When laying tile, whether on a floor or on the wall, there are no set rules for how thick the grout lines has to be. In most cases it is more of a design choice than a requirement. That being said, there are a few important facts and some guidelines you should know when trying to decide how thick your grout lines should be.
In some cases you may have seen floors or walls where the tiles are laid so close to each other that it almost looks like there are no grout lines. The only way to achieve this look is to use what they call rectified tiles. Rectified tiles are evenly cut and uniform on all sides. The average tile make look the same in terms of color, but are almost never exactly the same shape and size throughout (hence the need for grout lines). Rectified tiles have been put through the extra step in the process to ensure they are even, but they also will cost more.
Grout Lines for Floor Tiles
As a general rule, floor tiles should have grout lines of 1/8 inch or larger, depending on the type and size of tile; most floor tiles are 8 inches by 8 inches, but larger versions are available. Special types, such as slate or quarry tiles, often do not have edges uniform enough to look good with tight spacing. These tiles usually are spaced with grout lines 3/16 inch or wider. Plastic spacers are usually used at tile corners to form even grout spaces. Some really large-format tiles, 18 by 18 inches square or larger, look good with very narrow grout lines that are filled with a grout colored to match the tile. This technique produces a floor that looks like a single slab.
Grout Lines for Wall Tiles
Small tiles, such as the 4-inch by 4-inch style commonly used on bathroom and kitchen walls, often are self-spacing. They have small protrusions called lugs that fit against adjoining tiles and create an automatic space for grout, normally 1/32 to 1/16 inch wide. Grout lines on walls usually do not exceed 1/16 inch because the tiles need to be stacked to help support them during installation.