How the numbers are produced
- Every calculator shows its formula, a worked example and the assumptions behind the result.
- Constants such as bag yields carry a source and the date it was checked. Pure geometry is labelled as math, not sourced.
- Waste and similar factors are visible inputs. The result names every factor it applied.
- Only items sold whole, such as bags and sheets, are rounded up. Bulk volumes are shown exactly.
Math and sourced constants
We separate two kinds of number. Geometry, such as the area of a rectangle or the volume of a cylinder, is plain math and needs no source. Anything that depends on a product or a trade convention, such as how much a bag of concrete yields or how big a sheet of drywall is, comes from a manufacturer, a trade association or a university extension service, and is recorded with the date we checked it.
Units and rounding
Calculations are done in your chosen units and converted only at the end, so switching between US and metric does not change the result beyond display rounding. Volumes that you buy by the cubic yard are shown to two decimal places. Counted items, such as bags, sheets and pavers, are rounded up to a whole number, because you cannot buy part of one.
Estimates, not advice
Results are estimates for planning a purchase. Real jobs lose material to uneven ground, spillage, cuts and breakage, and suppliers differ in what they sell and how they measure it. Waste allowances are inputs you can change, and the default is a starting point, not a promise. Nothing here replaces local building codes, a supplier’s specifications or the judgement of a qualified professional for structural work.
Checking our own work
Each calculator is tested against a shared set of worked cases, and the same cases run against both the version in your browser and the version on our server, so the two cannot drift apart. The cases for each tool are shown on its page.
Sources and review dates
Every constant, its source and the date it was last checked is listed below.
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Corrections
When we change a number or a source, it is logged here with the date. To report a mistake, use the contact page.
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