Tools for makers
Clay shrinkage calculator
Clay shrinks as it dries and again in every firing. Enter the size you want the finished piece to be, and this works out how big to make it while the clay is still wet.
Results
Wet clay sizes
Enter a finished size and a shrinkage figure, then press Calculate wet sizes.
How it works
Shrinkage is a share of the wet size, so the wet size is the finished size divided by what is left. A bowl that should finish 25 cm across, in a clay that shrinks 12%, needs to be thrown at 25 ÷ 0.88 = 28.41 cm. Adding 12% to 25 cm would give 28 cm, and the bowl would come out small.
Separate stages are multiplied, not added. 6% drying, 4% bisque and 3% in the glaze firing is 12.47% overall, not 13%.
A clay maker's figure is a fair start. Your own test tile, measured wet and again after the glaze firing, is better, because it accounts for how you work and how hot your kiln really fires.