Tile Grout Gap Guide: How Wide Should Your Grout Lines Be?

What the grout joint does (and doesn't do)

No two tiles come out of the kiln exactly the same size. The grout joint takes up those size differences so the joint lines stay straight from one end of the room to the other. That is why the minimum joint width is tied to the size variation of the tile you actually bought.

What the joint does not do is absorb expansion and contraction. Cement grout cures hard. Movement is taken up by separate movement joints filled with a flexible sealant (see movement joints below).

Minimum grout joint width (ANSI A108.02)

ANSI A108.02 section 4.3.8 sets the minimum joint width. The joint you lay out has to meet every rule that applies:

The “variation in facial dimensions” is the difference between the largest and smallest tile in the lot. To find it, measure tiles from several different boxes and subtract the smallest measurement from the largest.

Example: if your tiles range from 599 mm to 600 mm, the variation is 1 mm, so the joint must be at least 3 mm. In inches, a 1/16″ variation needs a joint of at least 3/16″.

Rectified tiles have their edges ground to size after firing, so they usually vary less and can take a narrower joint. Calibrated (pressed, non-rectified) tiles are sorted by size but not ground, so they usually need a wider joint. Sheet-mounted mosaics come with the joint width already set by the backing.

Lippage is not the same as size variation

Lippage is the difference in height between the edges of two adjacent tiles after they are installed: one edge sits higher than its neighbor. You can feel it with your hand or see it with a straightedge, and the ANSI tile standards limit how much is allowed.

Three different terms often get mixed up:

• Facial dimension variation: tiles that differ in length or width. The grout joint takes this up.
• Warpage: a tile that is not perfectly flat (bowed edges or a curved face).
• Lippage: the height step between installed tiles. Warpage, an uneven substrate or uneven setting can all cause it.

A wider grout joint does not fix lippage. It is controlled by a flat substrate, careful setting and the tile's own warpage. Warpage is also behind A108.02's separate joint widths for offset layouts of tile longer than 15″, where the bowed middle of one tile lines up with the edge of the next.

Grout is not a movement joint

Tile, substrate and building all move with changes in temperature and moisture. Rigid cement grout cannot take up that movement. The TCNA Handbook method EJ171 calls for movement accommodation joints instead, filled with a flexible sealant over a backer rod rather than grout:

• At the perimeter, where tile meets walls and other restraining surfaces
• At inside corners and other changes of plane
• Over joints in the substrate below
• In the field of the tile, about every 20–25 ft (6.1–7.6 m) indoors and every 8–12 ft (2.4–3.7 m) outdoors. Indoor tile exposed to direct sunlight or moisture needs the closer spacing.

ANSI A108.02 requires movement joints to be kept free of setting and grouting materials. On a designed project, the architect or engineer shows the joint locations on the drawings.

How grout gap affects tile count

A wider joint means you need fewer tiles, not more. Each tile covers its own size plus one joint width, so a wider joint makes each tile cover more of the floor:

The effect is small for large tiles and bigger for small ones. Going from a 3 mm to a 6 mm joint changes the count by:

For example, a 12 m² floor in 60 × 60 cm tile needs 12 ÷ 0.603² = 33.002, rounded up to 34 tiles with a 3 mm joint, but 12 ÷ 0.606² = 32.68, rounded up to 33 tiles with a 6 mm joint (both before waste). For large tiles the difference is smaller than a typical 10% waste allowance.

Putting it together

• Measure tiles from several boxes. The joint must be at least 3× the size variation and never under 1/16″ (1.6 mm).
• For offset layouts of tile longer than 15″, use at least 1/8″ for rectified and 3/16″ for calibrated tile, on average.
• Follow the manufacturer's joint recommendation if it is wider.
• Leave movement joints at the perimeter, inside corners and in large fields, and fill them with sealant, not grout.

Once you've chosen a joint width, our tile calculator uses it to work out how many tiles and boxes you need, including a waste margin. For the full installation process, see how to lay tile step by step.

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