Large-format tiles
Minimum joint width by tile size (BS 5385-1/-3) and size variation (ANSI A108.02)
- where
- tile face area (m²)
- longest and shorter tile sides (mm)
- BS 5385 minimum: floors 3 mm; walls 2 mm (A < 0.1 m², L ≤ 600), 3 mm (A ≤ 1 m², L ≤ 1200), otherwise max(3, 5 × L / 3000) mm
- joint from size variation (mm)
- facial size variation across the tiles supplied (mm)
- recommended minimum joint (mm)
Explain this equation
Use this when setting out to choose the narrowest joint that is still acceptable. It takes the larger of the British Standard minimum for the tile size and location and the joint needed to hide the size differences between tiles in the batch.
Jbs is the BS 5385 figure: 3 mm on all floors, and on walls 2 mm for small tiles, 3 mm up to 1200 mm long, more for panels. dv is the measured difference between the biggest and smallest tile; ANSI A108.02 says the joint should be at least three times that so the variation does not show as wandering joints.
Joints exist to take up variation and movement. If tiles differ by 0.6 mm, a joint has to swallow that on each side while staying visually even, which is why the variation is multiplied by three. Whichever rule asks for more wins.
A 1200 × 600 floor tile with 0.6 mm variation: BS gives 3 mm, variation gives 1.8 mm, so set 3 mm. A small wall tile varying by 0.8 mm needs 2.4 mm rather than 2 mm.
Watch out: on floors BS 5385-3 also says ceramic joints should not be wider than the tile is thick. The pro-rata rule for panels over 1200 mm is scaled from the 3 m → 5 mm example.
Gives the narrowest joint you should set out for a tile, from the British Standard size bands and from the actual size variation in the batch.
Enter your values to see the result.
Good to know
Walls (BS 5385-1:2018): tiles under 0.1 m² with no side over 600 mm, 2 mm; tiles up to 1 m² with no side over 1200 mm, 3 mm; bigger panels pro-rata (3 m panel, 5 mm). Floors (BS 5385-3): never under 3 mm, and for ceramic not wider than the tile thickness. ANSI A108.02 adds that the joint should be at least three times the actual variation in face size across the tiles supplied, and never under 1/16 in (1.6 mm). Measure several tiles from different boxes with a steel rule or calipers to get the variation.
Common mistake
Asking for 1–2 mm “credit card” joints on large rectified floor tiles. The joint has to absorb size variation, bow and movement.
Limits
Minimums only. External tiling, heated floors and stone may need wider joints; follow the tile and grout data sheets. The pro-rata panel rule above 1200 mm is a direct scaling of the 3 m → 5 mm example and is an interpretation.
Sources
- Tile Foundations, Change in British Standards (BS 5385-1:2018: 2 mm minimum joint up to 600 mm, 3 mm up to 1200 mm; 3 m panel 5 mm)
- The Tile Association Tiling Guide, joint widths: BS 5385-1 (3 mm for tiles 0.1–1 m² with no side over 1200 mm; panel joints pro-rata, 3 m panel → 5 mm) and BS 5385-3 (floors not less than 3 mm, not more than tile thickness)
- The Tile Association, External floor tiling incorporating movement joints (BS 5385-3:2024; floor joints never under 3 mm)
- Stone World / CTEF Tile Tip, Being realistic about grout joint sizes (ANSI A108.02 4.3.8: joint ≥ 3 × actual facial-size variation, never less than 1/16 in)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.