How Much Primer Do I Need?

Don't buy the same amount as the paint

	Coverage per gallon
Paint	300–400 (350 is the working figure)
Primer	200–300

At 350 against 250, the same wall needs 40% more primer than paint.

A 12×12 room — 344 sq ft after deducting a door and a window — is 0.98 gallons of paint and 1.38 gallons of primer. Buy one gallon of each and you run out of primer with a third of the room to go.

That's the whole reason this has its own calculator, and why it shows both figures side by side.

Why it covers less

Because of what it's for.

Primer is formulated to soak into and seal a porous surface rather than sit on top of one. So a genuinely thirsty surface drinks it — new drywall, bare wood, plaster and masonry absorb far more than a previously painted wall does.

On bare work, use the low end of the range. That absorption is the primer doing its job, not a defect.

And your tin's own figure beats any default, including this one. Primers sold for a specific problem — stain blocking, adhesion to gloss, masonry sealing — state their own rate. Read it and enter that.

Measuring the wall

Enter the perimeter (add up the walls you're priming — a 12×12 room is 48 ft) and the wall height, then deduct doors and windows.

Rough figures for the deduction: a door is about 21 sq ft, an average window about 15. Being approximate here is fine — the deduction is smaller than the difference between the top and bottom of the coverage range, so precision in it buys you nothing.

What's worth being exact about is which walls. Priming three walls and not the fourth is common enough (a feature wall, a chimney breast) and it's an easy quarter of the job to over-buy.

For a small job the quarts output is the useful one. A door, a patch of repaired plaster or a single window reveal rarely needs a gallon, and primer is sold in quart tins — buying a gallon for a door is how half-used tins accumulate in a garage until they're unusable.

When you actually need it

Yes, not optional:

• New drywall, bare wood, plaster, masonry. Unprimed, these absorb the topcoat unevenly and the finish stays patchy however many coats go on.
• Stains, smoke, water marks, knots — and specifically a stain-blocking primer.
• Glossy or slick surfaces, where the problem is adhesion rather than absorption.
• A big colour change, especially dark to light — a tinted primer does this far more cheaply than three coats of finish paint.

Usually not: repainting a sound, clean, previously painted wall in a similar colour. Washing it matters more than priming it.

One coat is usually right

Primer is a preparation layer, not a finish. Two coats is unusual outside severe stain blocking or very porous masonry.

If it still looks patchy after one coat, that's normal. The topcoat is what evens it out. Adding a second coat of primer because the wall looks uneven is usually spending money to solve a problem the paint was going to solve anyway.

Stains really do come back

Sometimes months later, which is what makes this worth taking seriously.

Water marks, smoke damage, tannin bleeding from knots in bare wood, and some inks will come through ordinary paint and ordinary primer alike. They need a stain-blocking primer — usually shellac or oil-based.

The common failure is spot-priming only the visible mark when the surrounding area is affected too. If in doubt, prime the whole surface.

Self-priming paint

A real product, and an oversold one.

On a sound, previously painted wall in a similar colour it genuinely does the job and saves a step.

On new drywall, bare wood, or over a stain it does not replace a dedicated primer, whatever the tin claims. Stain blocking in particular is a specific chemistry, not a thicker paint. The usual outcome of trusting it on bare work is more coats of expensive finish paint than a cheap primer would have cost.

Print the priming plan — it has a "do you actually need it?" table, because the first decision is whether to prime at all.

Sources: Clare — paint coverage · KILZ primer calculator