Estimate how many tiles you need for a room, including wastage allowance.
Tiles needed = (room area ÷ tile area) × (1 + wastage%). Always round up and buy a bit extra for cuts and breakage.
The area of a floor and the number of tiles you should buy are two different numbers, and the gap between them is wastage — tiles cut, broken, or left over at odd edges. For a straight, simple rectangular room, 10% extra over the calculated tile count is the industry standard and usually enough. An L-shaped room, or one with alcoves, pillars or a lot of fixed furniture to cut around, pushes that to 12–15%. Diagonal-laid tiles, herringbone or other complex patterns can waste 15–20%+, because more of every cut tile ends up unusable rather than reusable as a smaller piece elsewhere.
Grout joint width (commonly 2–5mm) has a smaller effect than layout, but it isn't zero — wider joints mean slightly more tile-to-tile spacing and marginally lower coverage per box, and DIY installers should lean toward the higher end of any wastage range since first-time cuts have a steeper learning curve than a professional tiler's. Buy tiles by the box, not the piece — most suppliers won't split a box, and running short by half a box mid-job on a discontinued batch is a common and avoidable problem, since dye-lot color can shift between production runs.
10% for a simple, straight-laid rectangular room; 12–15% for L-shaped rooms or ones with alcoves; 15–20% or more for diagonal or complex patterns. Professional installers with tighter cutting skills can often work closer to the low end; DIY projects should budget toward the higher end.
Wider grout joints (toward 5mm) add a small amount of extra spacing across a floor, which very slightly reduces how much floor area a fixed number of tiles will cover compared to a narrow 2mm joint. The effect is minor next to layout-driven wastage, but it's part of why two identical rooms can need slightly different tile counts depending on the chosen joint width.
By box. Suppliers price and often sell by the box (a fixed number of pieces or fixed area per box), and most won't split one open for a partial sale. Round your total requirement (area plus wastage) up to the next full box, not down, to avoid a mid-project shortfall.
Tile color and finish are produced in dye lots, and even the same product code can shift slightly in shade between production runs. Ordering everything — including your wastage buffer — in a single batch avoids a visible mismatch if you have to reorder a box months later and the new stock is from a different lot.