Paint Primer Calculator
Calculate how many gallons of paint primer you need for a room from the wall area, doors, windows and the surface: painted walls, new drywall, bare wood or a dark color.
By CalcBuilt Editorial TeamUpdated September 11, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Primer needed
- 1.11 gal
- 1 coat at 300 sq ft per gallon
- Primer to buy
- 1 gallon + 1 quart
- Rounded up to whole gallons and quarts
- Wall area
- 334 sq ft
- After deducting 1 door and 2 windows
- Area to prime
- 334 sq ft
- 1 coat
Paint Primer formula
Wall area = 2 x (Length + Width) x Height - (Doors x 20) - (Windows x 15)
Effective coverage = Coverage per gallon x 0.8 for bare wood or patched walls
Gallons = Wall area x Coats / Effective coverage
Coats: 1 for painted walls and new drywall, 2 for a dark to light color changeWorked example
A 15 x 12 ft bedroom with 9 ft ceilings, 2 doors and 2 windows, walls heavily patched: 2 x 27 x 9 = 486 sq ft, minus 40 for doors and 30 for windows = 416 sq ft. Patched and bare areas absorb more, so coverage drops from 300 to 240 sq ft per gallon. 416 / 240 = 1.73 gallons, so buy 1 gallon and 3 quarts, or simply 2 gallons.
If instead the walls were dark red going to off-white, two coats at 300 sq ft per gallon: 416 x 2 / 300 = 2.77 gallons, so buy 3 gallons.
Quick reference
| Room size | Wall area | Gallons per coat | Buy for 1 coat |
|---|---|---|---|
| 10 x 10 ft | 270 sq ft | 0.90 | 1 gallon |
| 10 x 12 ft | 302 sq ft | 1.01 | 1 gallon + 1 quart |
| 12 x 12 ft | 334 sq ft | 1.11 | 1 gallon + 1 quart |
| 12 x 15 ft | 382 sq ft | 1.27 | 1 gallon + 2 quarts |
| 14 x 16 ft | 430 sq ft | 1.43 | 1 gallon + 2 quarts |
| 15 x 20 ft | 510 sq ft | 1.70 | 1 gallon + 3 quarts |
| 20 x 20 ft | 590 sq ft | 1.97 | 2 gallons |
How to calculate primer
Primer is estimated the same way as paint: find the wall area by multiplying the perimeter by the ceiling height, subtract about 20 square feet per door and 15 per window, and divide by the coverage on the can. Primer typically covers less than finish paint, around 300 square feet per gallon, and porous surfaces pull it in faster. Bare wood, fresh joint compound and unpainted plaster can cut coverage by 20 percent or more.
One coat is normal. New drywall gets one coat of PVA primer (KILZ PVA or any can labeled drywall PVA) to even out the suction between paper and joint compound. Without it the seams flash as dull stripes in anything above flat. Going dark to light, or white to a deep red, yellow or orange, plan on two coats of primer or one coat tinted toward the new color. Paint stores tint primer to a numbered gray shade matched to the topcoat, which saves two coats of finish.
Which primer to use
Pick the primer by the problem, not the brand. New drywall: PVA. Sound painted walls, patches and color changes: a general latex primer such as Bulls Eye 1-2-3. Glossy paint, oil enamel, laminate, tile or factory cabinet finishes: scuff with 220 grit, wipe, then a bonding primer (STIX, KILZ Adhesion). Water stains, smoke, markers and tannin bleed from cedar, redwood and pine knots: shellac (Zinsser BIN) blocks everything and dries in 45 minutes. It has strong fumes, so use it for spots; oil-based (Cover Stain, KILZ Original) is the whole-wall choice for smoke and for trim.
Skip the primer only when the existing paint is sound, the same sheen and a similar color. Paint-and-primer in one is a marketing term for thicker paint; it works on that same sound wall and nowhere else. Patches are the usual failure: fresh joint compound is more porous than the paint around it, so it drinks the topcoat and shows as a dull halo called flashing. Spot-prime every patch with the same primer, and on eggshell or higher with many patches, prime the whole wall.
What this calculator leaves out
Wall primer only. Not included: the ceiling, which is its own area and usually gets flat ceiling paint with no primer when the old paint is sound. Trim, doors and casings are separate too: about a quart of bonding or oil primer per 100 square feet, each door side about 20 square feet. The finish paint is in the paint calculator. Prep is the bigger cost: a TSP-substitute wash for grease and nicotine, spackle and sanding, caulk at the trim, and drop cloths. A quart of shellac for spot stains is typically $20 to $30 in 2026 and cleans up with ammonia or denatured alcohol, not water.
Mistakes that cost money
- Latex primer over a water stain, cedar tannin or a pine knot bleeds through two topcoats; use shellac (Zinsser BIN) or oil-based (Cover Stain).
- White primer under a deep red, yellow or orange means four topcoats; have the primer tinted to the gray shade the paint chip calls for.
- Paint over unprimed new drywall: paper and compound absorb differently, so every seam flashes through eggshell and satin, and more paint does not fix it.
- Priming over glossy trim, laminate or oil enamel without scuff sanding and a bonding primer; the new coat peels off in sheets within months.
- Spot-priming patches with a different product than the wall leaves halos in any sheen above flat; prime the whole wall with one product.
- Trusting paint-and-primer in one on new drywall, patches or stains: it is thicker paint, not primer, and the seams flash and stains bleed anyway.
Key facts
- Most primers cover 250 to 400 sq ft per gallon; PVA drywall primer is at the low end because bare drywall soaks it up.
- One coat of primer is enough for new drywall and for repainting a similar color; a dark to light change usually needs 2 coats or a tinted primer.
- A standard interior door is deducted as 20 sq ft and an average window as 15 sq ft.
- Latex primer is dry to the touch in about 30 minutes and can be topcoated in 1 to 3 hours; shellac and oil primers need longer.
- Tinting primer toward the finish color can save a full coat of paint on a strong color change.
Frequently asked questions
How much primer do I need for a 12x12 room?
With 8 foot ceilings, one door and two windows the room has 334 square feet of wall. At 300 square feet per gallon that is 1.11 gallons for one coat, so buy a gallon and a quart. A dark to light color change needs two coats, 2.23 gallons, so buy 2 gallons and a quart.
Can I skip primer if I use paint and primer in one?
Only on a sound, previously painted wall in a similar color, where you could skip primer anyway. Paint-and-primer is thicker paint, not a primer. New drywall still needs PVA, patches still flash without spot priming, stains still bleed without shellac or oil, and glossy surfaces still need a bonding primer.
Do I need two coats of primer?
One coat is enough for new drywall or repainting a similar color. Use two coats, or a tinted primer, when covering a dark or strong color with a light one, and over heavy stains.
Do I need to prime new drywall?
Yes. Bare drywall paper and joint compound absorb paint at different rates, so without a PVA primer the seams and patches flash through as dull stripes.
Sources and references
- KILZ 2 All-Purpose Primer product page and coverage (KILZ)
- KILZ PVA Drywall Primer product page and coverage (KILZ)
- Zinsser Bulls Eye 1-2-3 Primer product page (Rust-Oleum (Zinsser))
Next steps for this project
The usual order for a paint & walls project. See all paint & walls calculators.
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.