How many tiles do I need?
Work out the area to tile in square feet. Then work out the space one tile fills once it is laid: the tile length plus one grout joint, times the tile width plus one grout joint. Divide the area by that, add an allowance for cuts and breakage, and round up, because tiles are sold whole.
A 120 sq ft floor in 12 x 12 in tiles with a 1/8 in joint needs 118 tiles before waste, and 130 tiles with a 10 percent allowance.
The formula
- Area: length x width for a rectangle. The calculator also does circles, triangles and L-shapes, the same way as the square footage calculator. If you already know the area, pick I know the area and type it.
- One tile with its joint: (tile length + joint) x (tile width + joint). Each tile owns one joint along one long side and one short side, so the joints are counted once.
- Tiles: area divided by the space one tile fills.
- Waste: add the allowance, then round up to a whole tile.
- Boxes (optional): the area including the allowance, divided by the coverage printed on the box, rounded up.
Mixed units are fine. The calculator converts every measurement to feet first, using 1 in = 2.54 cm exactly.
Worked example: a 12 x 10 ft kitchen
The floor is 12 ft by 10 ft, the tiles are 12 x 12 in, and the joint is 1/8 in (0.125 in).
- Area: 12 x 10 = 120 sq ft.
- One tile with its joint: 12.125 x 12.125 = 147.02 sq in, which is 147.02 ÷ 144 = 1.021 sq ft.
- Tiles: 120 ÷ 1.021 = 117.5, so 118 tiles with no allowance.
- With 10 percent: 117.5 x 1.10 = 129.3, rounded up to 130 tiles.
- Boxes: if the box label says it covers 15 sq ft, the area with the allowance is 120 x 1.10 = 132 sq ft, and 132 ÷ 15 = 8.8, so 9 boxes.
Worked example in metric
A 4 m by 2.5 m bathroom floor is 10 m². In 30 x 30 cm tiles with a 3 mm joint, one tile fills 30.3 x 30.3 cm, which is 0.0918 m². 10 ÷ 0.0918 = 108.9, so 109 tiles before waste, and 108.9 x 1.10 = 119.8, rounded up to 120 tiles with 10 percent.
Grout joint width
The joint is the gap between tiles that the grout fills. It makes each tile take up a little more room, so a wider joint means slightly fewer tiles. With small tiles the joint matters more, because there are more joints per square foot.
The calculator starts at 1/8 in (3 mm in metric). That is a starting value you can change, not a recommendation: use the joint your tile maker or installer gives you. Enter 0 for tiles laid edge to edge.
Waste allowance
The calculator starts at 10 percent. That is a planning estimate, not a published figure. Rooms with many corners, cut-outs or small spaces need more cut tiles than a plain rectangle. Diagonal and herringbone layouts cut a tile at every edge of the room, so they need a bigger allowance than straight rows. Ask your installer for the figure for your layout, and keep a few spare tiles for repairs, because a later batch may not match.
Tiles to boxes
Tiles are usually sold by the box, and the label says how much area one box covers. Type that into Box coverage. The calculator divides the area including the allowance by the coverage and rounds up once, on the total.
Box coverage is usually worked out from the tile faces alone, without joints, so the box count can be slightly higher than the tile count divided by the tiles in a box. That leaves you a small margin rather than a shortfall.
What the calculator leaves out
- Grout and adhesive: the amount depends on the product, the joint depth and the trowel. Use the coverage on the bag.
- Trim and edge pieces: bullnose, skirting and transition strips are bought by length, not area.
- Underlayment and backer board: these are sheets, bought by area. The square footage calculator gives you that area.
Common mistakes
- Leaving out the joint. Dividing the area by the bare tile size counts tiles as if they touched, which gives a few too many.
- Typing the joint in the wrong unit. 1/8 in is 0.125 in, not 1.8. Check the unit menu beside the joint.
- Rounding down. Tiles and boxes are sold whole, so always round up.
- Buying the bare area. Cuts at walls and around fittings use up tiles. Add the allowance before you order.