Primer Calculator
Calculate how many gallons of primer you need for a room's walls before painting, plus total cost at your price per gallon.
How this calculator works
Primer typically covers about 300 square feet per gallon — slightly less than finish paint, since it’s formulated to soak into and seal a more porous surface. This calculator multiplies wall length by wall height to get total area, then divides by 300 sq ft per gallon.
Formula: Gallons = (Total wall length ft × Wall height ft) ÷ 300 sq ft per gallon.
Worked example
A room with 40 ft of total wall length and an 8 ft ceiling (320 sq ft), $25/gallon:
- Wall area: 40 × 8 = 320 sq ft
- Gallons: 320 ÷ 300 ≈ 1.07 gallons
- At $25/gallon, that’s $26.67 for one coat.
Coverage reference
| Total wall area | Gallons needed (1 coat) |
|---|---|
| 80 sq ft | 0.27 gallons |
| 320 sq ft | 1.07 gallons |
| 1,280 sq ft | 4.27 gallons |
How to measure your room
Add up the length of every wall you’re priming and multiply by ceiling height, the same way you would for a paint calculation. New or heavily patched drywall may absorb more than the 300 sq ft/gallon standard rate, so round your final gallon count up rather than down if the surface is especially porous.
Frequently asked questions
How much wall area does one gallon of primer cover?
About 300 square feet per gallon, slightly less than finish paint's typical 350 sq ft/gallon rate, because primer is formulated to be more absorbent and adhesive rather than fully sealed and washable. This calculator uses the 300 sq ft/gallon figure.
Why does new drywall need more primer than a repainted wall?
Fresh drywall paper and joint compound are highly porous and will soak up primer unevenly, especially over patched or sanded seams — some painters plan for a second primer coat or a higher-build primer on new construction specifically to even out that absorption before topcoat paint goes on.
Can I skip primer if I'm using a self-priming paint?
Sometimes — self-priming (paint-and-primer-in-one) products can work for a same-color repaint on an already-painted, sound wall, but they're not a substitute for real primer over bare drywall, patched areas, stains, or a drastic color change, where a dedicated primer coat gives more reliable, even coverage.
Does primer need to match the topcoat's sheen or color?
No — primer's job is adhesion and sealing, not final appearance, so any interior primer works under any finish sheen. Tinting the primer toward your topcoat color (many stores will tint primer gray or a mid-tone) can reduce how many topcoat coats you need, especially over a dramatic color change.
Why is there no waste toggle for primer?
Primer is rolled on at a fairly consistent, thin, single-coat rate across the whole wall with minimal cutting or piecing waste, unlike a material like tile or shingles that's cut to fit edges — so this calculator gives you the straight coverage math without an arbitrary waste percentage.
Estimates only. Verify quantities with your supplier before purchasing.