In general, there is not a specific standard for the size of a grout joint.  However, there are many relevant parameters that should be considered.

What is the amount of variation from tile to tile?

Are the edges of the tile linear or irregular (e.g. “hand-molded”)

How big is the tile?

What is the surface of the tile; can it be easily scratched?

Where is the tile being used?

Is the surface level?

Ultimately, the project owner should choose the grout joint they desire, keeping in mind that a tighter grout joint will show more variation from tile to tile (or two times the maximum variation) tends to look irregular and poorly installed.

Also, grout accommodates differences in the angle of the surface.  Where the floor or wall is not level, the grout will slope from one tile to another.  When tiling over a hump, the grout joint opens up; and when tiling across a depression, the top of the grout joint narrows.

Tile spacing is measured between tile edges – not from the top edge of the bevel on the tile.  The majority of tile made today has a beveled edge and grout should not be installed on the bevel.  The bevel is often a glazed surface which is intended to transition from on tile to another.  On some tiles, the installer must “hand-tool” the grout to keep grout off of the bevel.

A common mistake is to set the tile too close.  Often, the finished results look sloppy due to variations in the floor or wall and in the tile.  Even small variations can throw off the pattern of the tile if adjustments are not made in the grout joint.  Although 1/16” of an inch may seem unimportant (for example, on a 12” tile), it represents a 50% variation in a grout join 1/8 inch wide.  This would be immediately noticeable and unattractive.

What is standard for variations in grout joints?

When evaluating grout joints, it is important to consider that the grout is used to adjust for differences in the following:

Variations in the size of the tile

Changes in the plane of the substrate

Changes in the thickness of the tile (often this applies to hand-molded tile)

Variations in the rustic profile of the tile

The standards for the manufacture of tile allow for variation from tile to tile.  While the standard details this exactly, it is not uncommon for some manufacturers to ship tile with about 3/32” difference between the largest and smallest tiles in a box.

Grout must adjust for these differences between tiles so understandably there can be some variation in the width of a grout joint.

Generally, it is advisable to use a grout joint at least two times the average difference between the largest tiles and the smallest tiles.  A smaller joint will exacerbate the differences between tiles and the human eye can readily see very small differences as a percentage of the total grout joint.  For example, while a difference of a 1/16” of an inch may seem small on a 12” tile, this is readily apparent compared to a 1/8” grout joint.  As the plane of the tile changes, the grout joint allows for this change.  Should tile go over a hump in the floor, the grout joint will open; when tile follows a depression in the floor, the grout joint will narrow.