the openings you won't paint, then convert square feet to gallons using the coverage printed on the can.

1Measure the width and height of each wall in feet and multiply them for that wall's area.
2Add every wall together, then subtract ~21 sq ft per door and ~15 sq ft per standard window.
3Multiply the paintable area by the number of coats you plan to apply (usually two).
4Divide that total by 350 sq ft per gallon and round up to the next whole gallon.

Exemple résolu

En chiffres

Four walls (12 ft × 10 ft room, 8 ft ceilings)	448 sq ft
− 1 door (~21 sq ft) + 2 windows (~30 sq ft)	≈ 400 sq ft
× 2 coats	800 sq ft
÷ 350 sq ft per gallon	≈ 3 gallons

≈400 sq ft paintable × 2 coats = 800 sq ft ÷ 350 ≈ 3 gallons.

Précision et hypothèses

Doors (~21 sq ft each) and windows (~15 sq ft each) are subtracted from the wall area.
Coverage varies with surface and porosity — bare drywall, wood, and stucco drink more paint than a sealed wall.
Primer is estimated and bought separately; it isn't included in your finish-coat gallons.
Two coats is typical for even color and full coverage, especially over a new or contrasting color.

Conseils et règles clés

A US gallon covers about 350–400 sq ft per coat on a smooth, sealed surface; use 350 to stay safe.
Rough or textured surfaces (stucco, brick, popcorn ceilings) can drop coverage to 200–300 sq ft per gallon.
Ceilings are measured as floor area — length × width — and estimated separately from the walls.
Buy paint from the same batch (lot number) so the color matches across every can you open.

Cas d'usage

À qui s'adresse cette calculatrice

Interior walls

Measure each wall's width × height, subtract doors and windows, and plan on two coats for a clean finish.

Ceilings

Estimate the ceiling as its floor area (length × width) and add it as a separate line from the walls.

Exterior and siding

Measure the outside walls the same way, then add extra for textured siding, trim, and soffits that soak up more paint.

How to calculate square footage for painting

Paint is estimated from the area of the surface you're covering, so the starting point is wall area, not the square footage of the floor. Measure the width and height of each wall in feet, multiply them, and add the walls together. A 12 ft by 10 ft room with 8 ft ceilings has two 12 ft walls and two 10 ft walls, which comes to (12 + 12 + 10 + 10) × 8 = 448 square feet of gross wall area before you take anything out.

From that gross figure, subtract the openings you won't paint. A standard interior door is about 21 square feet and a typical window is about 15 square feet, so one door and two windows remove roughly 50 square feet, leaving about 400 square feet of paintable wall. Enter each wall and each opening in the calculator above and it does the subtraction for you, so you're pricing paint against the surface you'll actually roll.

Turning square feet into gallons

One US gallon of interior paint covers roughly 350 to 400 square feet in a single coat on a smooth, primed surface. Manufacturers print a coverage figure on the can, but 350 is a safe planning number because it leaves room for touch-ups and the inevitable spots that need a little more. Multiply your paintable area by the number of coats — two is standard — then divide by 350. For our 400-square-foot room that's 800 square feet of coverage, or about three gallons.

Coverage falls fast on thirsty surfaces. Bare drywall, new wood, and porous masonry can pull a gallon down to 250 square feet or less, which is exactly why a coat of primer first saves finish paint. Prime separately, then estimate your color coats against a sealed surface for a more reliable count.

Exterior and siding estimates

Exterior walls are measured the same way — width times height, minus the doors and windows — but the surface texture matters even more outside. Lap siding, shakes, and stucco have far more actual surface than a flat wall of the same dimensions, so they can cut coverage to 200 to 300 square feet per gallon. Add a margin for that texture, and estimate trim, fascia, and soffits as their own areas since they're often a different color and finish.

Once you know the gallons, drop your paint area and price per square foot into the square footage cost calculator to turn the measurement into a project budget.

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FAQ

FAQ de la calculatrice Paint

Multiply the width by the height of each wall and add them together, then subtract about 21 sq ft per door and 15 sq ft per window. For a 12 ft × 10 ft room with 8 ft ceilings, the walls total 448 sq ft; removing one door and two windows leaves about 400 sq ft of paintable area.

A US gallon of interior paint covers roughly 350 to 400 square feet in one coat on a smooth, primed surface. Use 350 for planning to leave room for touch-ups. Rough or unsealed surfaces like stucco, brick, and bare drywall cut coverage to 200–300 sq ft per gallon, so buy extra for texture.

Yes. Subtract about 21 sq ft for each standard door and about 15 sq ft for each window from your gross wall area, since you won't paint those surfaces. Skipping the subtraction usually just means you buy a little extra paint, which many painters do on purpose to keep a spare can for touch-ups.

Measure each exterior wall's width × height, add them, and subtract doors and windows, just like indoors. Then account for texture: lap siding, shakes, and stucco can drop coverage to 200–300 sq ft per gallon. Estimate trim and soffits separately, and add a margin so a rough surface doesn't leave you short.

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