A waste factor is not a hedge against sloppy sawing — it is a prediction about where cuts land. Every row of plank or tile ends at a wall, and what happens to the offcut decides the waste: in a straight lay the end cut from one row usually starts the next, so material recirculates and about 10% covers what is genuinely lost. Turn the same material 45° and every row starts and ends with an angled cut that often fits nowhere else, pushing the allowance to 15%; herringbone and parquet sit at the expensive end, 20%, because the pattern chops material into short directional pieces.
The arithmetic applies the factor to the net area before rounding to purchasable units: 280 ft² of plank laid straight plans 280 × 1.10 = 308 ft² and buys 14 boxes at 22 ft² per box, while the same floor in herringbone plans 336 ft² and needs 16. Some materials are counted on net area with no layout waste at all — underlayment, for instance, is not cut to the pattern.
And some waste is geometric rather than statistical, where no flat percentage works: broadloom carpet sells as fixed-width rolls, so a 10 × 12 ft bedroom buys a full 12-ft-wide strip and eats 20% waste by geometry alone. There the honest number is computed — (bought − laid) ÷ laid — not assumed.