How much primer do I need?
A gallon of primer covers about 200 to 300 sq ft, less than the 350 to 400 sq ft of paint. One coat of primer on a 12 × 12 ft room with 8 ft walls, one door and two windows covers 334 sq ft, which at 250 sq ft per gallon plus 10% extra is 1.47 gallons: buy 1 gallon + 2 quarts.
Worked example: a 12 × 12 ft bedroom
- Wall area: 2 × (12 + 12) × 8 = 384 sq ft.
- Subtract one door (20 sq ft) and two windows (2 × 15 sq ft): 384 − 50 = 334 sq ft.
- One coat of primer: 334 sq ft to cover.
- At 250 sq ft per gallon: 334 ÷ 250 = 1.34 gallons.
- Add 10%: 1.47 gallons, so buy 1 gallon + 2 quarts.
Primer needed by room size
One coat at 250 sq ft per gallon with 10% extra, for 8 ft walls with one door and two windows.
| Room | Wall area to prime | Gallons | Buy |
|---|---|---|---|
| 10 × 10 ft | 270 sq ft | 1.19 | 1 gallon + 1 quart |
| 12 × 12 ft | 334 sq ft | 1.47 | 1 gallon + 2 quarts |
| 12 × 14 ft | 366 sq ft | 1.61 | 2 gallons |
| 14 × 16 ft | 430 sq ft | 1.89 | 2 gallons |
| 16 × 20 ft | 526 sq ft | 2.31 | 2 gallons + 2 quarts |
| 20 × 20 ft | 590 sq ft | 2.60 | 3 gallons |
When do you need primer?
- New drywall or new joint compound, which soaks up paint unevenly.
- Bare wood, especially knotty or resinous wood that can bleed through.
- Covering a dark or bright color with a much lighter one.
- Stains from water, smoke or marker, which need a stain-blocking primer.
- Glossy surfaces, where paint needs something to grip.
Repainting a clean, sound wall in a similar color usually needs no primer, or a paint-and-primer product is enough.
How the calculator works
Wall area = 2 × (length + width) × wall height
Area to prime = wall area − (doors × 20 sq ft) − (windows × 15 sq ft)
Gallons = area to prime × coats ÷ coverage × (1 + extra %)
The result is rounded to the cans stores sell: whole gallons first, with quarts for the remainder. For the topcoat, use the paint calculator; for the ceiling, the ceiling paint calculator with coverage set to your primer.
Frequently asked questions
How much does a gallon of primer cover?
About 200 to 300 sq ft, according to Sherwin-Williams. New drywall and bare wood are at the low end.
How many coats of primer do I need?
One coat is usually enough. Use two over very dark colors, heavy stains or very porous surfaces.
Do I need primer on new drywall?
Yes. New drywall and joint compound absorb paint at different rates, so a coat of drywall primer gives an even finish.
Is paint and primer in one enough?
For repainting a sound, clean wall in a similar color, often yes. For new drywall, bare wood, stains or a big color change, use a separate primer.
Sources
- Sherwin-Williams, How to Use Paint Calculator
- Bob Vila, How Much Paint Do I Need?