Measure the diameters of both the inside and outside of the circular area. The inner diameter is the measurement from one inside edge to the opposite inside edge of the tile circle. The outer diameter is the measurement from one outside edge of the tile to the opposite outside edge of the tile circle.
Figure the outer circumference of the circle by multiplying the exterior diameter by pi, or 3.14. If your outer diameter, for instance, is 10 inches, the circumference of the circle equals 31.4 inches.
Divide the circumference of the circle by the width of the tiles you want to use to tile the circle. If you plan to use 2-inch tiles, for instance, divide 31.4 inches by 2 to get approximately 16 tiles needed for the space. Don't forget to factor in the distance that you must maintain to grout between the tiles.
Figure the inner circumference of the circle by multiplying the diameter by 3.14. If the inner circumference is 6 inches, for instance, the inner circumference equals approximately 19 inches.
Divide the inner circumference of the circle by the number of tiles you figured you needed to tile the circle to get the approximate size for the interior of each tile. If the inner circumference is 19 inches, for instance, and you need 16 tiles to cover the exterior of the circle, you need the interior of each of those tiles to be approximately 1.2 inches wide.
Measure the width you calculated for the interior edge of the tile on each tile for the circle by measuring in the same distance from each edge. To trim a 2-inch tile down to 1.2 inches, for example, measure in .4 inches from each side of the tile on the back of the tile and make a mark. Then, lay a ruler on the tile, so that it touches the mark on the right and lines up with the right corner on the opposite side of the tile, and repeat for the left side of the tile.
Cut the tiles on these lines with a tile saw to create tiles that angle in on both sides. Once cut, lay the tiles out along the circle. If the inner parts of the tiles are slightly too large, use the tile saw to trim them down a little bit at a time until they fit in the circle and enough space is left between the tiles for grouting.
Install the tiles with tile adhesive and let the adhesive dry. After the adhesive dries, grout between the tiles as you would grout any tile job.