How we calculate

Every number on this site is produced the same way: a pure calculation function, fed constants that live in one file, printed through one formatting layer. No figure is typed into a page by hand, which means a wrong constant has exactly one place to be corrected and cannot survive in a corner of the site nobody looks at.

Where the constants come from

Densities, bag yields, coverage rates and waste factors are taken from manufacturer data sheets and published trade standards, and each is cited on the page that uses it. We deliberately do not aggregate figures found across the web. A density averaged from a dozen unattributed sources looks authoritative and is accountable to nobody; a figure attributed to a named product sheet can be checked, disputed and corrected.

Citations on this site carry the source they came from. Where a URL and retrieval date are still blank, that is a figure awaiting verification against the published document rather than one we have quietly decided to trust. We would rather show you an unfinished citation than a confident one we cannot stand behind.

Rounding

Volumes and areas are shown to two decimal places. Purchase quantities always round up, without exception. You cannot buy 61.1 bags of concrete, and a calculator that reports 61 is telling you to make a second trip. Rounding happens once, at the end — never part-way through a calculation, and never per element before totalling, which would inflate a multi-part job.

Waste allowances

A waste allowance is not padding. It is the material that genuinely does not end up in the finished work: concrete left in the drum, offcuts too small to hang, tile corners cut away at a diagonal border. Each allowance here comes from trade practice for that specific material and job, which is why they differ — a straight tile lay takes ten percent and a herringbone takes twenty, and giving both the same figure would be wrong twice.

Uncertainty

Some materials genuinely do not have a single density. Crushed stone runs from 2,600 to 2,900 pounds per cubic yard depending on the rock and how wet it is. Screened topsoil varies more, and has no published typical figure at all. Where a range exists we calculate at the published typical and show you the spread. Where no typical exists we show no weight and ask for your supplier's number, because averaging the ends of a range produces a figure that is not true of any actual material.

What this site will not do

These are quantity calculators. They will not size a beam, check a span, rate a fixing, or tell you how thick a slab needs to be to carry a load. Structural questions are governed by local building codes and by engineers, and a calculator that answered them would be doing real harm. Where a figure depends on code — slab depth, fastener spacing, frost depth — we say so and send you to the code rather than guessing.

Who writes this, and how it is checked

MaterialMath is a small independent site. It sells nothing, takes no affiliate commission from any supplier, and is funded by display advertising alone — so it has no reason to tell you to buy more than you need, and every reason to be the calculator you come back to.

The calculators are written and maintained by Team MaterialMath, which is how every page is bylined. We have chosen not to put an individual's name and qualifications on the site, and it is worth being straight about what that costs you: a named expert you can look up is a stronger reason to trust a number than a team name is, and this page does not pretend otherwise.

What replaces it is meant to be better than a name, not a substitute for one. Every physical constant here is attributed to the document it came from, with a link, so you can check the figure against the manufacturer rather than against us. Everything below describes a process you can inspect. A credential asks to be believed; a citation asks to be checked, and we would rather be checked.

Every calculator is built against the manufacturer data sheet or published standard it cites, and the figures it uses are listed at the bottom of the page with a link to the source. Where two publishers disagree, we use the one whose product the answer is actually buying — taking a coverage rate from one and a container size from another produces a number neither of them stands behind.

The arithmetic is covered by an automated test suite that runs on every change: known-answer tests per calculator, and cross-cutting properties checked over randomised jobs — that a bigger job never buys less, that more waste never buys less, and that no answer ever tells you to buy too little. Every input on every form is swept to prove it actually changes the answer, which is how a field that looked wired and was not gets caught.

Each page carries the date it was published and the date it was last checked, both taken from the version history rather than from a clock, so neither can move without the work having been done. When a figure turns out to be wrong we change it and say so on the page rather than quietly editing it — there is a note on the brick and joint compound pages doing exactly that.

If you find a figure you think is wrong, please tell us. Include the source you are working from; a citation we can check is worth more than a correction we cannot.