Tile Calculator

“How many tiles do I need” has a subtly different answer from what most people expect, because tiles are not laid edge to edge. Every tile sits inside a grout joint, so the space one tile actually occupies is its own size plus the joint on each side. On a 600 mm tile with a 3 mm joint that is a 1% difference and barely matters; on a 100 mm mosaic tile with a 3 mm joint it is 6%, which is the difference between one box and two.

This calculator includes the joint, adds a cutting allowance, and converts the result into boxes — since that is how tiles are actually sold, and a part box is not something you can buy.

Why the grout joint is in the formula

A tile plus its joint forms a repeating module, and that module — not the tile itself — is what tiles the floor. Divide the room area by the bare tile area and you will come out slightly short every time, because you have ignored the width the joints take up.

The effect scales with tile size. A 600 × 600 mm tile with a 3 mm joint occupies 603 × 603 mm, which is 1.0% more than the tile alone — negligible. A 300 × 300 mm tile with a 3 mm joint occupies 303 × 303 mm, 2.0% more. A 100 × 100 mm tile with the same joint occupies 103 × 103 mm, 6.1% more. Small formats and wide joints are exactly where the shortcut goes wrong.

Joint width itself depends on the tile. Rectified porcelain — cut to an exact size after firing — can take a 2 mm joint, and looks best with one. Standard ceramic wall tiles usually want 5–10 mm, partly because their edges are not perfectly straight and the joint hides the variation. Floor tiles in a busy area sit in the 3–5 mm range.

Cutting waste: the figure that decides your order

Some tiles will be cut. That is unavoidable — rooms are not multiples of tile sizes. The question is only how much of each cut tile is thrown away, and that depends on the layout:

• Straight lay in a plain rectangular room — 10%. Most offcuts go at the perimeter and many can be used on the opposite wall.
• Diagonal or diamond lay — 15%. Every perimeter tile is a triangle, so half of every cut tile is waste by definition.
• Herringbone or chevron — 15–20%. These patterns generate offcuts at both ends of every course.
• Rooms with many obstacles — add 5%. Pipes, columns, door reveals and awkward returns around a bathroom or kitchen all consume tiles.

Always keep the surplus. Tiles are made in dye lots and the shade shifts slightly between production runs; a repair in two years' time with a tile from a different batch may not match, and a discontinued tile cannot be matched at all. A spare box in the loft is cheap insurance.

Worked example

A room 12 feet by 10 feet is 120 square feet, which is 11.15 square metres. With 600 × 600 mm tiles and a 3 mm joint, each module covers 0.3636 square metres, so 11.15 ÷ 0.3636 gives 30.7 tiles before any waste.

Add 10% for cutting and it becomes 33.7 tiles — 34 tiles. At four tiles per box that is 8.5 boxes, rounded up to 9 boxes, giving 36 tiles and a genuine spare pair plus a few cuts.

The calculator reports two coverage figures and they are not the same number. The tile area in a box is 1.44 square metres or about 15.5 square feet — that is what the manufacturer prints on the packaging, and it counts the tiles only. The floor a box covers is a little larger, 1.454 square metres, because once the tiles are laid with their joints the grout lines are floor too. Both are correct; they just answer different questions. Use the floor figure to check whether a box will cover the room, and the tile figure to compare packaging between suppliers.

With 600 mm tiles the gap between the two is about 1%, so it hardly matters. With small mosaics it can exceed 6%, which on a bathroom floor is the difference between one box and two.

Buying, checking and storing

Order the boxes from the same batch in one go, and check the batch code on every box as it is delivered. A mixed delivery is a genuine problem and much easier to refuse at the door than to argue about later.

Open a box and dry-lay a few tiles before fixing anything. Shade variation between tiles in the same batch is normal and often part of the look, but it needs to be seen before the adhesive is mixed, not after. Check the calibration too — tile sizes are nominal, and laying tiles from two different boxes alternately across a floor is the standard trick for disguising minor variation.

Store spares flat and dry, and keep the box with the batch code. That one detail is what makes a future repair possible.