Primer calculator
Primer evens out the absorption of a wall so the next layer — skim coat or paint — bonds well and dries evenly. It's cheap, but running out halfway through a room means the walls absorb paint unevenly.
Enter the room and the coverage of the ready-to-use (diluted) primer.
Your measurements
Result
- Wall area (one coat)
- 33.40 m²
- Total area to cover
- 33.40 m²
- Primer needed (ready to apply)
- 4.2 L
For a concentrate, divide by the dilution on the label: 4.2 L of mix at 1:4 is about 0.85 L of concentrate.
How the calculation works
- Wall area = 2 × (length + width) × height, minus doors and windows.
- Area to cover = wall area × number of coats (usually one).
- Litres needed = area to cover ÷ coverage of the diluted primer (m²/L).
Worked example
For a 4 × 3 m room with 2.6 m walls, 33.4 m² remain after deducting the door and window. One coat at 8 m² per litre is 33.4 ÷ 8 = 4.2 litres of ready-to-use primer.
What the calculator assumes
- 8 m²/L is an average for a wall with normal absorption; on aerated concrete, fresh plaster or very absorbent skim coat it can drop to 5–6 m²/L.
- One coat is enough on most surfaces; very absorbent or dusty ones need two.
- The ceiling is not included.
When to adjust the result
- Concentrated primer is diluted as per the label: 4.2 L of mix at 1:4 is about 0.85 L of concentrate.
- Primers are not interchangeable: quartz-sand primer goes on smooth surfaces before skimming or tiling, absorption-control primer before paint.
Choosing the right primer
Primer is not one product. A penetrating primer consolidates dusty or chalky plaster; a sealer evens out absorption on new plaster or filled patches; an adhesion primer gets paint to stick to tiles, glossy paint or laminate; a stain-blocking primer stops water marks, smoke or tannins bleeding through.
New plaster must be fully dry before priming — often several weeks — and many manufacturers recommend a thinned first coat (a mist coat) of emulsion. Do a simple tape test on old paint: if paint comes off with the tape, it needs removing or a suitable primer before repainting.
Frequently asked questions
Do I really need primer before painting?
On new plaster, fresh skim coat, repaired patches and chalky old paint, yes — otherwise the paint is absorbed unevenly and you need more coats. On a sound, previously painted wall it's often not needed.
How much primer per square metre?
Typically 0.1–0.2 litres of ready-to-use primer per square metre, depending on how absorbent the wall is.
Is primer always needed?
Not on sound, previously painted walls in the same type of paint. It is needed on new plaster, filler, stains, glossy or very absorbent surfaces.
Can I use paint as primer?
A thinned mist coat of emulsion works on new plaster; for tiles, gloss or stains use a dedicated primer.