Calculators that answer in things you can buy

The problem this site exists to fix

Search for a material calculator and most of what comes back is published by somebody who sells the material. Those tools are usually well made and often accurate. But they share a structural limitation that no amount of good faith gets around: they cannot tell you to buy less.

A merchant's calculator will not tell you that the half-pallet in your garage covers the job. It will not tell you to order a truckload instead of ninety-four bags, or that 50 mm of topsoil over an existing border is plenty when you asked for 250. It will not tell you that the deep bedding layer you are planning is the reason your patio will go wavy, and that the fix is less sand and more sub-base.

Every one of those is a sentence that costs the publisher money. All of them appear on this site, because advertising pays for it and the advertising does not care how much you order.

• Answers in purchase units, never raw quantities. Bags, tins, bundles, boards, pallets, boxes, tonnes, cubic yards — whatever the supplier actually sells. A result ending in "3.7 m³" is an unfinished answer.
• No prices, ever. Prices go stale within months and vary 30–50% by region. Bake one in and the calculator is wrong and confidently so. Instead the site accepts your price: type in what you were quoted, and it does the arithmetic on your number, on the day, for your supplier.
• Quantities never round down. Anything you might compare against a requirement rounds up, every time. Running short mid-job costs a second trip, a mismatched batch or a cold joint. A little spare costs a little money.

Every calculator rests on assumed figures — how far a litre of paint goes, what a bag of concrete yields, what a cubic metre of topsoil weighs. Most calculators keep those hidden. Ours are all published, each with its source and the reason that value was chosen over the alternatives.

• We prefer primary sources. Standards bodies, manufacturer data sheets and measured studies, ahead of trade rules of thumb — and where only a rule of thumb exists, the page says so.
• We round against ourselves. Where a genuine range exists and being short is costly, we take the cautious end and tell you that we did.
• We do not average away disagreement. When two credible sources give different figures, that usually means they are describing different material in different conditions. Splitting the difference produces a number that describes neither.
• We correct in public. Figures that turn out to be wrong get changed and the change is written up on the sources page, including what we had before. Two densities and a bag weight have been corrected that way already.
• Your product's label always wins. Every figure here is a default for when you do not have the product in front of you. Every calculator lets you type in the number on your tin, bag or box — and when you do, ours is ignored entirely.

A tool that never says "don't rely on me here" is not being straight with you.

• Anything structural needs an engineer, not a web page. Foundations, retaining walls, load-bearing framing, anything holding back ground or holding up a building. These calculators will get you a quantity for ordering and budgeting. They are not a design and they are not a check on one.
• Building control has the final say. Where a figure touches a code limit — stair geometry, baluster spacing — we show the limit we checked against. Codes vary by jurisdiction and get amended, and your inspector's copy is the one that counts.
• Densities move with the weather. Loose materials weigh more wet. Where a yard sells by weight, that changes what turns up. We say so on the pages where it matters.
• Some figures genuinely do not exist as single numbers. Tile adhesive coverage swings with trowel notch, tile size, joint width and substrate flatness. Rather than invent an average, those pages ask for the inputs and compute from them.

The site is static files. There is no application server, no database, no login and no user data — the arithmetic runs in your own browser and nothing you type is transmitted anywhere. That is not a privacy feature bolted on afterwards; it is simply how the thing is built, and it is why the privacy page is as short as it is about our own side of things.

Every calculator shares one engine, so a correction to the arithmetic reaches every page at once rather than being fixed on the page somebody happened to complain about.

Length Width Depth is built and maintained in the United Kingdom by a small independent publisher. It is not owned by, funded by, or affiliated with any builders' merchant, manufacturer or supplier, and it carries no affiliate links or sponsored placements. Its only income is the display advertising on the page.

The calculators cover both British and American practice, because the units, the product names and the standards genuinely differ and a tool that quietly assumes one market will be wrong in the other.

Tell us. A bad density produces a bad answer no matter how good the arithmetic, and figures are the part of this site most worth checking — which is exactly why they are all published with their sources. If your supplier's data sheet disagrees with ours, theirs is right for your product, and we would rather correct ours than keep publishing something that sends someone home short. The contact page has the address; there is no form, and no tracking on it.