How to Work Backward From Finished Size to Greenware Size
One of the most common mistakes in clay work is sculpting a piece at exactly the size you want it to end up. Clay first shrinks as moisture leaves during drying, then shrinks again during firing as the particles bond more tightly under heat. High-fire clays like porcelain can have total shrinkage of 12 to 15%, meaning the finished piece can end up noticeably smaller than what you originally sculpted.
This calculator works in reverse. Enter your desired finished size and the clay type you're using, and it works backward to tell you how large to sculpt the piece right now to account for shrinkage. For example, targeting a 4-inch finished piece in high-fire stoneware (about 11% shrinkage) means you need to sculpt it at roughly 4.49 inches. For materials with negligible shrinkage like polymer clay, you can simply sculpt at your target size.
Actual shrinkage rates vary somewhat by clay brand and firing temperature, so for precision work it's worth checking the specific product specs for the clay you're using.
Frequently Asked Questions
As clay dries, moisture leaves and particles pack closer together, then firing at high heat bonds the particles even tighter, causing further volume loss.
Polymer clay contains almost no moisture and doesn't shrink meaningfully when cured in an oven, so no adjustment is needed.
For a homogeneous clay body, shrinkage is generally similar in all directions, so this calculator applies the same rate to length, width, and height.