About this calculator
This calculator works out how much primer to buy before plastering, putty, paint, tiles or a floor screed. List the surfaces — each with its length, height and how absorbent it is — or type a total area. The three substrate classes each have their own consumption, because a gypsum ceiling soaks up far more primer than a dense concrete wall.
For a primer sold as a concentrate, the calculator also splits the mixed primer into concentrate to buy and water to add.
Formula
primer (l) = Σ area × consumption of its substrate (l/m²) × coats
with waste = primer × (1 + waste %)
containers = ceil(with waste / container size)
concentrate 1 : N
concentrate = with waste / (1 + N)
water = with waste − concentrate
containers = ceil(concentrate / container size)
Worked example
Room walls 14 × 2.6 m on cement plaster (normal, 0.15 l/m²) and a 4 × 3 m gypsum ceiling (absorbent, 0.30 l/m²), one coat, 10 % waste, 5 l containers:
walls 36.4 m² × 0.15 = 5.46 l
ceiling 12.0 m² × 0.30 = 3.60 l
total 9.06 l; +10 % = 9.97 l
containers = ceil(9.97 / 5) = 2
The same job with a concentrate diluted 1 : 4 needs 9.97 / 5 = 1.99 l of concentrate (two 1 l bottles) and 7.97 l of water.
FAQ
Which substrate class should I choose? Strongly absorbent: aerated concrete, gypsum plaster, lime plaster, old chalky surfaces. Normally absorbent: cement plaster, plasterboard, screed. Weakly absorbent: dense or smooth concrete, old paint. If a drop of water soaks in at once, the surface is strongly absorbent.
Where do the default consumptions come from? They are values chosen inside the ranges primer data sheets give: Ceresit CT 17 states 0.1–0.5 l/m² depending on how even and absorbent the substrate is; Knauf Tiefengrund 70–100 ml/m² per coat on plaster; Meffert's deep primer 100–200 ml/m² on smooth, slightly absorbent surfaces. No sheet states one figure per class, so check your own product and, if in doubt, measure the consumption on a small area.
How many coats? Usually one. Manufacturers ask for another coat if the surface still soaks up primer after the first one dries; Knauf recommends two coats wet on wet on strongly absorbent surfaces.
What about dense concrete or old paint? A deep-penetrating primer cannot soak in there. Such surfaces are normally treated with a contact primer containing quartz sand, whose consumption is on its own data sheet — enter it as the weakly absorbent consumption.
How is a concentrate diluted? As the data sheet says: Ceresit CT 17 Concentrate, for example, from 1 : 4 to 1 : 9 depending on the substrate. Consumption is entered for the diluted, ready-to-use primer.
Which standard applies? None. Primer consumption is a product figure; the quantity is area × consumption × coats.
Assumptions and limits
No standard is cited: the take-off is plain area × consumption.
Defaults to replace with your product's figures: consumptions 0.30 / 0.15 / 0.10 l/m² per coat (chosen within data-sheet ranges), one coat, 5 l containers, a 1 : 4 concentrate and 10 % waste.
Area: surfaces are rectangles; openings are not deducted — enter wall pieces between openings if you want them left out. All surfaces get the same number of coats.