How we calculate
Two kinds of number appear on this site, and they deserve different amounts of trust.
Geometry is exact
Areas and volumes are arithmetic. A rectangular slab is length times width times thickness; a cylinder is its cross-section times its height. There is no judgement in these steps and no source to cite, which is why every calculator shows its formula: you can check the result by hand.
Unit conversions are also exact. An inch is defined as 25.4 millimetres, a foot as 0.3048 metres, a pound as 0.45359237 kilograms. These are definitions rather than measurements, so we use them at full precision and never round the factors themselves.
Material figures are not exact
How much a cubic yard of gravel weighs depends on the rock, the gradation and how wet it is. How much concrete comes out of a bag depends on the product. How much a compacted base settles depends on the material and the effort put into compacting it. Anyone quoting one of these to four significant figures is inventing precision.
So we do three things instead. We show a range where the value genuinely varies. We list the figure we used, in public, on the page that used it. And we say whether anyone has actually checked that figure against the source we cite.
What the source status means
Source-recorded means the registry contains a citation and capture record for the figure. It does not assert that the value is current or replace product documentation. Needs-source-check means the figure is a working placeholder tied to a candidate authority.
A needs-review figure is a transparent starting value, not a hidden claim. It is allowed on a search-visible calculator only when its source, limitation and applicability are disclosed and the reader can replace it with product or supplier data. The build enforces those rules.
Registry inventory: 6 of 19 assumptions have source records, 13 still need a source check, across 14 registered sources.
What this site will not do
- Quote live material prices. Prices move locally and constantly; you enter your own.
- Tell you how thick a slab should be, or what reinforcement it needs. That is design.
- Present an estimate as a specification, or as a code-compliance determination.
- Hide whether a planning figure has a recorded source or still needs a source check.