How We Check the Numbers

We are not licensed contractors, engineers or building inspectors, and we do not claim to be. So rather than ask you to take our word for anything, this page sets out exactly what we do to every number on this site — and how to check it yourself.

Every figure traces to a published source

No coverage rate, spacing rule or conversion factor on this site is invented or copied from another calculator. Each one comes from one of three places:

• Building code. The International Residential Code sets stair riser and tread limits, minimum footing dimensions, stud spacing rules and much else. Where a calculator applies a code limit, it names the section.
• Industry standards and trade bodies. The Tile Council of North America handbook for adhesive and grout, the Interlocking Concrete Pavement Institute for paver base depths, the Metal Construction Association for panel coverage, ANSI and ASTM for material specifications.
• Manufacturer coverage data. Bag yields, spread rates per trowel notch, sheet sizes and package counts, taken from the published product data rather than from memory.

The sources used are listed at the bottom of every calculator page, under Where these figures come from. Not a generic list at the end of the site — the specific ones that page relies on.

Every calculator is recomputed independently

This is the part that actually catches mistakes, and it is unusual enough to be worth explaining.

Each calculator's arithmetic is written twice. Once as the tool you use on the page, and once separately, from the source figures, as its own program that shares no code with the first version. The two are then run against each other on a set of test cases and compared value by value.

If they disagree, one of them is wrong, and the page does not go live until we know which. At the time of writing that comparison covers 61 separate checks across the calculator library.

One we caught

The paver calculator originally had the 4 × 8 inch paver at 0.44 square feet. The correct figure is 32 square inches divided by 144, which is 0.22 — exactly half.

Nobody reading the page would have spotted it. The tool would have told you a 110 square foot patio needed 250 pavers when it actually needs 496, and you would have found out when you ran out halfway through laying it. The independent recomputation disagreed, we checked, and it was corrected before the page was published. That is the entire point of doing the arithmetic twice.

The tables are generated from the tool, not typed

Most calculator sites have a written example or a reference table somewhere on the page that quietly disagrees with the calculator above it — usually because the tool was updated and the prose was not.

Every table and worked example here is produced from the calculator's own formula, so the two cannot drift apart. When a figure changes, the tables change with it.

Every page is tested in a real browser

Before a calculator goes live it is loaded in an actual browser and checked automatically for a working primary result, a visible breakdown of the arithmetic, a sources list, at least six answered questions, and zero JavaScript errors. It is also loaded at phone width and checked for anything spilling off the side of the screen, because most people use these standing in a store aisle.

What happens to what you type

Every calculation runs inside your own browser. Your measurements are not sent to a server, not stored, and not seen by us.

There is one exception, and it only happens if you ask for it. If you use the Send me this list box on a calculator page, that one calculation is sent to us so we can put it in the email — the numbers you entered, the quantities that came out, and your email address. We keep the address so we can send it and so you can unsubscribe. We do not keep the measurements afterwards. Ignore that box and nothing you type ever leaves your device.

A practical side effect of running in the browser: the calculators keep working on a bad signal at the back of a lumber yard, which is where they are most needed.

What we cannot do for you

Being clear about the limits is part of being trustworthy about the rest.

• We cannot see your site. Soil conditions, what a wall is carrying, whether a joist is sound — those need somebody physically present.
• Codes are local. We cite the model code, but your jurisdiction may amend it, and frost depth in particular is entirely local. Your building department is the authority, not us.
• Products vary. We use standard package sizes and typical coverage. The bag in your hand carries the number that actually applies. Check it.
• Anything structural needs a professional. Header sizing, span tables, retaining walls over a few feet, load-bearing changes. We will tell you when a question has crossed that line rather than guess at it.

When we get it wrong

We will get something wrong eventually. Everyone who publishes numbers does.

There is a Think a number here is wrong? link at the bottom of every calculator that reports the specific page you were on. Every report gets read and checked against the source. If the page is wrong we fix it and change the date at the top, so you can see when it last changed. If the page is right we will write back and explain why — that reply is often more useful than the correction would have been.

What we will not do is quietly edit a number and pretend it was always that way.

How the site is paid for

Advertising, and nothing else. We do not sell anything, we are not paid by any manufacturer or retailer, and no figure on this site is chosen to make you buy more of something.

That matters more than it sounds for a site like this, because the easiest way to monetize material calculators would be to inflate the quantities slightly. The waste allowances here are the trade standard figures, every one of them is editable, and the arithmetic is shown on the page so you can see exactly where each number came from.

Questions about any of this are welcome — the contact page reaches us directly.