Pottery Clay Shrinkage Calculator
Enter a size and your clay body's total shrinkage. The calculator shows the fired size, or works backwards to the size to throw or build so the finished piece comes out right.
The formula
Worked example. a piece made 10 cm tall in a clay with 12% shrinkage fires to 8.8 cm; to finish at 10 cm, make it about 11.4 cm.
How to use it
- Choose whether you know the wet size or the fired size you want.
- Enter the total shrinkage from your clay's data sheet. Use the wet-to-final-firing figure, not just drying or bisque shrinkage.
- For lids, sets or anything that must fit a fixed object, measure your own clay with a test tile and use that number instead.
- Apply the scale factor to every dimension; shrinkage is the same in all directions for most bodies, though tall forms can slump slightly.
Questions
Why measure my own clay?
Published figures are averages. Your firing temperature, kiln and how wet you work the clay all shift the result by a percent or two, which matters for lids, tiles and pieces made to fit hardware.
Is shrinkage the same in every direction?
Roughly, for most clay bodies. Thrown forms can shrink slightly more in height than width, and large flat pieces may warp, so test with a form similar to the final piece when accuracy matters.
Does glaze change the size?
Not measurably. Glaze adds a fraction of a millimetre of thickness. Account for it only where a lid or insert must slide into a glazed opening.