How We Calculate

Every formula on this site is published on the page that uses it. Here is why, and what our rules are.

Last updated: 11 August 2026

Most material calculators are a black box. You put in a room size, a number comes out, and there is no way to tell whether it accounted for the grout joint, what waste factor it used, or whether it thinks your tile is sold by the box.

We do the opposite. Every calculator on this site publishes four things.

1. The formula

Written out in full, on the page, in the same form the calculator executes. If you want to check the result by hand, you can. If you think we have it wrong, you can point at the line.

2. The assumptions

Every calculator has a section headed What this calculator assumes. It lists the judgement calls: whether the grout joint is included in the coverage, what waste allowance a given layout gets, whether the perimeter has an expansion gap, what happens on the last row.

These are where estimates go wrong, so they are stated rather than buried.

3. Where the numbers come from

Where a figure comes from a published standard — the IRC for spans and stair geometry, ANSI A108 for tile coverage — we name it.

Where a figure is a typical trade allowance rather than a standard, we say that too. Waste percentages by layout are a good example: there is no authority that decrees herringbone gets 20%. It is what experienced tile setters use, and calling it a specification would be dishonest.

4. Whether the manufacturer beats us

For consumables — grout, thinset, adhesives — coverage varies between products, and the printed chart on the bag includes real-world losses that pure geometry does not.

On those pages we calculate the geometry, show you the coverage our answer implies, and tell you plainly that if the bag disagrees, the bag wins. Several of those calculators have an override field so you can enter the manufacturer's figure and have it take precedence.

We would rather give you a number you can interrogate than a confident one you cannot.

What we round, and which way

Materials are sold in indivisible units, so almost everything rounds up: whole tiles, whole boxes, whole sheets, whole bags. Where a calculation rounds twice — tiles and then boxes — the result can jump more than you expect, and we flag it where that happens.

Where a count could plausibly be rounded down, we still round up. Being one sheet over costs a few dollars. Being one sheet short costs a day.

Units

The site is imperial-first because it is written for the United States and, where the terminology matches, the United Kingdom. Feet, inches and fractions are the working units, and every measurement field accepts what you would actually write: 12, 12', 12' 6", 12ft 6in, 150" and 12 6 all mean the same thing.

Conversion happens inside the calculation engine, never in the text, so nothing gets converted twice.

Corrections

If a formula here is wrong, we want to know, and we will fix it and say so.

hello@materialsmath.com