Paint Calculator

How to use the paint calculator

Measure the room with a tape and round to the nearest half foot; precision past that never changes how many cans you carry to the car. You need three numbers — length, width, and wall height — and the height is the one people guess wrong, so measure floor to ceiling in a corner rather than assuming the standard eight feet.

Enter them and the calculator builds the same rectangular room model a painter sketches on a notepad: perimeter times height for the gross wall, minus the openings, times the number of coats.

If you are painting a single accent wall rather than a whole room, skip the model and do it directly — wall length times height, doubled for two coats, divided by your coverage. A 12 by 8 ft accent wall is 192 sq ft across two coats, which sits comfortably inside one gallon with enough left over for touch-ups.

Count every door and every full-size window, closet doors included. The calculator subtracts 21 square feet per door — a 3 by 7 ft slab — and 15 square feet per window, the takeoff values painters have used for decades.

Whether that deduction matters at all depends on how close your job already sits to a whole gallon. On the default 12 by 10 ft room the openings remove 51 square feet per coat, which is 102 square feet across two coats, or nearly a third of a gallon at 350 sq ft per gallon; on a job hovering near the 2-gallon line, that is exactly the difference between two cans and three.

Count a sliding glass door as two doors, and count a picture window or a wall of glass as two or three windows, so the deduction stays honest instead of optimistic.

Leave coats at 2 unless you have a specific reason not to. Dulux instructs two coats on its Easycare Washable and Tough Matt and warns that a strong color change may need more, which is the honest general case: one coat rarely hides evenly, dries with visible roller lap marks, and leaves a thinner film that scuffs and washes out sooner.

Turn on Include ceiling only when the ceiling is going the same color as the walls; a different white is its own job with its own cans. Then read the result as whole gallons, with the exact figure beside it — when that exact number only just clears a whole can, a quart may cover the overage.

How much does the coverage setting change the answer?

The Advanced section holds the coverage setting, and it is the single input with the most leverage over the answer. Behr publishes 250 to 400 square feet per gallon for its Premium Plus interior line and attributes the spread to application method and substrate porosity, which is exactly the right way to think about it.

Smooth, previously painted drywall sits near the top of that band, while knockdown texture, stucco, brick, and bare drywall have more real surface area and more thirst, and belong near the bottom. Drop the setting to 300 or lower for those surfaces and raise it toward 400 only for walls in genuinely good condition.

The formula, worked line by line

Painting is an area problem with a multiplier bolted onto it. Everything starts from the gross wall — the perimeter of the room times the wall height — because paint covers surface, not volume, and the perimeter of a rectangular room is simply twice the length plus twice the width.

From there you subtract the openings you will not roll over, add the ceiling only if you are painting it, multiply by the number of coats, and divide by whatever a gallon covers on your particular walls.

The two places this goes wrong are both multiplications rather than measurements. Coats multiplies the entire area, so moving from one coat to two does not add a little paint at the margin, it doubles the job outright.

Coverage divides it, so the gap between the 250 and the 400 that Behr prints for the very same product is a factor of 1.6 on your final can count. Neither of those is a rounding error, and both are decided before you leave the house.

wall area = 2 × (length + width) × height − (doors × 21) − (windows × 15)
ceiling area = length × width   (only if painting the ceiling)
total area = (wall area + ceiling area) × coats
gallons = total area ÷ coverage, rounded UP to a whole can

Now run the same room for primer and watch why it needs its own pass. Primer normally goes on in a single coat, so the area drops back to 301 sq ft rather than 602 — but Sherwin-Williams rates a gallon of primer at only 200 to 300 sq ft.

Take the middle of that at 250 and you need 1.2 gallons, which is two cans if you buy gallons, or one gallon plus a quart, since Sherwin-Williams puts a quart at about 100 sq ft. Fewer coats and lower coverage do not cancel out, and assuming they do is how people prime three walls and run dry on the fourth.

If you are working in litres, one square foot per gallon is 0.0245 square metres per litre, so the familiar US 350 to 400 band is roughly 8.6 to 9.8 m2 per litre. European tins routinely quote higher numbers for the same kind of wall — Dulux prints up to 13 m2 per litre on Easycare Washable and Tough Matt, which converts to about 530 sq ft per gallon.

Read that as a difference in how the figure is stated rather than a difference in the paint: an up-to figure is a best case on smooth, sealed plaster, while the US ranges describe typical results across substrates. Either way this is a shopping estimate, not a contractor’s takeoff — a painter measures every wall individually and prices trim, doors and ceilings as separate line items.

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