How the primer calculator works
Primer works like paint math: net wall area times coats, divided by the label coverage, plus an allowance. The difference is the coverage. Primer labels commonly list a range that depends on how porous the surface is, so use the lower end for new drywall or bare wood.
One coat is the usual starting point. Stain-blocking over heavy stains or tannin-rich wood may call for two; the product instructions decide.
Formula and assumptions
- coverage
- square feet per gallon from the primer label
- coats
- usually 1; more for heavy stains or porous surfaces
Assumptions and limits
- The 300 sq ft default is an illustrative midpoint; primer labels commonly list about 200–400 sq ft per gallon depending on porosity.
- Walls only.
Worked example
The 12 × 10 ft room with 8 ft walls and 20 sq ft of openings, one coat, 300 sq ft per gallon, 10% allowance.
- Primer area = 332 sq ft.
- 332 ÷ 300 = 1.11 gal; with 10% allowance = 1.22 gal.
- Round up: buy 2 gallons.
These are the calculator’s default values, so the live result above matches this example until you change an input.
How to measure correctly
- Measure the same walls you will paint.
- If only patches need spot priming, estimate the patched area instead of the whole wall.
- Use your primer’s stated coverage for the surface type (drywall, masonry, previously painted).
Primer Calculator FAQ
How much area does a gallon of primer cover?
Primer labels commonly list about 200–400 sq ft per gallon, sometimes more on smooth sealed surfaces. Porous surfaces such as new drywall sit at the low end. Use your product’s figure.
Do I need primer before painting?
Prime new drywall, patched areas, stains, glossy surfaces, and big color changes. A sound, clean, previously painted wall in a similar color often does not need it; follow the paint label.
Is paint-and-primer-in-one enough?
It can be for repainting sound surfaces. For bare drywall, stains, or bare wood, many manufacturers still recommend a dedicated primer.
How many coats of primer?
Usually one. Severe stains, tannin bleed, or very porous surfaces may need two; check the product directions.
Should I tint primer?
A tinted primer toward the finish color can help coverage on a dramatic color change. It does not change how much primer you need.
Can I use the same estimate for a ceiling?
Use the ceiling area (length × width) instead of the walls. The ceiling paint calculator gives that area.