Wall Putty Calculator

Filler coverage is quoted at 1 mm on a flat, sealed surface. Real walls after plastering deviate by 3 to 5 mm over 2 m, and the filler goes into that deviation rather than into a uniform layer. That is why the substrate matters more than the brand on the bag.

Ready-mixed filler goes on up to 3 mm per pass, powdered gypsum up to 5 mm. Beyond that it shrinks, cracks and shells off, so a 5 mm correction is two passes, and two passes is roughly double the material of the single-layer figure.

The first coat takes the geometry, so its quantity is set by the deepest point of the wall. The second coat is a skim of 1 to 1.5 mm that closes the surface for paint. Adding the 1 mm rate twice and calling it 2 mm ignores the fact that the first coat is never 1 mm.

Sand between coats and prime after the first. Filler that has not been sealed pulls moisture out of the second coat unevenly, and the result shows through a silk paint as patches.

Area × first-coat thickness × rate gives the first coat; area × 1.5 mm × rate gives the second; add 10 % and divide by the bag weight. For 45 m² of wall, a 3 mm first coat and 1 kg/m² per mm, that is 135 kg plus 67.5 kg — about eight 25 kg bags.

Buy per room rather than for the whole flat. An open bag absorbs moisture and sets into lumps within weeks, so the last room should be bought when it is about to be done, not with the first delivery.