Clay shrinks as it dries and again when it is fired. This calculator helps you measure your clay’s shrinkage from a test tile and work out how big to make a piece so it ends up the size you want.
Clay shrinkage
1. Measure your clay (optional)
2. Size your piece
How it works
Shrinkage % = (wet length − fired length) ÷ wet length × 100. Draw a line of known length, such as 100 mm, on a flat test tile, then measure it again after firing.
To end up with a given size, make the piece at fired size ÷ (1 − shrinkage). For example, with 10% shrinkage, divide by 0.9.
Worked example
A 100 mm line measures 90 mm after firing, so the clay shrank 10%. To get a 30 cm tile after firing, make it about 33.33 cm wide when wet. A piece made at 30 cm wet will end up about 27 cm.
Good to know
- Shrinkage depends on the clay body, how wet it is and the firing temperature. Always test the clay and kiln you will actually use.
- Fire the test tile at the same temperature as the real piece. A hotter firing usually shrinks clay more.
- Big or thick pieces can warp or shrink unevenly, so allow a little extra where fit matters, such as lids.
Frequently asked questions
Why do I need to make pottery bigger than the final size?
Clay loses water as it dries and tightens further in the kiln, so the piece shrinks. Making it larger at the start makes up for this.
How do I measure clay shrinkage?
Draw a line of known length on a wet test tile, let it dry, fire it, and measure the line again. The difference divided by the wet length gives the shrinkage.
Is shrinkage the same for every clay?
No. Different clays and firing temperatures shrink by different amounts, which is why a test with your own clay is the best guide.