Work out how much concrete, gravel or topsoil a job needs — with the density figures, bag yields and conversions written out so you can check the answer yourself.
Most material calculators give you a number and nothing else. You cannot see which density they assumed, whether they added a waste allowance, or why the figure differs from what your supplier quotes. When the answer is wrong you find out on delivery day.
These calculators publish the numbers they use. Every density range, bag yield and conversion factor is printed on the page with a named source, and each one shows a worked example you can repeat on paper. If our figure disagrees with your supplier's, you can see exactly where the difference comes from.
Three things move the real number. Subgrade: ground that has been over-excavated or left soft absorbs more than the nominal dimensions suggest. Compaction: loose aggregate settles, so the volume you order is larger than the volume you end up with. Supplier increments: ready-mix is sold in quarter-yard steps and bulk aggregate by the ton, so you round up to how the material is actually sold, not to how it was calculated.
Each calculator has a waste allowance you can set, and each page explains what allowance is normal for that material and why.
Figures are printed on the page that uses them with the source named, so you can open the document and check. That coverage is currently uneven — the gravel and topsoil pages cite a document, edition and section per row; the concrete page does not yet. The About page says plainly which pages have been independently fact-checked and which have not.
The calculators are plain client-side arithmetic. No data leaves your browser and nothing is stored. If you find a figure you think is wrong, corrections are welcome.