Calculator — Paint & Walls
Primer Calculator
Gallons of primer for a room, by surface type — new drywall, previously painted or textured — with doors and windows deducted.
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Results
Adjust the inputs to see your result.
About this calculator
Primer seals porous surfaces so the topcoat covers evenly, and it covers fewer square feet per gallon than paint on exactly the surfaces that need it. This calculator works from room size, coats and surface type — 300 sq ft per gallon on previously painted walls, 250 on new drywall, 200 on textured walls or masonry.
What this number means
Primer is not thinned-down paint: it seals porous surfaces so the topcoat goes on evenly instead of soaking in patchily. New drywall is the classic case — bare paper and joint compound absorb paint at very different rates, so an unprimed wall shows every seam and screw patch through the finish coat.
Coverage is where primer differs from paint. A gallon of paint covers about 350 sq ft; primer covers less on the surfaces that need it most — 250 sq ft per gallon on new drywall, only 200 on textured walls or masonry, because the surface drinks it. That is why a room that takes 2 gallons of paint can take 2 gallons of primer too, and why skipping primer to save a gallon usually costs a second coat of the more expensive paint.
The formula
- Wall area = 2 × (Length + Width) × Ceiling height (sq ft)
- Openings = Doors × 21 + Windows × 15 (sq ft)
- Paintable area = Wall area − Openings
- Gallons = Paintable area × Coats ÷ coverage per gallon, rounded up
- Coverage: previously painted smooth 300, new drywall 250, textured or masonry 200 sq ft per gallon
Worked example
A 12 × 15 ft room, 8 ft ceilings, 1 door, 2 windows, 1 coat, new drywall at 250 sq ft per gallon.
- Wall area: 2 × (12 + 15) × 8 = 432 sq ft
- Openings: 1 × 21 + 2 × 15 = 51 sq ft
- Paintable area: 432 − 51 = 381 sq ft
- Gallons: 381 ÷ 250 = 1.524 → 2 gallons
Assumptions and limitations
- Ceilings are not included. If the ceiling is being primed too, add its area (Length × Width) to the paintable area — for the worked example that is 180 sq ft, which would still round to 3 gallons at 250 sq ft per gallon.
- Door and window deductions use standard sizes (3 × 7 ft door, 3 × 5 ft window). Oversized openings should be measured and deducted exactly.
- Coverage figures are label-typical for primer on each surface; heavily patched or stained walls can drink a second coat regardless of surface type.
- Stain-blocking primer (for water marks, smoke or tannin bleed) is a different product with its own coverage — usually lower — and is not modelled here.
- Primer is counted for walls only. Trim is primed separately and uses very little product per linear foot.
Frequently asked questions
On new drywall, patched walls, stains or a big color change, primer earns its gallon: it seals porous spots so the topcoat covers evenly in two coats instead of three. On a previously painted wall in good condition in a similar color, a quality paint-and-primer product can skip a separate primer coat.
With 8 ft ceilings, 1 door and 1 window: 352 sq ft of wall minus 36 sq ft of openings = 316 sq ft. On new drywall at 250 sq ft per gallon that is 1.26 gallons — buy 2.
Primer goes onto the surfaces that absorb the most — bare drywall, fresh plaster, masonry. More of it soaks in rather than sitting on the surface, so the same gallon covers 200–300 sq ft instead of paint's roughly 350.
One coat is standard for new drywall and repaints. Use two over water stains, smoke damage, dark colors being covered with light ones, or bare masonry — and let the first coat dry fully before the second.
Yes when the topcoat is a deep or vivid color: tinting the primer toward the finish color (most stores do this free) can save a full topcoat. For whites and light neutrals, untinted white primer is fine.
Run this one first when the surface is new, patched, stained or changing color — it tells you the primer buy. Then run the Paint Calculator for the topcoat gallons; the two quantities are often close but they are different products.