How it works
The calculator finds the floor area, adds your waste allowance, and divides by the space one tile takes up including its grout joint.
Area = Length × Width
Area with waste = Area × (1 + Waste ÷ 100)
Tiles = ⌈ Area with waste ÷ ((Tile length + Joint) × (Tile width + Joint)) ⌉
Boxes = ⌈ Tiles ÷ Tiles per box ⌉ or ⌈ Area with waste ÷ Coverage per box ⌉
⌈ ⌉ means rounding up to the next whole number: you cannot buy part of a tile or a box. Rounding happens only at the end, so small fractions are not lost in intermediate steps.
All dimensions are converted to one internal unit before multiplying, so you can mix feet for the room with inches for the tile.
Worked example
Inputs
- Measure by
- Room length × width
- Room length
- 12 ft
- Room width
- 10 ft
- Tile length
- 12 in
- Tile width
- 12 in
- Grout joint width
- 1/8 in
- Layout
- Straight
- Waste allowance
- 10 %
- Box size given as
- Tiles per box
- Tiles per box
- 10
Result
- Floor area
- 120 sq ft
- Area incl. waste
- 132 sq ft
- Tiles
- 130
- Boxes
- 13
Common mistakes
- Using the tile’s nominal size instead of the actual size printed on the box. A tile sold as “12 × 12” can measure slightly less.
- Forgetting the grout joint. With small tiles the joint adds noticeably to the space each tile covers – and leaving it out overstates the count.
- Buying exactly the calculated amount with no waste. Cut pieces at walls, breakage and a few spares for future repairs all come out of the same boxes.
- Mixing boxes from different production lots. Shade and size can vary between lots; buy everything at once and check the lot number.
Assumptions and limits
- The room is a rectangle, or you have entered its total floor area. Each tile plus its share of the joint covers (tile length + joint) × (tile width + joint).
- The count is an area estimate. It does not plan individual cuts, so rooms with many corners or very large tiles may need more.
- Diagonal layouts are counted by the same area; the extra cutting is covered only by the waste allowance you enter.
- Boxes are rounded up to whole boxes. Without a box size, only the tile count is shown.
Frequently asked questions
How much extra tile should I buy?
It depends on the layout, the tile size and how many cuts the room needs. Manufacturers’ installation guides and your retailer are the best source for a figure; straight layouts in simple rooms need the least, diagonal and patterned layouts the most. Many installers also keep a few spare tiles for repairs.
Does the grout joint change how many tiles I need?
Yes, slightly. Each tile occupies its own size plus one joint width in each direction. For 12 × 12 in tile with a 1/8 in joint that is 12.125 × 12.125 in, about 2 % more area per tile, so you need about 2 % fewer tiles than an area-only calculation suggests.
Can I use this for wall tile?
Yes. Enter the wall’s width as the length and its height as the width, or enter the wall area directly, then subtract windows by measuring them separately.
Sources
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 2, 2026