Wet to fired, both ways
Clay shrinkage calculator
Shrinkage is (wet length − fired length) ÷ wet length × 100. Mark 100 mm on a wet test tile; if the marks are 87.5 mm apart after firing, your clay shrinks 12.5%, and a mug you want 90 mm tall should be thrown 102.9 mm tall.
Result
Test tile: 100 mm wet, 87.5 mm fired
12.5%linear · 33% by volume
Divide each fired size by 0.875 to get the size to make
- Linear shrinkage
- 12.5%
- Volume shrinkage
- 33%
- Make it larger by
- 14.3%
- Drying (wet to dry)
- 6.25%
- Firing (dry to fired)
- 6.67%
- Drying + firing, added
- 12.92% (not the total)
| Dimension | Fired size you want | Make it wet at |
|---|---|---|
| Height | 90 mm | 102.9 mm |
| Width or diameter | 80 mm | 91.4 mm |
| Capacity | 300 mL (10.1 US fl oz) | 448 mL (15.1 US fl oz) |
Shrinkage ruler
Measure wet clay with it and read the fired size. Print at 100% (actual size) and check the 50 mm line against a real ruler.
How to measure your clay's shrinkage
Roll a test tile of the clay you will use, press two marks exactly 100 mm (or 4 in) apart while it is wet, then measure again when it is dry and after the glaze firing. Fire it to the cone you will use, because clay shrinks more the closer it gets to maturity.
- Size to make divides each fired size by 1 − shrinkage. Fired size multiplies by it.
- Volume shrinkage is 1 − (1 − s)³, assuming the clay shrinks the same amount in every direction. At 12.5% linear, a mug loses about a third of its capacity.
- Drying and firing shrinkage don't add up to the total, because firing shrinkage is measured on the dry length. Digitalfire's example: 6.25% plus 6.66% looks like 12.9%, but the total is 12.5%.
Sources
- Shrinkage measured on the wet (plastic) length: Digitalfire, Drying Shrinkage; fired shrinkage on the dry length: Digitalfire, SHAB shrinkage and absorption test; the 6.25% + 6.66% example: Digitalfire, “Drying shrinkage + firing shrinkage ≠ total shrinkage”.
- Example clay body: Plainsman Clays, M370 (dry 5.5–6.9%, fired 6.0–7.4% at cone 6, checked 2 November 2026). The total range is our sum by Digitalfire's method. Your clay's data sheet will differ.
- Volume shrinkage, 1 − (1 − s)³, is the cube of the linear factor for a clay that shrinks evenly in all directions. 1 in = 25.4 mm and 1 US fl oz = 29.5735 mL: NIST Guide to the SI, Appendix B.8.
Questions
How do I calculate clay shrinkage?
Shrinkage % = (wet length − fired length) ÷ wet length × 100. Marks 100 mm apart that fire to 88 mm mean 12% shrinkage.
How big should I throw a mug?
Divide the fired size you want by 1 − shrinkage. With 12% shrinkage, a mug that should be 90 mm tall after firing is thrown 90 ÷ 0.88 = 102.3 mm tall.
How much does a pot's capacity shrink?
Capacity shrinks with the cube of the linear factor. At 12.5% linear shrinkage, 0.875³ is about 0.67, so a pot that held 450 mL wet holds about 300 mL fired.