Tile Calculator: How Many Tiles You Actually Need

Running out of tiles two rows from the end is a genuinely expensive mistake, because the replacement box comes from a different batch and the shade will not match. The arithmetic that prevents it takes about a minute.

The calculation, and the part people leave out

Area to cover, divided by area of one tile, gives the bare tile count. That is the easy half. The half that decides whether the job finishes is the waste allowance, and it is not a fudge factor. Every tile cut to fit an edge produces an offcut that usually cannot be used anywhere else, so a room with lots of short walls, alcoves or boxed-in pipework burns through more than a plain rectangle.

A straight grid layout wants 10%. Diagonal or herringbone patterns want 15%, sometimes 20%, because almost every perimeter tile is cut at an angle. Large-format tiles in a small room want more again, since one broken 600 mm tile is a much bigger loss than one broken 150 mm tile. Then round up to whole boxes and buy the full box, not the exact count.

A worked bathroom

The floor

A bathroom 2.4 m by 1.8 m is 4.32 m². Using 300 x 300 mm tiles at 0.09 m² each: 4.32 / 0.09 = 48 tiles. Add 10% and you need 53, so 54 or whatever the next whole box gives you. If the tiles come nine to a box, that is six boxes.

The walls

Say you tile three walls to 1.2 m high. Their lengths are 2.4 m, 1.8 m and 2.4 m, totalling 6.6 m of run, so 6.6 x 1.2 = 7.92 m². Take off a window opening of 0.6 m by 0.9 m, which is 0.54 m², leaving 7.38 m². With 200 x 250 mm tiles at 0.05 m² each that is 148 tiles, and 170 once you allow 15% for all the cutting round the window and the corners.

Using the calculator

You need three things. The area, either as room length and width or as a total you have measured yourself. The tile size, entered in whatever unit is printed on the box, usually millimetres in the UK and inches in the US. If your tiles are sized in inches, convert before dividing: a 12 by 24 inch tile is 288 square inches, and a square foot is 144 square inches, so each one covers exactly 2 square feet. And the waste percentage, which the calculator will suggest but which you should raise for awkward rooms.

Grout joints are a small bonus in your favour. A 600 mm tile with a 3 mm joint actually occupies 603 x 603 mm, which is 0.3636 m² rather than 0.36, so you need roughly 1% fewer tiles than the raw division suggests. Not enough to skip the waste allowance, but real. Measure the room properly first, because every error here gets multiplied: our guide to measuring square footage and the square footage calculator walkthrough both deal with irregular rooms.

Common questions

How many 12 by 12 inch tiles cover 100 square feet? Each tile is exactly one square foot, so 100 tiles bare, and 110 with the standard 10% allowance.

Should I subtract doors and windows? Subtract window and door openings from wall areas, yes. Do not subtract a bath or a vanity unless the tiling genuinely stops at it, and even then keep the offcuts, because tiles from behind a fitting are the ones you will want when something leaks in five years.

How much adhesive and grout will I need? Adhesive coverage depends on trowel notch size and is printed on the bag, typically enough for 4 to 5 m² per 20 kg at a 6 mm notch. Grout depends on tile size and joint width, and small tiles with wide joints use several times more than large tiles with thin ones.

Is 10% waste enough for a first-timer? Usually not. If it is your first tiling job, go to 15% even on a straight layout. The extra box costs less than a second delivery and a shade mismatch.