Understanding your result
The headline is the primer for the coats you chose, and the sub-line is what to buy and which product class the surface needs: PVA for drywall, high-hide for a colour change, bonding for gloss, stain-blocking for stains. The three figures show the area, the coverage actually used (the typical figure for the surface, or your can’s figure if you typed one) and the cans.
The breakdown shows the wall area with the openings taken out, the ceiling if you included it, the surface and its product, the typical coverage for that surface, and then the usual per-coat, allowance, total and can lines. Comparing the “typical coverage for this surface” row with the “coverage per coat” row tells you whether your can’s figure was used.
The finish paint is a separate quantity. The wall paint calculator takes the same room and gives the paint; the accent wall calculator and the cabinet paint calculator include their own primer lines because those jobs nearly always need it. If you are unsure whether the surface needs primer at all, the coats calculator answers that first.
How we calculate this
The area is measured the same way as for wall paint: perimeter times height with the standard openings subtracted (20 sq ft per door, 15 per window, from the standard sizes page), plus the ceiling as length times width if you tick it. The typical coverages come from primer data sheets, which quote lower figures for porous surfaces: about 300 sq ft per gallon for bare drywall, stain blockers and high-hide primers laid on thick, 350 for patched walls and bonding primers, 400 for a sound, previously painted wall. The coverage rates table lists them with the products they come from.
Your can’s figure, if you enter one, replaces the typical one for the arithmetic but not for the “typical” row, so you can see both. The primer guide explains why each surface gets its own product, and when the honest answer is no primer at all.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| New drywall, PVA primer | 300 sq ft per gallon | PVA drywall primer data sheets (Kilz PVA, Sherwin-Williams, Behr) quote 300–400 on bare drywall; the low end allows for the joint compound |
| Patched or skim-coated | 350 sq ft per gallon | Multi-purpose latex primer data sheets, 350–400 |
| Previously painted, sound | 400 sq ft per gallon | The upper figure on the same sheets for sealed surfaces |
| Dark colour going light | 300 sq ft per gallon | High-hide primers (Kilz 2 All-Purpose, Zinsser Bulls Eye 1-2-3) quote 300–400; tinted and applied full, use the low end |
| Glossy or hard to stick to | 350 sq ft per gallon | Bonding primers (Zinsser Bulls Eye 1-2-3, Kilz Adhesion, Sherwin-Williams Extreme Bond) quote 300–400 |
| Stains | 300 sq ft per gallon | Stain-blocking primers (Zinsser B-I-N shellac, Kilz Original oil) quote 300–400 |
| Coats | 1 | Manufacturers’ recommendation for all but heavy stains |
| Allowance | 5% | Roller loss and a touch-up jar |
Assumptions last reviewed October 7, 2026.
The calculator does not model bare wood, which drinks far more than drywall and is a different product (wood primer or oil primer on tannin-rich woods), or exterior surfaces, which the exterior calculators handle with their own factors. It does not decide whether you need primer: the coats guide and the coats calculator do that from the colour, sheen and surface.
Two worked examples
A newly drywalled bedroom
A 12 × 12 room with 8 ft walls, one door, one window, fresh drywall with the seams taped and sanded, walls only.
- Walls: 2 × (12 + 12) × 8 − 20 − 15 = 349 sq ft
- Surface: new drywall, PVA primer at 300 sq ft per gallon
- Per coat: 349 ÷ 300 = 1.163 gallons
- Needed: 1.163 × 1 × 1.05 = 1.22 gallons, buy 1 gallon + 1 quart
PVA primer is cheap enough that most people buy two gallons and prime the ceiling and closet with the second. Ticking the ceiling adds 144 sq ft and brings the figure to 1.73 gallons.
Dark green walls and ceiling going pale, two coats of tinted primer
Same room, ceiling included, the whole surface going from a deep green to an off-white.
- Area: 349 + 144 = 493 sq ft
- Surface: dark colour going light, high-hide primer at 300 sq ft per gallon, tinted
- Per coat: 493 ÷ 300 = 1.643 gallons
- Needed: 1.643 × 2 × 1.05 = 3.45 gallons, buy 3 gallons + 2 quarts
Two coats of primer is the expensive end of priming, and it is still cheaper than the three or four coats of finish paint the change would take without it. For comparison, 800 sq ft of glossy trim and doors given one coat of a bonding primer whose can says 400 sq ft per gallon comes to exactly 2 gallons, and the result shows the typical 350 alongside the 400 you typed.
Where to find your inputs
Room dimensions and openings. As for wall paint: perimeter times height, with every door and window counted. Tick the ceiling if it is new drywall or stained; previously painted ceilings in a similar colour do not need priming.
Surface. Look and touch. Bare drywall is grey paper with white seams. Patches are the white or pink spots of filler. Glossy means the old paint shines and water beads on it. Stains are yellow or brown rings (water), an overall yellow film (smoke, nicotine) or pink-brown bleed on woodwork (tannin). If several apply, choose the most demanding: stains over gloss means a stain blocker that also bonds, usually shellac.
Coverage from your can. Optional. Printed on the back of the can or on the data sheet; use it when it differs from the typical figure.
Coats. One unless the surface says otherwise. The stain note on the result says when two are needed.
Common mistakes
- Using PVA primer as a stain blocker. It seals drywall and nothing else. Stains need shellac or oil.
- Priming over gloss without sanding. Bonding primer helps, but it is not a substitute for a scuff-sand or a de-glosser.
- Skipping primer on patches. The filler absorbs paint differently and the patch “flashes” as a dull spot through every coat. Spot-prime it.
- Priming everything out of caution. Sound, similar-colour walls do not need it; the money is better spent on a second finish coat.
- Forgetting to tint. Grey primer under a dark colour, or white under a light colour over dark walls, saves a whole finish coat.
- Painting over primer a week later. Many primers have a recoat window; after it, they need a light sand. Check the can.
Questions people ask
- How much primer for a 12 × 12 room?
- With 8 ft walls, a door and a window, the room has 349 sq ft of wall. New drywall takes PVA primer at about 300 sq ft per gallon, so one coat is 1.16 gallons, 1.22 with a 5% allowance; buy a gallon and a quart. Adding the ceiling makes it 493 sq ft and 1.73 gallons.
- Do I need primer on previously painted walls?
- Usually not, if the paint is sound and the new colour is similar. Spot-prime the patches so they do not flash, and use two coats of a self-priming paint. Prime the whole wall when the colour changes a lot, when the old paint is glossy, when it is stained, or when it has been washed down to bare patches.
- Which primer for new drywall?
- A PVA (polyvinyl acetate) drywall primer, the cheapest in the store, applied in one coat. It seals the paper and the joint compound so they absorb the finish paint evenly. It is not a stain blocker and it does not stick to gloss; use it only on bare drywall.
- Which primer covers water stains?
- A stain-blocking primer, oil-based or shellac-based for serious water, smoke or tannin stains, water-based for light ones. Ordinary primer and extra coats of paint let the stain bleed back through within weeks. Spot-prime the stain, then prime the whole surface if the sheen would show the patch.
- Can I tint the primer?
- Yes, and you should for any big colour change. The store adds colourant so the primer is grey or a muted version of the finish colour, and the first finish coat then has far less to hide. It saves a coat of paint for the price of a little colourant.
- How many coats of primer?
- One, nearly always. Two for heavy stains, for a very dark colour going light, or for bare wood that drinks the first coat. More than two is a sign the wrong primer was chosen.