How Many Tiles Do I Need?

Divide the surface area by the area of one tile, then add for cuts and breakages. Here's the arithmetic, how much waste to allow, and why you buy a spare box.

Two numbers, one division

How many tiles you need is the area you're covering divided by the area of one tile. Everything else is unit conversion and allowing for the ones you'll ruin.

The catch is that rooms get measured in feet and tiles get sold in inches.

Your details

Surface length(ft)

Surface width(ft)

Tile width(in)

Tile height(in)

Waste allowance(%)

10% laid square, 15 to 20 for diagonal or herringbone.

Result

Tiles to buy

Rounded up, waste included.

Tiles to cover the areaBefore any allowance for cuts or breakages.: 80
Surface areaSquare feet being tiled.: 80
Area of one tileSquare feet covered by a single tile.: 1

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The arithmetic

A floor 10 by 8 feet, with 12 by 12 inch tiles.

Surface area:

10 × 8 = 80 square feet

Area of one tile. The tile is in inches, so convert:

12 × 12 = 144 square inches 144 ÷ 144 = 1 square foot

That 144 is the number of square inches in a square foot — 12 × 12. It's the conversion that does all the work here, and it's the one people forget.

Divide:

80 ÷ 1 = 80 tiles

Add waste. At 10%: 80 × 1.1 = 88 tiles.

A 12-inch tile is a friendly example because it lands on exactly 1 square foot. A 6 × 24 inch plank is 144 ÷ 144 = 1 square foot too. A 4 × 4 inch tile is 16 ÷ 144 = 0.111 square feet, so the same floor takes 720 of them.

How much waste to allow

This is the number worth thinking about, because it's the difference between one trip and two.

10% — a rectangular room, tiles laid square to the walls. Most jobs.
15–20% — diagonal or herringbone. Every single edge tile is a cut, and each cut wastes most of a tile.
More again — rooms with alcoves, pipework, a bay window, or lots of corners. Every obstacle is more cuts.

Large-format tiles also push it up: a bad cut on a 24-inch plank wastes far more material than a bad cut on a 4-inch square.

Buy the spare box. Really.

The most common regret in tiling isn't miscounting, it's not keeping spares.

Tiles are made in batches, and the colour shifts between batches. It's called the dye lot, and it's printed on the box. Two boxes of the same tile from different lots can be visibly different side by side under a window.

So when a tile cracks in two years — dropped pan, settling floor, unlucky chair leg — you can't just buy one. The line may be discontinued, and if it isn't, the batch will be different. A replacement that doesn't match draws the eye far more than the crack ever did.

One spare box, in the loft, for the life of the floor. It's the cheapest insurance in the job.

Check the dye lot at the shop, too

Not just for spares. If your order spans two batches you can end up tiling half a room in one shade and half in another, which is invisible in the box and obvious on the floor.

Check the lot number on every box before you leave, and if they don't match, ask for boxes that do.

What about grout joints?

Ignore them.

Technically each tile plus its joint occupies slightly more space than the tile alone, so accounting for joints would reduce the count very slightly. That means ignoring them errs towards buying a few extra — which is exactly the direction you want to be wrong in.

Tiles are sold by the box

Your final number isn't tiles, it's boxes. Boxes list both the tile count and the square feet they cover.

Take your tiles-to-buy figure, divide by the tiles per box, and round up. Again. Rounding up twice feels wasteful and is the correct move both times.

Where people get it wrong

Forgetting the 144. Dividing 80 square feet by "144" (the tile in square inches) instead of by 1 square foot gives 0.55 tiles. Somewhere in the mistake, the units stopped matching.

Measuring the room instead of the surface. A bathroom floor isn't the room's footprint — the bath, the vanity and the shower tray aren't getting tiled. Measure what you're actually covering.

Buying zero waste. There is no layout where the tiles land perfectly at every wall. The cuts are guaranteed; the only question is how many.

Ignoring the layout. Starting at a wall and working across leaves a sliver of a tile at the far side, which looks wrong and is fragile to cut. Find the centre line, plan outward, and put the cuts where furniture or a door will sit.

One number for a whole bathroom. Floor and walls are different surfaces, often different tiles, always different waste. Work each one separately.

The short version

Area ÷ area-of-one-tile = tiles, then add 10% laid square or 15–20% on the diagonal. Convert tile inches to square feet by dividing by 144, round up to whole tiles, round up again to whole boxes, and keep one box back for the day you need it.