Tile Layout Calculator

Work out where the cuts land before you set the first tile — tiles across and down, the size of each edge cut, and how many boxes to buy, with the grout joint counted properly.

The tile is not the size on the box

A 12 × 24 in tile is not 12 × 24 in. It is 12 × 24 in nominal, which is a mold size, and the tile that comes out of the box has been fired and may be a sixteenth of an inch or two off in either direction. That is not a defect. It is the reason grout joints exist, and it is why the number you lay out with is not the number on the label.

The number you lay out with is the module: the tile plus one joint.

• Module = tile size + grout joint width

This looks like a rounding detail and it is not. A 12 in tile with a 3/16 in joint has a module of 12 3/16 in, and across a 12 ft run that is eleven joints — 2 1/16 in of drift between the first course and the last. Nearly two inches, on a floor that a tape measure will tell you is a whole number of feet. Lay out on the nominal size and the last course lands somewhere you did not plan, which is exactly how a floor ends up with a sliver of tile along one wall.

Here is what the joint is worth at each of the common widths, over that same 12 ft run:

Which width to use is not a free choice. It is printed on the adhesive bag, and it is a minimum rather than a suggestion — the manufacturer sets it because that is the width their product will bond and their tile will tolerate without lipping. Large-format tile usually wants a wider joint than small tile, not a narrower one, which is the opposite of what most people assume: a big tile is stiffer, so it is harder to bed flat, and the joint is what absorbs the difference.

Where the cuts land

Once the module is right, the layout is a fencepost problem — the same one as a fence, with the joint standing in for the gap. A row is a whole number of modules less the joint at the far end, and the leftover is shared between the two walls.

There is one real decision, and it is made before the first tile goes down: centre the layout, or start square to a wall?

Centring is almost always right, and not for reasons of taste. Solve the row for the edge cut and it comes out as:

• Edge cut = ((tile − joint) + d × (tile + joint)) ÷ 2, where d is how far into the last module the room falls

d is always between 0 and 1. So the edge cut is always between half a tile and a full tile — it is arithmetically impossible for a centred layout to leave a thin strip. That is the whole argument for centring, and it is why the calculator here checks the square layout rather than the centred one: the centred layout cannot be the one at fault.

Start square to a wall instead and there is no such floor. The strip at the far end is whatever the room leaves after a whole number of modules, and it can come out arbitrarily thin. A 12 ft room with 12 in tile lands almost exactly on ten tiles; a 12 ft 6 in room does not, and if you start square you get a strip under a third of a tile wide along one wall. Along one wall only, at the far end, under a window, where you will see it every time you walk in.

So: centre it, and set a few tiles dry before you commit. Centring gives two equal cuts, which read as deliberate. One wide cut at one wall and a full tile at the other reads as a room somebody measured wrong, even when it is not.

Tiles across and down

The two directions are laid out independently — each gets its own count, its own edge cuts, and its own answer to the centring question. These are real outputs from the calculator for a 12 × 24 in tile with a 3/16 in joint, 10% waste, and a 10 sq ft box:

Now hold the room fixed and change the tile. This is the table that surprises people:

That is worth sitting with, because it is the thing that makes a tile estimate feel wrong. The number of tiles is almost meaningless on its own — 30 or 525 depending on what you pick — and the number that goes on the receipt is the same either way. Buy by area, and count pieces only to check you have enough.

A diagonal layout does not take more tile

This is the one thing on this page that most tile calculators get wrong, so it is worth being blunt about it.

Turn the grid through 45° and the tiles still cover the floor. Same tiles, same size, same density — rotating a grid does not change how much area it covers, any more than turning a sheet of paper changes its area. So the number of tiles does not go up by 41%, which is what you get if you multiply by the square root of two and stop there. That figure is the increase in the spacing each row sees, not the increase in tile.

What does change is the shape of what is left over. In a straight lay, an edge cut is a strip, and a strip off one wall is often wide enough to be turned and used somewhere else. In a diagonal lay every edge cut is a triangle, and a triangle off one wall fits essentially nowhere else: it is the wrong shape for the opposite wall and the wrong shape for the ends. On top of that, the awkward pieces in a diagonal lay land at the corners, where a straight lay quietly hides them along a wall.

So the cost of a diagonal lay is a waste cost, not a tile count. That is why the trade rule is "add another five percent for diagonal" rather than "add forty percent" — and why the waste box on this calculator is the place to put it, and 15% is a more realistic starting point than 10% if you are setting tile on the diagonal.

The one thing a diagonal lay does genuinely change on this page: centring matters more, because a triangle that is too small to work with is far more visible than a strip that is too small to work with. Whatever the layout, set it out dry before you mix anything.

Worked example: a 12 ft × 10 ft floor

A 12 ft × 10 ft floor, a 12 × 24 in tile laid with the 12 in side along the length, a 3/16 in joint, 10 sq ft to the box, and 10% waste. This is what the calculator loads with.

First the module. 12 + 3/16 is 12 3/16 in, and 24 + 3/16 is 24 3/16 in. Both directions get the same treatment; neither is the nominal size.

Then the length. Twelve feet is 144 in. At a 12 3/16 in module that is just under twelve modules, so the row takes 12 pieces — 10 full tiles and a cut at each end. The cut works out at 11 in. Across those twelve pieces there are eleven joints, and eleven joints at 3/16 in add 2 1/16 in. A layout done on the nominal 12 in would have been two inches out by the last course.

Then the width. Ten feet is 120 in. At a 24 3/16 in module that is five pieces — 3 full tiles and a 23 5/8 in cut at each end. Two equal cuts, both nearly a full tile, which is what a centred layout is supposed to give you.

The count. Twelve pieces by five is 60 tiles on the layout. Every piece costs a whole tile, because an edge cut comes off a whole one, so that is the number to cover.

And the buy. The floor is 120.0 sq ft. Add 10% and you are buying 132.0 sq ft of tile, which at 10 sq ft a box is 14 boxes. Sixty tiles at 2 sq ft each is exactly 120 sq ft, so the boxes cover the layout with the waste allowance on top — and the two figures agreeing is the check worth doing at the counter.

Would the square layout have been better here? The calculator checks it: starting with a full tile at one wall would leave 9 15/16 in at the far end of the length and 23 1/4 in at the far end of the width. Nothing wrong with either of those numbers. But it is luck — a slightly longer room would have put a three-inch strip along one wall — and centring gives the same two cuts without having to be lucky.

Is tile layout in the building code?

No — and this is the one page on this site where that is the honest answer. There is no table of tile spacings to look up, because there is nothing structural about a tile floor. Tile is a finish: it carries nobody, it holds nothing up, and no code exists to size it. What governs it is the manufacturer's written instructions and the industry standards they are written against, and when an inspector or a warranty claim asks a question, the bag and the box are what get read.

The framing underneath it, though, is a different story, and this is the connection worth making. Tile almost never cracks because the tile was wrong. It cracks because the floor under it moved. A subfloor that deflects too much between joists, a joist that is overspanned, a floor that is not flat — none of those are tile problems, and no amount of care with the layout will save a floor that flexes. That is real, prescriptive, tabulated framing: joist spans and subfloor spans are in the code, and if the tile is going on a floor above a room, the framing page covers the wall side of it.

Three things worth knowing that are not on this page:

• Grout joint width is a minimum from the manufacturer, not a preference. It is set by the tile and the adhesive, and it is usually wider on large-format tile rather than narrower. It is also the input that moves this page's numbers the most.
• Dye lots. Tile is made in batches and the shade moves between them. Buy the whole floor in one order, keep the spare tiles, and never assume you can top up later — this is the single most expensive thing people get wrong on a tile job, and it cannot be fixed afterwards.
• Flatness of the substrate. Large-format tile needs the floor flat, not merely sound, and the tolerance is far tighter than most floors are built to. Check it before you order the tile, because the fix is a levelling compound and it changes your floor height and therefore your door clearances.

What is genuinely outside any book is the decision this page is built around: where the cuts land, and whether to centre the layout or start square. No manufacturer knows the dimensions of your room, and no code has an opinion. It is arithmetic, and it is the part that decides whether the finished floor looks deliberate.

Frequently asked questions