Know what to buy, and what it will cost

63 free calculators that finish the job — quantity, purchase units, waste allowance and a price you can take to the merchant.

How we calculate

Every figure comes from a manufacturer data sheet or a published trade standard, cited on the page that uses it. Purchase quantities always round up, because you cannot buy part of a bag. Where a material's density genuinely varies by supplier, the range is shown rather than quietly averaged.

Read the methodology