Paint guide
How much primer do I need?
Estimate primer the same way as paint (area × coats ÷ coverage), but use the coverage on the primer label. Primer often covers less area per gallon than a topcoat, especially on porous surfaces.
When primer is part of the job
- New drywall and fresh joint compound.
- Patched or repaired areas.
- Water, smoke, or tannin stains.
- Glossy surfaces that need better adhesion.
- Big color changes, especially dark to light.
A clean, sound wall that is already painted a similar color often does not need a separate primer. Follow the directions on your paint.
The formula
For a 12 × 10 ft room with 8 ft walls and 20 sq ft of openings, the area is 332 sq ft. At 300 sq ft per gallon, one coat needs 1.11 gal, or 1.22 gal with a 10% allowance, so buy 2 gallons. If your label lists 400 sq ft per gallon, it is 0.91 gal and one gallon may be enough.
Why primer coverage varies
Primer labels commonly list around 200–400 sq ft per gallon, and the spread depends heavily on porosity. Bare drywall paper and new compound soak up more than a previously painted wall. KILZ, for example, lists a range for its PVA drywall primer that depends on the surface rather than one number.
One coat or two?
One coat is the usual plan. Heavy stains or bleeding wood may call for two, and the product instructions decide. Spot-priming patches uses far less than priming the whole wall: estimate just the patched area.
Then estimate the topcoat with the paint calculator.
References: KILZ PVA Drywall Primer; KILZ 2 All-Purpose product page