Home Improvement Calculator

The arithmetic in a DIY calculation is simple. The mistakes are not, and they share a shape: you cannot buy a fraction of a unit. Paint comes in tins, tiles in boxes, concrete in bags. Working out that a room needs 3.7 litres and buying three is how a job stops with a visible line across the wall, so every quantity here rounds up to what a shop will actually sell you.

The second recurring error is that coverage figures are per coat. A tin claiming 12 m² per litre means one coat, and almost nothing covers in one — so two coats over 40 m² is 80 m² of painting. Multiplying before dividing is the difference between enough paint and a second trip.

Surface matters more than most calculators admit. Bare plaster absorbs roughly half again as much as a previously painted wall, and exterior masonry can take twice as much. Each surface here carries its own coverage figure rather than sharing one optimistic number from a tin label.

Tile wastage is not an allowance for clumsiness — it is the offcuts the layout itself produces. A straight grid wastes about 10%; a diagonal or herringbone pattern cuts every edge tile at an angle and wastes 15–20%. Buying to the exact area guarantees running short.