The formula
tiles = area × (1 + allowance) ÷ area of one tile
Where it comes from
The allowance goes onto the area before dividing, not onto the tile count afterwards. The order matters: rounding first and adding the allowance second almost always produces one tile too many, and where tiles are priced by the box that can mean a whole unnecessary box.
How to work it out by hand
- Work out the area of one tile in square metres
- Add the allowance for cuts to the area you are covering
- Divide by the area of one tile
- Round up and work out the boxes
What is worth knowing
A 10 % allowance is enough for straight laying in a regular room. Go to 15 % for diagonal work, for rooms with many corners, or where the tile has a pattern that has to line up. Always buy from the same batch and keep three or four spare: two years later, finding the same reference to replace a cracked one is next to impossible.