How the calculator works it out
It adds up the areas, adds the waste allowance for your layout, and divides by the space one tile takes up including its grout joint. Tiles are rounded up, then boxes. Grout is worked out from the length of joint in each square foot, times the joint width and the tile thickness.
Tiles = area × (1 + waste %) ÷ ((tile length + joint) × (tile width + joint) ÷ 144), rounded up
Boxes = tiles ÷ tiles per box, rounded up
Grout (cu in) = area × 144 × joint × thickness × (length + width) ÷ (length × width)
Worked example: a 10 × 12 ft kitchen
120 square feet with 10% for a straight layout is 132 square feet. A 12 inch tile with a 1/8 inch joint takes up 12.125 × 12.125 = 147.0 square inches, which is 1.021 square feet, so 132 ÷ 1.021 = 129.3: buy 130 tiles, or 13 boxes of 10. Grout: 144 × 0.125 × 0.375 × 24 ÷ 144 = 1.125 cubic inches per square foot, 135 cubic inches for the room, about 8.75 lb at a typical 112 lb per cubic foot. Switch the layout to herringbone and the waste goes to 20%: 144 square feet, 142 tiles, 15 boxes. This kitchen is loaded when the page opens.
Waste by layout
| Layout | Starting waste | Why |
|---|---|---|
| Straight (grid or offset) | 10% | Cuts only along the edges of the room. |
| Diagonal (45°) | 15% | Every edge tile is cut on an angle, and triangles rarely fit elsewhere. |
| Herringbone | 20% | Angled cuts at every wall, with short offcuts. Planks waste the most. |
These starting figures are conventions used by installers and tile retailers. No tile manufacturer or the TCNA handbook sets one waste percentage, because it depends on the room, the tile size and the installer. Change the number if your installer or the tile maker gives you a different one.
What the numbers are based on
- Tile count. Plain geometry: the area each tile covers including half the joint on each side, rounded up to whole tiles and whole boxes.
- Grout volume. The joint length per square foot comes from the tile size, and the joint depth is taken as the full tile thickness, the standard way grout makers size their coverage charts.
- Grout weight. 112 lb per cubic foot (about 1.8 kg per litre) is a typical figure for mixed cement grout, not a manufacturer specification, so it is editable. Epoxy and urethane grouts are sold by volume or by kit; use the cubic inches instead.
- Units. One square foot is exactly 0.09290304 square meters; one pound is exactly 0.45359237 kg.
Not included
- Thinset mortar: coverage depends on the trowel notch, so use the chart on the mortar bag.
- Underlayment, backer board, edge trim and transition strips.
Questions people ask
How much extra tile should I buy?
A common rule is 10% for a straight layout, 15% for tiles laid on the diagonal and 15 to 20% for herringbone. These are conventions used by installers and tile shops, not a standard from any maker, and the calculator labels them that way. Go higher for small rooms, lots of corners and cuts, or large tiles where one broken piece is a big share of a box. Keep a few spares for future repairs, since later batches rarely match.
How many tiles do I need for a 10 × 12 room?
The room is 120 square feet. With 10% waste for a straight layout that is 132 square feet. A 12 × 12 inch tile with a 1/8 inch grout joint covers 12.125 × 12.125 = 147 square inches, or 1.021 square feet, so you need 132 ÷ 1.021 = 129.3, rounded up to 130 tiles. At 10 tiles a box that is 13 boxes.
Does the grout joint change how many tiles I need?
Slightly. Each tile plus its share of the joint covers a little more than the tile alone, so wider joints mean fewer tiles. With 12 inch tiles a 1/8 inch joint saves about 2% and a 1/4 inch joint about 4%. With small mosaics the effect is much bigger. The calculator counts tiles on the tile-plus-joint footprint.
How much grout do I need?
Grout fills the joints, so the volume is the joint length times its width times the tile thickness. Per square foot of floor that is 144 × joint width × thickness × (tile length + tile width) ÷ (tile length × tile width) cubic inches. For 12 × 12 tile, 3/8 inch thick, with 1/8 inch joints, that is 1.125 cubic inches a square foot, about 8.75 lb for 120 square feet. Grout makers publish coverage charts on the bag, which are the better guide for your product.
Is herringbone really that wasteful?
It can be. Every row ends in an angled cut at the walls, and the offcuts are often too short to reuse on the other side. Long, narrow planks in a herringbone waste the most. If the pattern is set at 45° to the walls, plan on the high end of the range.
Should I buy tile by the box or by the square foot?
By the box. Tile is sold in whole boxes, and the calculator rounds up to whole boxes and tells you how many spare tiles that leaves. Check the box label for the number of tiles, not just the square feet, since the square-foot figure on the box usually ignores the grout joint.