Three numbers matter
Net area is the surface you intend to cover. Required area includes your chosen waste allowance. Purchased coverage is what you actually receive after rounding to whole packs. These numbers can be different without the arithmetic being wrong.
At 2.2 m² per pack: 9 packs cover exactly 19.8 m².
At 2.5 m² per pack: 8 packs cover 20 m².
Choose waste for your layout
There is no percentage that fits every installation. Long straight runs, diagonal layouts, borders, repeated patterns, damaged units and future repairs have different demands. The calculators start at 10% as an editable example, not a guarantee. Check your cut plan and supplier’s guidance.
Do not spend the same allowance twice
If a supplier’s quantity already includes waste, do not add the same percentage again without a reason. For paving, spare paving units and loose-volume allowance for ground layers are separate. A 10% paver allowance should not automatically increase the base by 10%.
Offcuts are not automatically reusable
A short piece may be too small, the wrong pattern or missing a required connection edge. The deck calculator buys full stock boards for each row and does not assume offcuts can be shared. A detailed cutting plan may reduce that conservative estimate.