How Is Clay Shrinkage Calculated?
Clay shrinks because water molecules occupying the spaces between clay particles evaporate during drying, and because silica particles fuse and densify during firing (vitrification). This two-stage process means a pottery piece fired to cone 6 will be considerably smaller than the original wet form. Shrinkage is expressed as a percentage of the original wet dimension and calculated separately for each stage.
Note: Total shrinkage is not simply the sum of drying + firing shrinkage, because firing shrinkage is calculated from the dry dimension, not the wet dimension. Total shrinkage is always measured wet-to-fired.
What Shrinkage Rate Should I Expect?
Typical total shrinkage rates vary significantly by clay body type and firing temperature. These are industry-standard reference ranges based on manufacturer data and studio testing:
| Clay Body Type | Typical Total Shrinkage | Common Firing Range |
|---|---|---|
| Porcelain | 12–16% | Cone 6–10 |
| Stoneware (grogged) | 8–11% | Cone 6–10 |
| Earthenware | 6–8% | Cone 06–1 |
| Terracotta | 5–7% | Cone 06–02 |
| Raku clay | 7–10% | Cone 06–6 |
Always test your specific clay body — moisture content and firing temperature affect final shrinkage. Use at least 3 test tiles and average the results.
Pottery Shrinkage Examples: Mugs, Lids & More
Throwing a Mug
You want a finished mug 100mm tall. Your clay has 12% total shrinkage. Use the Target Size Calculator to find you should throw at 113.6mm wet.
Fitting Lids
When throwing a lid and body separately, apply the shrinkage factor to the gallery measurement. Throw lids 2–3mm larger than calculated to allow for final fitting after firing.
Pro tip: Measure the gallery wet, not bone-dry, for the most accurate lid fit calculation.
Why Are My Pottery Shrinkage Results Inaccurate?
- cancel Measuring irregular shapes. Always use a standardized 100mm shrinkage bar or scribe a straight line on a flat test tile — not curved surfaces.
- cancel Ignoring directionality. Clay particles align during throwing and rolling, causing unequal shrinkage vertically vs. horizontally. Measure both axes for functional forms.
- cancel Using one test tile. Clay moisture varies batch-to-batch. Test at least 3 tiles and average the results for a reliable shrinkage figure.
- cancel Measuring too soon after firing. Let pieces cool completely before measuring. Thermal expansion while hot will give falsely low shrinkage numbers.