Primer Calculator — How Many Gallons of Primer Do I Need?
Enter your room size, surface condition and coats to get primer gallons for walls and ceilings, with doors and windows subtracted. No sign-up, no data stored.
Primer needed
How primer quantity is estimated
- Wall area (sq ft) = 2 x (length + width) x wall height.
- Prime area = wall area - doors and windows.
- Coverage = 250 sq ft/gal for normal surfaces, 200 for new drywall, 150 for stained or repaired walls.
- Gallons = prime area x coats ÷ coverage.
New drywall is thirsty: the paper face drinks primer, so coverage drops from 250 to about 200 sq ft per gallon. Stained or heavily patched walls need a stain-blocking primer and often a second coat.
Room size → primer needed reference
8-ft ceilings, 40 sq ft of openings, standard primer at 250 sq ft/gal, one coat.
| Room | Wall area | Prime area | Primer |
|---|---|---|---|
| 10x10 bedroom | 320 sq ft | 280 sq ft | 2 gal |
| 10x12 bedroom | 352 sq ft | 312 sq ft | 2 gal |
| 12x12 bedroom | 384 sq ft | 344 sq ft | 2 gal |
| 12x16 living room | 448 sq ft | 408 sq ft | 2 gal |
| 14x20 living room | 544 sq ft | 504 sq ft | 3 gal |
Last updated: October 2026 · Coverage varies by brand and roller nap. Add a quart when you paint ceilings or have tall walls.
Primer FAQ
For bare drywall, patched repairs, and color changes, yes - primer seals the surface so the topcoat covers evenly and lasts. Paint-and-primer-in-one saves a step on repaints, but new drywall still benefits from a dedicated primer coat.
Water stains, smoke damage and marker need a stain-blocking primer - shellac-based or an acrylic stain-blocker. Standard wall primer lets the stain bleed through within months.
You often spend more in the long run: skipping primer on bare drywall can take 3 coats of topcoat instead of 2, and patch spots flash through the sheen. Primer is the cheap layer; topcoat is the expensive one.
Latex primer is usually dry to the touch in 30-60 minutes and recoatable in about 1-2 hours. Oil and shellac primers take 2-4 hours or more - read the label and wait for full cure before the topcoat.
Yes - a flat or uneven primer base shows through gloss topcoats. Use the recommended primer for your topcoat sheen, and sand lightly between primer and paint for a smooth finish.
Related calculators & guides
How Many Gallons of Primer Do I Need?
Coverage chart, surface types and when to use stain-blocking primer.
Read the guide →