Why floor area alone cannot tell you how many tile boxes to buy
Dividing floor area by tile area is only a first estimate. A small edge piece may consume a whole new tile if its offcut cannot be reused elsewhere.
Area is not a layout
A row of n whole tiles has n−1 internal joints: length = n × tile length + (n−1) × joint width. Subtract the wall gap at each end separately.
Inspect the edges first
B&Q recommends a dry layout and checking narrow cuts before fixing tiles. Centring balances opposite edges but does not always use fewer tiles. Rotate rectangular tiles and compare both the layout and box count.
References: B&Q — How to tile a floor
Turn positions into an order
A conservative grid counts one source tile per position, including cut positions, without assuming offcut reuse. Add reserve, then round up to whole boxes. Obstacles, diagonal patterns and non-square corners need another layout; confirm actual measurements and product instructions.
Worked example: joints change the overall size
With 600 × 600 mm tiles and 6 mm joints, five columns span 3024 mm and four rows span 2418 mm, excluding wall gaps. That makes 20 positions with no cuts.
Adding 10% reserve gives 22 tiles. At 4 tiles per box, buy 6 boxes containing 24 tiles, not 5 boxes rounded down from 5.5. The reserve is an example choice, not a universal installation rule.