Paint Primer Calculator
To prime a 12 ft wide, 9 ft tall wall with 1 coat you need about 108 sq ft of coverage (with 10% waste) — 1 gallon, based on 250 sq ft of coverage per gallon per coat.
Reviewed by The ponderloop teamUpdated 2026-07-11How we calculate
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0 sq ft + 0% waste = 0 sq ft
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How this number is calculated
0 sq ft (no waste added) ÷ 250 sq ft per gallon = 0 gallons
Shopping list
- Drywall primer-sealer
- Stain-blocking primer
- Roller kit (roller, tray, frame)
- Angled sash paintbrush
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Methodology & assumptions
Wall area is wall width times wall height, minus any door/window openings. That area is divided by coverage per gallon per coat — a 250 sq ft/gal baseline, slightly below typical topcoat figures since primer runs thinner-bodied and porous surfaces absorb more — divided by your number of coats, then multiplied by your chosen waste percentage and rounded up to the next whole gallon.
- Manufacturer coverage specs (~250 sq ft/gal per coat, drywall/stain-blocking primer)
- Typical US primer retail price range, 2026
Worked examples
| Room | Result |
|---|---|
| 12 × 9 ft wall, 1 coat, +10% waste | 108 sq ft + 10% waste ≈ 118.8 sq ft → 1 gallon |
| 30 × 9 ft wall, 2 coats (bare drywall), exact | 270 sq ft, no waste → 3 gallons |
| 18 × 8 ft bedroom wall, 1 coat, +10% waste | 144 sq ft + 10% waste ≈ 158.4 sq ft → 1 gallon |
Frequently asked questions
- How many gallons of primer do I need?
- Multiply the wall width by height in feet to get the square footage, subtract any doors and windows, then add 10% for touch-ups if you want a buffer. Divide by the coverage per gallon per coat (about 250 sq ft) divided by your number of coats, and round up to the next whole gallon.
- Do I actually need to prime before painting?
- Priming is worth it in a few common cases: bare or new drywall (unprimed drywall paper soaks up topcoat unevenly and looks patchy without primer), water or smoke stains you want to block from bleeding through, a big color change (especially going from dark to light, where primer cuts the number of topcoats needed), and glossy or previously oil-painted surfaces where primer helps the new coat adhere. Repainting a similar color over sound existing paint usually does not need primer.
- Does the number of coats change how much primer I need?
- Yes — coverage per gallon is divided by the number of coats. Primer is usually applied as a single coat (this calculator's default), but heavy stains, very porous new drywall, or a drastic color change can call for a second coat, which roughly doubles the gallons needed for the same wall area.
- Is primer coverage the same as paint coverage?
- Roughly similar, but primer is often a bit thinner-bodied and can run slightly lower than premium paint on porous surfaces like bare drywall or raw wood. 250 sq ft per gallon is a reasonable single-coat baseline; very absorbent surfaces may cover less.
- Should I round up or down when figuring out how many gallons to buy?
- Always round up to the next full gallon. Running out mid-wall means a trip back to the store before you can even start on your topcoat.