Primer Calculator

About the Primer Calculator

Primer is the coat that nobody sees and that decides how everything on top of it looks. It seals a surface so topcoats go on evenly, grip properly and cover in fewer coats. Skipping it on the wrong surface is the usual reason for patchy paint, peeling on new plaster, or stains bleeding through a fresh finish.

Working out how much primer to buy is like working out paint, with two differences. It is usually one coat. And its coverage is lower, because it goes onto the thirstiest surfaces — bare plaster, new plasterboard, raw wood and masonry — which soak up far more liquid than a sealed, painted wall.

This primer calculator works out the area from the room's size, applies the coverage for the surface you choose, and tells you which cans to buy. For new plaster, it can instead work out a mist coat — thinned emulsion — including how much water to add.

How to Use the Primer Calculator

Choose metres or feet and enter the room's length, width and wall height, and the area of doors and windows.

Tick include the ceiling if you are priming it too.

Pick the surface: new drywall, new plaster, bare wood, masonry, or stain-blocking over existing paint. Each has a typical coverage, which you can override with the figure from the tin.

Tick mist coat if you are sealing new plaster with thinned emulsion, and set the percentage of water.

How the Primer Is Worked Out

  area       =  walls − openings + ceiling
  primer     =  area ÷ coverage             (one coat)
  mist coat  =  that volume, made of paint + water in the chosen ratio

Step-by-Step Example

A 4 × 3 m room, 2.4 m walls, new plaster, mist coat with 15% water.

  Area:          2 × (4 + 3) × 2.4        =  33.6 m²
  Mist coat:     33.6 ÷ 8 m²/L            =  4.2 L
  Of which paint: 4.2 ÷ 1.15              =  3.7 L
  And water:     4.2 − 3.7                =  0.5 L
  Buy:           1 × 2.5 L + 2 × 1 L of matt emulsion

Typical Coverage by Surface

Very porous or rough surfaces can take even more; the tin's own figure should win when you have one.

Primer or Mist Coat?

On new plaster, many decorators skip dedicated primer and use a mist coat: ordinary matt emulsion thinned with 10 to 20% water. The thinned paint soaks into the plaster and seals it, then the full-strength coats go on top. Only use a plain, breathable matt emulsion for this; vinyl, silk or wipeable paints can form a skin that later peels.

On plasterboard, a drywall primer seals the paper and the jointing compound equally, so the joints do not show through as dull stripes — a problem called flashing.

On bare wood, a wood primer seals the grain and helps paint adhere; knots may need a knotting solution first to stop resin bleeding through.

Over stains, smoke or strong colours, a stain-blocking primer stops them showing through the new paint.

Let New Plaster Dry First

Fresh plaster is full of water. It needs to dry completely before being sealed, or the moisture is trapped behind the paint and can cause blistering and peeling. Dry plaster is pale and even in colour, without darker patches; drying typically takes two to four weeks, longer for thick coats or in cold, damp weather. Good ventilation and gentle heat help; rushing it with fierce heat can crack the surface.

Applying Primer and Mist Coats

Stir thoroughly. Primers settle in the tin, and the solids at the bottom are what do the sealing.

Work it in. Primer should be brushed or rolled well into the surface rather than laid on thickly; the aim is to seal, not to build up a film.

Keep a wet edge. Work in sections so each area blends into the last before it dries, avoiding lap marks that can show through the topcoat.

Let it dry fully. Follow the drying time on the tin before the first topcoat. Mist coats on plaster usually need several hours, longer in cold weather.

Check the result. A properly sealed wall has an even sheen with no dull, thirsty patches. Any patches that still look dry may need a second touch of primer.

Sand lightly if needed. A quick pass with fine paper removes any raised grain or nibs, giving the topcoat a smooth base.

Oil-Based or Water-Based

Water-based primers dry quickly, smell less and clean up with water, and suit most walls. Oil-based and shellac primers block stubborn stains and knots better but need ventilation and solvent for cleaning.

Understanding Your Result

Primer needed is the total primer, or the paint for a mist coat.

Cans to buy gives the combination of can sizes.

Mix shows the water to add for a mist coat.

Area shows the area and coverage used.

Worth knowing covers drying times and tinted primers.

When Should You Use This Calculator?

After plastering or boarding a room, before the first paint.

When painting bare wood, such as new doors or skirting.

Covering stains or a strong colour before redecorating.

Planning a renovation, alongside the paint calculator.

Common Mistakes

Priming damp plaster. Wait until it is pale and even.

Using vinyl emulsion for a mist coat. Use plain matt.

Assuming primer covers like paint. It covers less.

Skipping primer on plasterboard joints. They show through.

Thinning too much. Over 20% water leaves too little binder to seal.