Calculation methodology

Formulas

All calculations convert inputs to SI units (metres, square metres, cubic metres) before any arithmetic, then convert results back to your regional units. Conversion factors are exact legal definitions (for example 1 inch = 0.0254 m).

Purchasing suggestions round quantities UP to realistic ordering increments (whole sheets, whole bags, quarter cubic units) — you cannot buy 1.63 cubic yards of concrete.

Every formula has automated tests including hand-calculated worked examples. The formula version is displayed on each calculator page and incremented on any change.

Prices and taxes

This site does not publish live material prices. Price fields are blank and editable — enter a quote from your local supplier. Where an example figure appears it is clearly labelled as an illustration, not a market average.

Tax defaults are only pre-filled where a single national statutory rate exists (UK VAT 20%, Australian GST 10%). In countries with regional variation (US sales tax, Canadian GST/HST/PST) you must enter your local rate.

What estimates are not

Estimates are not quotations, structural designs, code-compliance checks or professional advice. Wastage, site conditions, and product variations mean real projects can differ. Always confirm quantities with your supplier before ordering.