Ceramic, or clay-based, tiles come in a wide range of sizes, from small-sized mosaics up to tiles as large as 60 cm (24 inches) in some cases. Because each size of tile requires a different mortar bed thickness, you must size your trowel to the size of the ceramic tile you are installing.
Mosaic ceramic tiles
While you will smooth out mosaic tile mortar prior to installation, you still must key it to the correct depth beforehand. For mosaics ranging from 9 mm (3/8 inch) up to 5 cm (2 inches) in size, use a V-notch trowel that measures 5 mm by 4 mm (3/16 by 5/32 inches). You can install 5 cm (2 inch) tiles using a 6 mm by 4 mm (1/4 by 3/16 inch) V-notch trowel as well.
If necessary, you can use a 6 mm (1/4 inch) square-notch trowel with 5 cm (2 inch) ceramic tiles, but avoid this size for mosaics any smaller.
Ceramic wall tiles commonly come in 10 cm (4 inch), 7.5 by 15 cm (3 by 6 inch), 15 cm (6 inch), 10 cm by 20 cm (4 by 8 inch) and occasionally 20 cm (8 inch) sizes. For the majority of ceramic wall tiles, a 6 mm (1/4 inch) square-notch trowel is adequate. You can also use a 6 mm by 5 mm (1/4 by 3/16 inch) V-notch trowel to install 10 cm (4 inch) tiles, but do not install that size of tile with a trowel any larger.
Install 10 cm by 20 cm (4 by 8 inch) and 20 cm (8 inch) wall tiles with a 6 mm by 9 mm (1/4 by 3/8 inch) or 1.2 cm (1/2 inch) square-notch trowel. A 6 mm by 9 mm (1/4 by 3/8 inch) square-notch trowel is also adequate for installing 15 cm (6 inch) square wall tiles.
Small floor tiles
Ceramic floor tiles are available in 10 cm, 15 cm, 20 cm (4 inch, 6 inch and 8 inch) sizes. Floor tiles are typically thicker than the corresponding wall tiles and need a slightly larger trowel to accommodate the mortar. Use a minimum of a 6 mm by 9 mm (1/4 by 3/8 inch) square-notch trowel. A 1.2 cm (1/2 inch) square-notch trowel is also fine for 20 cm (8 inch) floor tiles. U-notch trowels that are 6 mm by 8 mm (1/4 by 5/16 inches) are adequate for all ceramic floor tile sizes up to 20 cm (8 inches) square. You will require a larger trowel for rectangular-shaped tiles that exceed these sizes.
Large floor tiles
Ceramic floor tiles and some wall tiles are also available in 15 by 30 cm (6 by 12 inch), 30 cm (12 inch), 30 by 60 cm (12 by 24 inch) and 40 cm (16 inch) sizes. These thicker tiles require a much deeper mortar to beat the tile into place to prevent the corners from lipping.
Use a 6 mm by 9 mm (1/4 by 3/8 inch) square- or U-notch trowel for floor tiles up to 30 cm (12 inches) in size. For tiles over 30 cm (12 inches) in size, use a 6 mm (1/2 inch) square-notch, 6 mm by 1.2 cm (1/4 by 1/2 inch) U-notch or 1.8 cm by 1.5 cm (3/4 by 5/8 inch) U-notch trowel. The thicker the tile, the larger the trowel required; tiles thicker than 1.5 cm (3/8 inch) will require larger U-notch trowels.