Calibre is the size group a tile belongs to. After firing, tiles from the same design come out very slightly different in size, so the factory measures them and sorts them into groups, each marked with a calibre code on the box.
Tiles within one calibre lay together with even joints. Tiles from different calibres can differ by a millimetre or more, and mixing them in one floor gives joints that wander and corners that do not meet.
Why tiles vary in size
A tile shrinks as it is fired and vitrifies. The shrinkage depends on the body mix, how hard the powder was pressed, and the temperature in each part of the kiln. Small changes in any of these change the final size.
So even a well-run line produces a spread of sizes. The factory cannot remove that spread in firing, so it manages it in one of two ways: it sorts tiles into calibres, or it grinds the edges to one size. The second method is rectifying, explained in rectified vs non-rectified tiles.
How calibre is marked
Each factory uses its own codes. Some print a number, such as 1, 2 or 3, and some print a letter or the measured size in millimetres. The calibre usually sits beside the shade code and batch number on the box end.
Ask the factory for its key. You need to know which code means which size, and how wide each group is. Shade and batch are separate marks, covered in shade vs batch numbers.
What the standards allow
ISO 13006 and EN 14411 limit how far each tile can be from the work size the factory states. For porcelain tiles in group BIa, technical sheets referring to EN 14411 commonly state ±0.6% of the work size, to a maximum of ±2.0 mm.
That limit is wide enough for two tiles in the same tolerance band to differ noticeably. A tile can meet the standard and still not lay well beside a tile from another calibre. That is why calibre sorting matters even when every tile passes.
Does it matter for rectified tiles?
Rectified tiles are ground to one size, so most factories do not sort them by calibre. Some still mark a size code, especially on large formats. Grinding also wears the wheels, so size can drift slightly over a long run. Measure rectified tiles too, especially when an order is made on different days.
How to protect your order
- Write in the purchase order that each design ships in one calibre, as well as one shade and one batch.
- Ask the factory to state the calibre code on the packing list for every pallet.
- At loading, check box ends on each pallet to confirm the calibre matches the packing list.
- Measure a few tiles from different pallets with a calliper and compare them.
- Keep some spare stock of the same calibre for repairs and later sales to the same customer.
Mixed calibres are one of the common grounds for complaints, alongside mixed grades and shades. Our guide on avoiding mixed grades in one shipment covers the wider loading checks.
Next step
Calibre problems are easy to prevent and hard to fix once tiles are laid. We check calibre on every pallet at loading and record it for you. Book a free sourcing review to set up your order.