Tile calculator

The grout joint is not a preference

Most tile calculators ask you to type a joint width and take whatever you give them. The tile standards do not work that way. ANSI A108.02 sets a floor under it, and the floor depends on the tile:

“In no circumstance shall the grout joint be less than 1/16”,” is how the standard puts it. The wider minimum for a large tile exists because tiles are not all exactly the same size, and the joint is what absorbs the difference. The standard says that too: the joint “shall be at least three times the actual variation of the facial dimensions of the tile supplied”.

A calibrated tile — one that comes out of the kiln at whatever size it came out at, rather than being cut to size afterwards — varies more, which is why it needs the wider joint of the two.

This page works the minimum out from what you type above it and tells you when your joint is under it. It does not refuse to answer: it is your floor.

Two readings of the same clause

The Education Foundation quotes the over-15-inch rule as applying to any tile that size. A tile consultancy reads the same clause number as applying to offset and running-bond layouts, and adds that the joint should be widened further by the warpage of the actual tiles. The standard itself is behind a paywall, so this page takes the stricter reading — it applies the minimum whatever the pattern — and tells you both views exist rather than picking one quietly.

It is not a theoretical problem. A large retailer lists a rectified porcelain tile whose own spec sheet gives its length as 23.5 inches, and suggests a grout line of “1/16, 1/8, or 3/16”. The first of those is under the standard for a tile that size.

The joint is part of the tile count

A 12 in tile with a 1/4 in joint takes up 12.25 in of floor, not 12. Dividing the floor by the bare tile over-counts, and the error grows as the joint widens. This page counts the cells the tiles really sit in, then adds the pattern’s waste on top and rounds up.

Grout, from geometry rather than a constant

There is a formula circulating for grout quantity, with a constant of about 8.25 in it. It is not on any manufacturer’s page. Custom Building Products, Laticrete and TEC all publish coverage charts instead, and Laticrete says outright to use their own estimator rather than a formula.

So this page does the part that is arithmetic and stops. The grout is whatever the tiles leave uncovered, as deep as the tiles are thick:

grout volume = floor area × (1 − tile area ÷ cell area) × tile thickness

That is checkable, and it has been checked. Run it over every row of Custom Building Products’ published chart — tiles from 6 to 24 inches, two joint widths, two thicknesses — and every row agrees that one 25 lb bag holds about 6.33 litres, within half a per cent. Laticrete’s chart agrees with itself the same way, on 2.32 litres for an 8 lb bag.

Those two bags imply different densities, 1.79 kg per litre against 1.56. That is exactly why a single constant cannot be right for both products, and why this page gives you a volume and asks for your own bag’s coverage rather than inventing a bag count.

Waste

Ten per cent is Daltile’s published figure: “approximately 10% more tile than you anticipate needing”. For a diagonal or a herringbone this page offers 15%, which is not a published number — it is ours, because those patterns cut two edges off tiles a straight lay would have left whole. It is marked as an assumption on the page and you can change it.

The layout and its cut list

Below the order, the page lays each rectangular room tile by tile and draws it, with every cut piece shaded. Then it lists the cuts: which size, how many, how many come out of one tile, and so how many tiles they take. That is the waste for your room worked out rather than guessed, and the page sets its count beside the order by area and says so when the order falls short.

The grid starts one of two ways.

• Centred puts the same cut at both walls, and picks whichever centre line keeps that cut biggest. It is Daltile’s method: mark the centre of the room, and “If this layout leaves cuts smaller than 1/2 tile at walls, adjust the center line by snapping a new line 1/2 tile closer to the wall.”
• From a wall keeps whole tiles along one wall and puts the whole cut at the far one. The page says so when that cut is under half a tile.

A brick bond is the same rows with each one shifted along its length, so a shifted row has a cut at both ends. The shift is half a tile for most tiles and a third for long ones:

The standard’s own words are that “the running bond offset will be a maximum of 33% unless otherwise specified by the tile manufacturer”, and the makers above specify it from 15 in. The page starts at a third from 15 in, lets you choose a half, and says whose limit that passes.

A diagonal or herringbone floor has no grid to draw here: its tiles are counted by area in the order, with that pattern’s extra. And each group of identical pieces is cut from tiles of its own, so the count errs towards an extra tile. Using one group’s offcuts for another saves a little more.