How to read the clay shrinkage chart
The fired dimension is the result you want. The required wet dimension is larger because it is divided by the proportion that remains after firing—not simply increased by the shrinkage percentage.
100 mm ÷ (1 − 11.5%) = 112.99 mm wet
Choose a row to prefill the ruler generator with that clay body's estimate, then replace the estimate when you have measured your own clay's actual shrinkage rate.
Why clay shrinkage varies
The values in the chart are widely-used starting estimates. Your actual shrinkage can differ because of:
- Clay formulation — the ratio of ball clay, kaolin, feldspar, and silica.
- Forming method — throwing, extruding, and hand-building can compress clay differently.
- Drying conditions — slow drying reduces stress cracking but can slightly affect final dimensions.
- Firing temperature and schedule — higher temperatures increase vitrification and total shrinkage.
- Glaze — glaze doesn't shrink the same way clay does and can affect overall dimensional change.
Common clay shrinkage rates
Typical shrinkage ranges by clay body type:
- Earthenware: 5–9% (lower-fire clays, more open body)
- Stoneware: 9–14% (mid to high-fire, denser after firing)
- Porcelain: 12–16% (fine particle size, high shrinkage)
- Raku and low-fire bodies: 4–7% (often open, groggy bodies)
Use these ranges for rough planning. For production work or precision fitting (lids, inserts, sets), always test your specific clay, glaze, and firing combination. Use the clay shrinkage calculator or print a ruler for your measured rate.