Art and Craft Supplies
Clay shrinkage.
Enter the drying and firing shrinkage from your clay's data sheet to see the fired size of a wet piece, or the wet size to make for a target fired size.
Total shrinkage = 1 - (1 - drying) x (1 - firing). The two percentages do not add. With 6.25% drying and 6.66% firing shrinkage the total is about 12.5% (12.49% unrounded), not about 12.9% (12.91 unrounded).
Shrinkage figures
Sizes
Fill the form and press Calculate.
Worked example
A 10 cm wide wet slab from a clay with 6.25% drying shrinkage and 6.66% firing shrinkage. After drying it is 10 x (1 - 0.0625) = 9.375 cm (9.38 on the tool, which rounds to 2 decimals). Firing takes 6.66% off the dry length: 9.375 x (1 - 0.0666) = 8.75 cm. The total shrinkage is (10 - 8.75) / 10 = about 12.5% (12.49% unrounded), which is less than the about 12.9% (12.91 unrounded) you would get by adding 6.25 and 6.66. To fire to exactly 10 cm, make the piece 11.43 cm wet. With only a total, 12% on a 30 cm piece fires to 26.4 cm, and a 25 cm target needs 28.41 cm wet.
How it works
Clay loses size twice. Drying shrinkage is measured from the wet size to the bone-dry size. Firing shrinkage is measured from the dry size to the fired size. Because the two start from different sizes, they cannot be added: the fired size is the wet size multiplied by (1 - drying) and then by (1 - firing).
Use figures from your own clay supplier and the temperature you fire to, since shrinkage changes with the body and the cone. Test tiles are the reliable way to confirm it for a particular clay and kiln. Lids, handles and feet that must fit later should be made from the same batch of clay.
Sources and basis (opened 2026-10-08): the rule that dry and fired shrinkage start from different sizes and cannot be added, with the 6.25 + 6.66 = 12.5 total example, from Drying Shrinkage (Digitalfire Reference Library).
FAQ
Can I add drying shrinkage and firing shrinkage?
Not exactly. They are measured from different starting sizes. With 6.25% drying and 6.66% firing the true total is about 12.5% (12.49% unrounded), a little less than the simple sum of about 12.9% (12.91 unrounded).
How much will my clay shrink?
It depends on the clay body and the temperature it is fired to. Use the figures on your supplier's data sheet, or fire a measured test tile in your own kiln.
How big should I make a piece to fire to a given size?
Divide the target fired size by (1 - drying) x (1 - firing). For a 10 cm target with 6.25% and 6.66% that is 11.43 cm wet.
What if my data sheet only gives one shrinkage figure?
If it is the wet-to-fired total, choose "total shrinkage only". If it is a firing figure, use it with a separate drying figure.
Does the calculator work for height as well as width?
Yes. Shrinkage applies to every dimension, so run each length you care about, or use the same percentage for all of them.
Disclaimer
Results are arithmetic from the numbers you enter. Real clay varies by batch, forming method and firing. Always confirm with test tiles from your own clay and kiln, and follow the kiln and clay maker's safety instructions.