How the primer calculator works
A primer does not behave like a finish coat. Its job is to soak into a porous substrate, lock down dust, seal knots or rust and give the topcoat an even, grippy base, so the amount it spreads over depends far more on what is underneath than on the label. Thirsty new plaster can drink twice as much as a sealed, previously painted wall. That is why this tool asks for the substrate rather than a single coverage figure.
Primer = area × coats ÷ spread rate for that substrate × 1.10Imperial inputs are converted to square metres (1 ft² = 0.0929 m²), the result is computed in litres and then turned back into US gallons (1 gal = 3.785 L). The 10% on top covers the roller sleeve, which alone holds a fair amount of primer, cutting-in with a brush and patches that visibly absorb more.
Worked example
A newly skimmed living room has 40 m² of wall. Bare plaster takes about 7 m² per litre, so one coat needs 40 ÷ 7 = 5.71 L, or 6.29 L with the allowance. In 2.5 L tins that is three tins (7.5 L); a 5 L plus a 2.5 L tin gives the same 7.5 L and is often cheaper. The imperial equivalent: 430 ft² of fresh drywall is about 39.9 m². At 9 m² per litre (roughly 370 ft² per gallon) one coat takes 4.9 L, which is 1.29 gallons, so one gallon is not enough: buy two, or one gallon plus two quarts.
Spread rates by substrate
| Substrate | Rate used | Suitable primer type |
|---|---|---|
| New bare plaster | 7 m²/L (about 285 ft²/gal) | Watered-down contract matt as a mist coat, or a plaster sealer |
| New drywall, taped joints | 9 m²/L (about 365 ft²/gal) | PVA or acrylic drywall primer |
| Bare softwood, MDF | 10 m²/L (about 405 ft²/gal) | Acrylic or alkyd wood primer; shellac spot-prime on knots |
| Bare metal | 9 m²/L (about 365 ft²/gal) | Rust-inhibiting primer; etch primer on galvanised steel |
| Stains over old paint | 11 m²/L (about 450 ft²/gal) | Stain-blocking or shellac-based primer |
| Brick, block, render | 5 m²/L (about 200 ft²/gal) | Masonry stabiliser or alkali-resistant primer |
These are practical averages; if your tin states a lower rate for porous surfaces, trust the tin.
When primer is worth it, and when it is not
- Always on bare surfaces. Unsealed joint compound flashes through the topcoat as dull patches, and raw MDF edges swell and fur up without a sealing coat.
- Spot-priming saves money. Water marks, smoke stains and marker only need a stain blocker on the affected patch, feathered out by 10 cm (4 in), not the whole wall.
- Tint the primer grey under deep reds, navy or charcoal. It can save a full extra topcoat.
- Match the chemistry. Glossy oil-based trim needs sanding and a bonding primer before a water-based enamel will stick to it.
- Respect recoat times. Alkyd and shellac products need longer before topcoating than acrylics; painting too soon traps solvent and wrinkles the finish.
- Ventilate. Solvent and shellac primers give off strong fumes; keep windows open and follow the safety sheet on the tin.
Frequently asked questions
How much primer do I need for a 12x12 room?
A 12 ft × 12 ft room with 8 ft walls has about 384 ft² of wall. On new drywall at roughly 365 ft² per gallon, one coat plus 10% takes about 1.2 gallons, so buy one gallon and a quart or two gallons.
How many square metres does a litre of primer cover?
Around 9 to 11 m² on drywall or sealed surfaces, but only 5 to 7 m² on bare plaster, brick or render, which absorb much more.
Do I need one or two coats of primer?
One coat is normal. Use two on bare masonry, over heavy stains that bleed through, or on resinous knots in softwood.
Can I use paint instead of primer on new plaster?
Yes, a mist coat of contract matt emulsion thinned with about 10 to 20% water is a common alternative to a dedicated plaster sealer; vinyl paints are not suitable for this.
Is primer cheaper than paint?
Usually yes, per litre. Priming porous or patchy surfaces lets the more expensive topcoat spread at its full rate, which often saves a whole tin of finish paint.
Last reviewed: 2026-09-27 · How we calculate