Clay Shrinkage Calculator
Published shrink rates are a starting point; your studio's number is what counts. Measure a test bar before and after, enter both, and get the shrinkage percentage plus the scaling factor for sizing every piece afterwards.
Scribe a long, straight line — 100 mm or 4 inches is standard. Longer lines give a more precise percentage.
Formula & how it works
Shrinkage is (start − end) ÷ start × 100. The scaling factor for going the other way — sizing wet work to finish at a target — is start ÷ end, equivalently 1 ÷ (1 − shrinkage ÷ 100).
Worked example
Scribe a 100 mm line on a wet test bar. After firing it measures 88 mm, so shrinkage is (100 − 88) ÷ 100 = 12%. The scaling factor is 100 ÷ 88 ≈ 1.136, meaning a piece you want to finish at 4 inches should be built about 4.55 inches wet.
Measuring shrinkage properly
Make the test bar honest
Roll several bars from the same wedged clay, scribe a long straight line with clean end marks, and measure while the clay is still fully wet. Fire them exactly as you fire real work — same cone, same schedule, same position in the kiln — because all of those shift the result.
Drying and firing are separate stages
Clay loses size first as water leaves and again as the body vitrifies. Measuring at dry and bisque as well as fired shows where the movement happens, which is useful when work cracks: heavy drying shrinkage points to uneven moisture loss rather than anything in the kiln.
One number per clay, per cone
A shrink rate belongs to a specific clay body fired to a specific temperature. Switching from stoneware to porcelain, or from cone 6 to cone 10, changes it enough to ruin a piece meant to fit a lid or a shelf. Keep a note of each combination you use.