Clay Shrinkage Calculator
Calculate the wet clay size needed to achieve your target fired size.
Accounts for drying shrinkage and firing shrinkage for any clay body.
Why clay shrinks — and by how much:
Clay shrinks in two distinct stages: once during drying (as water evaporates from the clay body) and again during kiln firing (as the clay particles vitrify and fuse). Missing this calculation means your finished piece will be smaller than intended.
Total shrinkage formula:
Total shrinkage = 1 - (1 - Drying shrinkage) × (1 - Firing shrinkage)
Required wet size = Target fired size ÷ (1 - Total shrinkage)
Example calculation:
You want a fired bowl with a 10-inch diameter:
- Drying shrinkage: 6% (0.06)
- Firing shrinkage: 8% (0.08)
- Total = 1 - (0.94 × 0.92) = 1 - 0.8648 = 13.52% total shrinkage
- Required wet size = 10 ÷ 0.8648 = 11.56 inches
So you must throw or hand-build to 11.6 inches wet to end up with a 10-inch fired piece.
Typical shrinkage rates by clay body:
| Clay Type | Drying | Firing (Cone 6) | Total |
|---|---|---|---|
| Earthenware | 5–7% | 5–8% | 10–14% |
| Stoneware | 6–8% | 8–12% | 13–19% |
| Porcelain | 7–10% | 10–14% | 16–22% |
| Raku clay | 4–6% | 5–7% | 9–12% |
| Paper clay | 8–12% | 8–12% | 15–22% |
Important notes:
Shrinkage rates printed on clay bags are typical values — actual shrinkage varies based on:
- Throwing technique (walls compressed differently than pulled)
- Drying speed (too-fast drying warps and creates uneven shrinkage)
- Kiln temperature (cone 10 fires tighter than cone 6 for the same clay)
- Whether the piece is covered during drying
Test tiles:
Always make shrinkage test tiles from each new bag of clay. Score a 10cm line on a wet tile, dry it, fire it, then measure. The difference tells you the exact shrinkage percentage for your clay and firing temperature.
Actual shrinkage % = (Wet length - Fired length) ÷ Wet length × 100