Free construction tool
Paint calculator
Work out how much paint a room needs. Enter the wall perimeter, ceiling height, and the door and window counts, pick coats and coverage, and get paintable square feet, whole gallons to buy, and cost. The formula is printed under the results so you can check the math.
Formula: paintable area = perimeter x height, less 20 SF per door and 15 SF per window. Here 48 x 8 = 384 SF, less 1 door(s) (20 SF) and 1 window(s) (15 SF) = 349 SF. Gallons = ceil(area x coats / coverage) = ceil(349 x 2 / 350) = ceil(1.99) = 2 gallon(s). Paint is sold by the can, so the exact figure (1.99) always rounds up.
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What a paint calculator figures
A paint calculator turns a room's dimensions into the number of gallons to buy. It works on paintable area, not floor area: the surface you actually roll is the walls, so the calculator measures the perimeter of the room, multiplies by ceiling height, and subtracts the openings you skip. Enter perimeter, height, the door and window counts, how many coats, and the coverage rate for your surface, and it returns paintable square feet, whole gallons, and a cost you can price against a quote.
The formula
Paintable area = perimeter (feet) x height (feet), less 20 square feet per door and 15 square feet per window. Gallons = ceil(area x coats / coverage). Worked example: a 12 by 12 room with 8 foot ceilings has a 48 foot perimeter, so 48 x 8 = 384 SF gross. Deduct one door (20 SF) and one window (15 SF) and you have 349 SF of wall. Two coats at 350 SF per gallon is 349 x 2 / 350 = 1.99 gallons, which rounds up to 2. Paint is sold by the can, so the calculator always rounds up: a coat that runs a quarter-gallon short means a second trip and a visible lap mark where the wet edge died.
Coats, coverage, and the deductions
Three inputs decide the count, and all three follow standard estimating conventions. Coats: two is the standard for a color change or new drywall, one covers a same-color refresh, and three is safe going from a dark wall to a light one. Coverage: smooth drywall or plaster spreads about 350 SF per gallon, rough or textured surfaces closer to 275, and bare or primer-coat surfaces around 250 because the substrate drinks the first pass. Deductions: 20 SF per door and 15 SF per window are the trade's fixed allowances, so you do not have to measure every opening for a quick takeoff. These are planning numbers, not manufacturer specs, and you can overwrite the coverage rate with whatever the can states.
Gallons, quarts, and square feet
A gallon is the buying unit; a quart (a quarter gallon) is what you reach for on trim, a closet, or an accent wall. If the exact figure lands under one gallon, one gallon still covers it, but a quart may be the cheaper buy for a genuinely small job. Square feet is the working unit underneath everything: coverage is quoted in SF per gallon, so the whole calculation is really "how many SF do I have, times how many coats, divided by how many SF a gallon covers." Ceilings and floors are separate takeoffs; this tool sizes walls, which is where interior repaint budgets live.
From a gallon count to a painting bid
A gallon count is one line of a paint bid. The bid also carries prep, primer, trim, labor hours by surface, and the markup that keeps the job profitable, and every one of those scales off the same square-foot takeoff this calculator starts. Getting the wall area right is the foundation; pricing the whole scope against your own rates is the job. Ruh reads the plan set or the field measurements, builds the surface takeoff, and drafts a priced estimate on your own price book, so the paint line traces back to the room it came from. See how it works on the painting estimating software page.
Paint calculator FAQs
How much does a gallon of paint cover?+
About 250 to 350 square feet in one coat, depending on the surface. Smooth drywall spreads farthest, near 350 SF per gallon; rough or textured walls run closer to 275, and bare or primed surfaces around 250 because they absorb more.
How many gallons of paint for a 12x12 room?+
About 2 gallons for the walls at two coats. A 12 by 12 room has a 48 foot perimeter, so at 8 foot ceilings that is 384 SF gross, or 349 SF after one door and one window. Two coats at 350 SF per gallon is 1.99 gallons, rounded up to 2.
Do you subtract doors and windows when calculating paint?+
Yes. The standard estimating deductions are 20 square feet per door and 15 square feet per window. On a 384 SF room, one door and one window drop the paintable area to 349 SF, which can be the difference between one gallon and two.
How many coats of paint do I need?+
Two coats is standard for a color change or new drywall. One coat covers a same-color refresh, and three coats is the safe call going from a dark color to a light one. More coats multiply the gallon count directly.
Does primer count differently than paint?+
Yes. Primer covers roughly 200 to 300 square feet per gallon, less than finish paint, because bare or porous surfaces soak up the first pass. Set the coverage rate to about 250 SF per gallon for a primer coat.
Ruh prices the whole paint scope, not one wall.
AI takeoff reads your plans or field measurements, builds the surface takeoff, and drafts a priced painting estimate on your own price book.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.