Primer Calculator
Work out how much primer the walls of a room need — area with openings deducted, coats, and your primer's own coverage.
Primer is the coat nobody sees and everybody skips — until the topcoat peels. This calculator works out the primer quantity properly, with the lower coverage that primer actually achieves.
Use it for new plaster, bare drywall, or any surface the paint needs to grip.
Your results
Enter your measurements and press Calculate.
Quantities are rounded up to whole tiles, planks, boxes or bags, after the wastage allowance. Cost appears only when you enter a price.
Formula
All lengths are converted to a single unit before the calculation: 1 ft = 0.3048 m, 1 in = 25.4 mm, 1 ft² = 0.09290304 m², 1 yd² = 9 ft². Material quantities are rounded up — you cannot buy part of a tile, plank or bag.
How to use this calculator
- Measure the room and count the doors and windows.
- Enter the coats — usually 1 for primer.
- Enter the coverage from the tin (primer covers less than paint).
- Add a price per gallon or litre if you want a cost estimate.
Worked example
| Room | 12 ft × 15 ft × 8 ft high |
|---|---|
| Paintable area | 432 − 51 = 381 ft² |
| Primer (1 coat, 300 ft²/gal) | 381 × 1 ÷ 300 = 1.27 gallons |
| To buy | ⌈1.27⌉ = 2 gallons |
Check it by hand: every step above uses the formula in the box. If your numbers match the example inputs but the calculator gives a different answer, please report it via the contact page.
Frequently asked questions
Do I need primer before painting?
On new plaster, bare drywall, bare wood or a big colour change — yes. Primer seals porous surfaces so the topcoat covers evenly and does not peel. Over sound paint in the same colour, you can usually skip it.
How much primer do I need?
Primer covers less than paint — often 200–300 sq ft per gallon (5–7 m² per litre) — because it soaks into the surface. Enter the coverage from the tin and this calculator does the rest.
How many coats of primer?
One coat is usually enough. Two coats are needed on very porous surfaces (new plaster, skimmed walls) or over strong stains.
Can I use paint instead of primer?
Not on bare surfaces — paint soaks in unevenly and peels. Primer is formulated to seal and bond; paint is formulated to finish. They are different products for a reason.
Related calculators
Estimates only. Results are for planning. Measure twice, confirm box coverage and tile sizes on the packaging you buy, and follow the manufacturer's instructions. See the disclaimer.