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Calculate how much paint you need for walls and ceilings. Estimate gallons, coverage, coats, and cost for interior and exterior painting projects.
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Enter measured project values. Results update only when you choose Calculate.
Method
A 12 x 14 bedroom with 8-foot ceilings is 416 square feet of wall. Subtract one door (20 SF) and two windows (30 SF) and you are at 366 SF — two coats is 732 SF, which at 350 SF per gallon is 2.1 gallons, not the "gallon per room" a lot of crews still order by. Put that room on unprimed drywall drinking its first coat at closer to 275 SF per gallon and the shortfall gets worse. Multiply the miss across a 20-unit repaint and the paint order is off by cases, not quarts.
Painting subs use this to build material counts for repaint and new-construction bids; GCs use it to sanity-check a painter's quote before it goes into the estimate; production builders use it to standardize per-unit paint orders across a plan set. The discipline that matters is measuring walls, ceilings, and trim as three separate line items — they take different products at different coverage rates and different labor productivity, and a single blended square-foot number hides all of it.
Walls get wall paint, ceilings get flat ceiling paint, and trim gets enamel — three products, three coverage rates, three labor rates. Perimeter times ceiling height for walls, length times width for ceilings, and linear feet for base, casing, and crown. Lumping them into one number is how trim enamel ends up missing from the order.
The standard deductions are 20 SF per door and 15 SF per average window. On a room with one door and one window, many painters skip the deduction and let it ride as a built-in waste and touch-up factor — reasonable. On a wall of storefront glass, a garage door, or a slider, always deduct, or the count balloons.
The 400 SF per gallon on the can assumes a smooth, sealed, previously painted wall. Orange peel and light knockdown pull it to 300-350; heavy texture, bare drywall, and patched surfaces to 250-300; raw masonry and stucco can drink the first coat at well under 250. When in doubt, price the lower rate — the extra gallon is cheaper than a second trip.
New drywall gets a PVA primer coat plus two finish coats — no exceptions if you want uniform sheen. A deep color change (dark to light, or into saturated reds and blues) gets a tinted primer plus two coats. A same-color, same-sheen repaint in good condition may cover in one, but bid two: the one-coat jobs that actually cover in one are rarer than the ones that don't.
With 8-foot ceilings, a 12 x 12 room is 48 feet of perimeter times 8, or 384 SF of wall. Subtract a door (20 SF) and a window (15 SF) to get 349 SF; two coats is 698 SF, which at 350 SF per gallon is right at 2 gallons. The ceiling is another 144 SF — figure about half a gallon per coat in ceiling paint, ordered separately.
Plan on 250 to 400 SF per gallon depending on texture and porosity. The 400 number on the can assumes smooth, sealed, previously painted walls; light texture drops it to 300-350, and heavy texture, bare drywall, or masonry drops it to 250-300 or below on the first coat. Estimating at 350 for typical repaint work and 275 for unprimed or textured surfaces keeps you out of trouble.
For any color change, sheen change, or new drywall, yes — one coat will show roller marks, flashing, and the old color reading through. The only realistic one-coat scenario is a same-color, same-sheen refresh on walls in good condition, and even then most painters bid two coats and treat a one-coat outcome as margin. Two-coat math is simple: double the paintable area before dividing by coverage.
Four cases: new drywall (PVA primer, always), a dramatic color change (tinted primer saves a third finish coat), stains or tannin bleed (stain-blocking primer), and glossy or patched surfaces (bonding or spot primer so the finish coat adheres and dries uniform). Skipping primer on new drywall is the classic callback — the paper and the joint compound absorb paint at different rates and every seam telegraphs through.
Use 20 SF per door and 15 SF per average window. On rooms with only one or two openings, many pros skip the deduction and treat it as a built-in waste and touch-up allowance. Always deduct large openings — garage doors, sliders, storefront glass — because at that scale the deduction changes the gallon count.
Measure them separately from walls. Ceilings are length times width — a 12 x 14 ceiling is 168 SF, about half a gallon per coat of flat ceiling paint. Trim is counted in linear feet of base, casing, and crown; as a rule of thumb a gallon of trim enamel handles roughly 350-400 SF of brushed surface, and most rooms need only a quart or two — but it is a different product than wall paint, so it must be its own line.
Separate plan workflow
This calculator solves one bounded formula from the inputs shown. BuildVision AI supports reviewed plan takeoff, complete-document CSV, and editable quote lines; the estimator owns pricing and final bid approval.