Primer Calculator
How many gallons of primer you need
A 12 by 12 ft room with 8 ft walls, one door and two windows needs 1.21 gallons of primer for one coat, so buy 2. Primer covers noticeably less than finish paint because it soaks into the surface, and new drywall or bare wood takes more again. Enter your own walls to find how many gallons to buy.
How much primer do I need?
Add up the length of every wall, multiply by the height, subtract the doors and windows, then divide by the coverage on the can. One coat is normally enough; two only for heavy stains or a very porous surface.
The number people get wrong: primer does not cover as far as paint. Buying the same gallon count for both leaves you short on the primer coat.
Primer coverage per gallon
Coverage depends almost entirely on how porous the surface is. These are the practical ranges rather than the best case printed on the front of the can.
| Surface | Coverage per gallon | Why |
|---|---|---|
| Previously painted, sound | 300 to 350 ft² | Sealed already, little absorption |
| New drywall | 200 to 250 ft² | Paper and joint compound drink it in |
| Bare wood | 200 to 250 ft² | Grain absorbs unevenly |
| Skim-coated plaster | 250 to 300 ft² | Smooth but porous |
| Masonry or block | 100 to 200 ft² | Texture and heavy absorption |
| Glossy or previously oil | 300 to 400 ft² | Bond coat only, nothing absorbs |
Paint primer coverage per gallon is quoted optimistically on most cans, so this calculator uses 275 ft² per gallon as a working mid-point. If you're priming new drywall, drop it toward 225 and buy accordingly.
How much primer for new drywall
New drywall is the job that most often needs priming and the one where coverage is worst. Raw paper and dried joint compound absorb at different rates, so paint applied straight onto them dries patchy and flashes at every seam.
| Room | Wall area (8 ft ceiling) | Less openings | Primer |
|---|---|---|---|
| 10 × 12 ft | 352 ft² | 301 ft² | 1.3 gal |
| 12 × 12 ft | 384 ft² | 333 ft² | 1.5 gal |
| 12 × 15 ft | 432 ft² | 381 ft² | 1.7 gal |
| 15 × 20 ft | 560 ft² | 509 ft² | 2.3 gal |
Openings assume one door (21 ft²) and two windows (15 ft² each), the calculator defaults. A drywall primer or PVA sealer is cheaper per gallon than a general-purpose one and is formulated for exactly this.
How much a gallon of primer costs
Primer pricing is simpler than paint. Standard interior latex primer runs about $15 to $30 per gallon. Stain-blocking primers cost $25 to $45, and shellac-based products, which are the ones that genuinely stop smoke, water and marker bleeding through, run $40 to $60.
Priming is almost always cheaper than the alternative: primer costs less per gallon than finish paint, and one primer coat frequently saves a third finish coat on a colour change. Enter a price above to see the total.
When primer is worth it
Prime new drywall, bare wood, patches and repairs, glossy surfaces needing a bond coat, and stains like water marks, smoke or marker that bleed through. Over a sound wall in a similar colour you can usually skip it entirely.
Primer vs. paint-and-primer-in-one
"Paint and primer in one" is a thicker self-priming paint. It's fine over a sound, similar-coloured wall, and no substitute for dedicated primer on new, bare or stained surfaces. On new drywall it will still flash at the seams.
Tinting matters too: having the store tint your primer toward the finish colour, especially for deep or bold shades, lets the topcoat cover in fewer coats. Most primers accept a partial dose of colorant.
Primer mistakes
- Buying primer and paint in equal amounts. Primer covers less; you'll run short.
- Using the best-case coverage figure from the front of the can on a porous surface.
- Skipping primer on new drywall. The seams flash and no number of finish coats fully hides it.
- Latex primer over a heavy stain. Water and smoke need shellac; latex lets them bleed through.
- Not tinting for a deep colour. White primer under a dark red costs you an extra finish coat.
What the primer estimate assumes
Area is total wall length × height, minus openings, multiplied by coats. Openings deduct a standard 21 ft² per door and 15 ft² per window.
- Coverage defaults to 275 ft² per gallon, deliberately below the 300 to 400 ft² printed on common latex primers. Label figures assume a sealed surface, and primer mostly goes onto porous ones. Finish paint is modelled at 350 because it goes over a primed wall.
- Porous surfaces cover far less. New drywall, bare wood and masonry can drop real coverage to 200 ft² or below, which is why the range on the can is wide.
- Always check your own can. Shellac and masonry primers differ substantially from standard latex.
Both unit systems. Metric mode works in metres and litres: each door deducts 1.95 m² and each window 1.39 m² (the same openings), 275 ft² per gallon becomes 6.75 m² per litre, and paint is bought in 2.5 L tins.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Primer Calculator: frequently asked questions
How much does a gallon of primer cover?
Labels on common latex primers print 300 to 400 ft² per gallon, but that assumes a sealed surface. On typical interior walls plan on 250 to 300, which is why this calculator uses 275. New drywall and bare wood drop it to 200 to 250, and masonry can be as low as 100 to 200, because porous surfaces absorb primer rather than letting it sit on top.
How much primer do I need for new drywall?
Work on about 225 ft² per gallon rather than the figure on the can. A 12 by 12 ft room with 8 ft ceilings is 333 ft² after deducting a door and two windows, so about 1.5 gallons for one coat.
How much is a gallon of primer?
Standard interior latex primer is around $15 to $30 per gallon. Stain-blocking primers run $25 to $45, and shellac-based products, the ones that actually stop smoke and water bleeding through, are $40 to $60.
Does primer cover as much as paint?
On the label it looks similar, but in practice no, and this is the most common mistake. Primer goes onto the porous surfaces that soak it up, so it covers far less than the finish coat that sits on top of it. Buying equal amounts of both leaves you short on primer.
Do I need primer before painting?
Yes for new drywall, bare wood, patches, glossy surfaces and any stain that could bleed through. Over a sound wall in a similar colour you can usually skip it and save the coat.
Can I tint primer to match my paint?
Yes, and you should for deep or bold colours. Having the store tint the primer toward the finish shade lets the topcoat cover in fewer coats, which often saves more than the tinting costs.
How many coats of primer do I need?
One is enough for most jobs, including new drywall. Use two only over heavy stains, very porous masonry, or when going from a very dark colour to a very light one.