Always buy more than the calculation says
These figures are theoretical volumes. Real sites lose material to cuts, spillage, breakage and mistakes, which is why a wastage allowance of 10% is standard for tiles and 5% for concrete — more for complex layouts or diagonal tile patterns.
For tiles specifically, buy from a single batch. Colour varies subtly between production runs, so a replacement bought a year later will rarely match.
Paint coverage varies more than the tin admits
The 10 m² per litre default suits a smooth, previously painted wall. Bare plaster, textured render and porous masonry can drop that to 6 m² or less on the first coat, and dark-to-light colour changes often need three coats rather than two.
Windows and doors are not deducted here. On a room with large openings, subtract their area from the wall figure before buying.
Frequently asked questions
What mix ratio does the concrete estimate use? +
A nominal 1:2:4 by volume — one part cement, two sand, four aggregate — with a 1.54 dry-volume factor. Structural work should follow an engineer's specified mix, not a rule of thumb.