How do you calculate clay shrinkage?
Fired size = wet size × (1 − shrinkage rate). Working backwards, wet size = target fired size ÷ (1 − shrinkage rate). A 12% clay body means a 4 in wet rim fires to about 3.52 in.
Work out what a wet pot will measure after firing — or the wet size you need to hit an exact finished dimension. Pick a clay body or enter the rate from your own test tile.
The most common studio range: 11–13%.
Fired measurement
3.52 in
Loses 0.48 in at 12% shrinkage
Typical ranges. Always confirm against your supplier's data sheet or a fired test tile — rates vary by batch and by how hot you fire.
| Clay body | Typical cone | Drying | Total (wet to fired) |
|---|---|---|---|
| Earthenware / terracotta | Cone 04–06 | 5–6% | 8–10% |
| Buff stoneware | Cone 5–6 | 6–7% | 11–12% |
| Dark / iron stoneware | Cone 5–6 | 6–7% | 12–13% |
| Stoneware | Cone 9–10 | 6–7% | 13–14% |
| Porcelain | Cone 6–10 | 7–8% | 12–15% |
| Grogged sculpture clay | Cone 4–6 | 4–5% | 6–9% |
| Paper clay | Cone 5–6 | 7–9% | 13–16% |
Fired size = wet size × (1 − shrinkage rate). Working backwards, wet size = target fired size ÷ (1 − shrinkage rate). A 12% clay body means a 4 in wet rim fires to about 3.52 in.
Stoneware usually 11–13%, earthenware 8–10%, porcelain 12–15%. Roughly half happens during drying and the rest in the glaze firing.
Yes — the same body taken from cone 6 to cone 10 vitrifies further and typically shrinks another 1–3%.
Different clay bodies, thicknesses, or firings shrink at different rates. Make lids and their pots from the same batch, fire them together, and log the shrinkage rate you measured so repeats match.
ClayCal records clay body, wet and fired dimensions, weight, firing and glaze for every piece — so repeat orders and matching sets land the same size twice.