When clay dries, it shrinks. More specifically, it shrinks about 6% when it dries, and another 6% when it is fired to cone 10. Generally, the total shrinkage is between 10-14%, depending on the clay body and how much it is rolled out or thrown. This tool is designed so you can quickly estimate how much a clay body will shrink, so you can size and plan when you first roll it out.
This Shrink Ruler from Creative Industries has four clay shrinkage percentages so you can better match lids to bowls.
To use, measure the bowl, then calculate the percentage. Then use the same percentage on the lid.
The Creative Industries Shrink Ruler is flexible, moisture-tolerant, and easily cleaned.