The math behind your estimate
The inputs, allowance, and rounding stay visible.
- Area
- Area = Length × Width, summed over every tiled area
- Per tile
- Coverage per tile = (Width + joint) × (Length + joint)
- Tiles
- Tiles = Area ÷ coverage per tile, × (1 + waste %), rounded up
- Boxes
- Boxes = Tiles ÷ tiles per box, rounded up
Assumptions
- Grout joints are included in each tile's coverage, which is why a 12 × 24 in tile with a 1/8 in joint covers slightly more than 2 ft². On small jobs this changes the count by a tile or two; over a large floor it matters.
- Waste percentages are starting recommendations, not engineering facts. They reflect how much of a typical cut tile ends up unusable in that layout — your room's shape, the tile size and your cutting skill all move the real figure.
- Buying an extra box beyond the calculation is common practice: dye lots vary between production runs, so a replacement tile bought in two years rarely matches.
- Counts assume full tiles are cut to fit. Mosaic sheets and interlocking formats are sold by sheet coverage instead — use the tiles-per-box field with sheets if that is what you are buying.
Frequently asked questions
- How many 12 × 24 tiles do I need for a 10 × 8 ft floor?
- 80 ft² at 2 ft² per tile is 40 tiles before waste. With a 1/8 in grout joint each tile covers a touch more, and a straight layout at 10% waste puts you at 44 tiles — 5 boxes if there are 10 tiles per box.
- How much extra tile should I buy?
- 10% for a straight layout in a square room, 15% for a diagonal, and around 20% for herringbone or other patterns. Add more for small or awkward rooms: a bathroom with a toilet, vanity and doorway generates far more cuts per square foot than an open floor.
- Does grout joint width change how many tiles I need?
- Yes, slightly. Each tile plus its joint takes up more room than the tile alone, so wider joints mean marginally fewer tiles. It is a small effect on a bathroom floor and a real one across a large open plan.
- Should I buy tile by the box or individually?
- Almost all tile is sold by the box, and boxes rarely divide evenly into what you need — which is why this calculator rounds up to whole boxes and shows the leftover. Keep the offcuts and any spare tiles: matching a dye lot later is usually impossible.
Sources
- Common tile module sizes and grout joint widths (industry standard practice)