How Many Tiles to Buy for a Floor or Wall

Every tiling job comes down to one number you take to the shop: how many boxes. Get it wrong on the low side and you stop work mid-room for a second trip, where the new box almost never matches the original dye lot. Get it wrong on the high side and you have paid for tiles you will never lay. The good news is that the math is simple, repeatable, and something you can do on the back of an envelope before you ever load a calculator.

This guide walks through the whole chain: measure the area, divide by the size of one tile, add a waste margin for cuts and breakage, fold in the grout joint, and round up to whole boxes. Plug your own numbers into the tile calculator as you read and the same logic happens automatically.

Start With the Area, Not the Tile

The first measurement is the area you need to cover. For a plain rectangular floor that is just length times width. A room that measures 10 ft by 12 ft is 120 square feet. A 4 m by 3 m room is 12 square metres. Write that number down and keep it.

Real rooms are rarely a clean rectangle. The fix is to break the space into rectangles and add them up. An L-shaped room is two rectangles. A room with a bay window is the main rectangle plus the bay. A triangular niche is half its bounding rectangle. Calculate each piece on its own, sum the areas, and treat that total as your job size. The same trick handles walls: a backsplash that runs 3.6 m wide and 0.6 m tall is just 2.16 square metres.

One thing not to do is subtract small openings. It is tempting to deduct a doorway or a window from a wall, but the cuts you make around an opening produce off-cut waste anyway, so subtracting the opening usually leaves you short. Tile the full wall area and let the waste margin absorb the cut-outs.

The Core Formula: Area Divided by Tile Size

Once you have the area, the bare tile count is one division:

tiles = area to cover ÷ area of one tile

The only trap is keeping the units consistent. Tile sizes are quoted in millimetres or inches, while rooms are measured in metres or feet, so convert one to match the other before you divide. A 600 × 600 mm tile is 0.6 m × 0.6 m, which is 0.36 square metres each. A 12-inch tile is 1 ft × 1 ft, or exactly 1 square foot. If you ever need to move between metric and imperial cleanly, a unit converter keeps the tile size and the room in the same system so you do not silently divide square feet by square millimetres.

So for that 12 m² room with 600 mm tiles: 12 ÷ 0.36 = 34 tiles bare, before any allowance. For a 120 square foot floor with 12-inch tiles: 120 ÷ 1 = 120 tiles bare. Clean and obvious — but that bare count is not what you order.

Add 10% for Cuts and Breakage

No tile job uses every tile whole. The perimeter needs cut tiles, the off-cuts usually cannot be reused, and a few tiles crack during handling or setting. That loss is the waste margin, and you apply it as a multiplier on the bare count rather than tacking on a flat handful of tiles, so it scales with the size of the job.

The rule of thumb:

Straight lay, simple rectangular room: 5–10%. Ten percent is the safe default.
45° diagonal lay: 15%. Every perimeter cut is a triangle, so off-cuts pile up.
Herringbone or small mosaic: 20%. The more cuts a pattern demands, the more you waste.
Discontinued or one-off batch: add a couple of extra points on top. You cannot dye-lot match a top-up later, so buying the attic spare now is cheaper than re-tiling a patch in two years.

To apply 10%, multiply the bare count by 1.10. You can sanity-check that step against a percentage calculator if mental math is not your strong suit. A 120-tile bare count becomes 132 with the margin. The 34-tile floor becomes 38.

Grout Joints Cut the Count Slightly

Here is the part most calculators get backwards. A grout joint makes each tile occupy more floor than its bare size, not less. A 600 mm tile with a 3 mm joint sits in a 603 × 603 mm module, so the area each tile actually covers is a touch larger — which means a given floor needs slightly fewer tiles, not more.

The effect is small on a big tile, about 1% on a 600 mm tile at 3 mm, but it grows fast as tiles get thin or joints get wide. A 25 mm mosaic with a 3 mm joint loses over 20% of its count to the joints. An honest estimate builds the joint into the module before dividing, instead of pretending each tile is its bare size. It is a minor correction on a floor and a major one on a feature wall, so it is worth getting right when you are quoting a client.

A Worked Example: 10 × 12 ft Floor, 12-inch Tiles

Let me run the full job the way I do it on site, because the steps matter more than any single number.

The floor is 10 ft by 12 ft, so the area is 120 square feet. The tile is a 12-inch square, which is exactly 1 square foot each. Bare count is 120 ÷ 1 = 120 tiles.

It is a plain straight lay in a rectangular room, so I add the standard 10% margin: 120 × 1.10 = 132 tiles to buy. If the tiles come 8 to a box, that is 132 ÷ 8 = 16.5 boxes, which rounds up to 17 boxes — because no shop splits a box. Seventeen boxes is 136 tiles, leaving 4 spares.

Those four spares are not rounding error, they are the point. Store them in the attic. When one tile cracks two years from now, the exact colour batch will be long gone, and an exact-match spare is worth far more than the few dollars it cost. If I were laying the same floor on a 45° diagonal, I would bump the margin to 15% (138 tiles, still 18 boxes) and not think twice.

From Tiles to Boxes to Budget

The last step turns tiles into a purchase. Tiles ship by the box and shops will not sell loose tiles, so always round the with-waste count up to the next whole box. That leaves a handful of spares almost every time, which is exactly what you want for future repairs.

If you know the price per box, the cost is just boxes × box price — before grout, adhesive, and labour, which are separate line items. For a quick split of those shared finishing costs across rooms or housemates, a tip calculator handles the proportional math without a spreadsheet. The point of doing all of this before you leave home is simple: you buy once, you buy the right amount, and you keep enough spare to never re-tile a patch you could have matched.

Run your own room through the tile calculator and it does the area, the waste, the grout module, and the box rounding in one pass, with the coverage area shown so you can confirm your purchase clears the floor with margin.

Made by Toolora · Updated 2026-06-13