How to avoid buying more material than you actually need

A small reserve over the exact area — usually 5–10% for materials like tiles or flooring, to cover cuts and any defective pieces — is reasonable caution, not waste. Buying 'just in case' with a much larger margin and no clear technical reason turns a useful precaution into an unnecessary expense.

An exact calculation of the real area, taking into account the room's shape and any niches or awkward corners, is the correct base on which to add a reasonable safety margin. Estimating by eye without precise measurements often leads either to buying too little (needing a second order, possibly from another batch) or too much 'to be safe'.

Buying far more than needed as a 'reserve for the future' carries the risk that the remainder won't match anything available in a few years (batch change, discontinued model), reducing the real usefulness of that excessive reserve. Money spent on unjustified surplus stays tied up in stock that's rarely used up.

Materials with complex cuts (tiles on a surface with many corners, flooring laid diagonally) need a bigger margin than those on simple rectangular surfaces. Asking the supplier or installer for an explicit recommendation based on the real complexity of the surface gives a more precise estimate than a generic rule applied the same way to every project.