Clay Shrinkage Calculator (drying & firing shrinkage, size to throw)

Understanding the clay shrinkage calculator — concept and principle

This calculator measures how much your clay shrinks while drying and firing, using a marked test bar, and tells you how big to make a piece so it comes out of the kiln at the size you want. Potters use it for lids that must fit, sets of matching mugs, tiles and commissions with a fixed finished size.

Clay shrinks twice: with the default inputs the result is 22.73 cm wet (to finish at 20 cm).

How it is calculated

With Lp = plastic (wet) length, Ld = dry length and Lf = fired length between the same marks:

Drying shrinkage (%) = (Lp − Ld) ÷ Lp × 100
Total shrinkage St (%) = (Lp − Lf) ÷ Lp × 100
Firing shrinkage (%) = (Ld − Lf) ÷ Ld × 100, measured against the dry length
Wet size = finished size ÷ (1 − St)
Bone-dry size = finished size × Ld ÷ Lf
Volume shrinkage ≈ 1 − (1 − St)³

The principle

Clay shrinks twice. While drying, the water films between the clay particles leave and the particles pull closer together. During firing, the clay body sinters: glassy material forms and fills pores, so the piece shrinks again. Expressing shrinkage as a share of the original wet length makes the steps add up on one scale, which is how the ASTM test method reports it. To work backwards from a fired size you must divide by (1 − St), not multiply by (1 + St), because the shrinkage is a share of the larger wet size. Volume shrinks with the cube of the linear factor if the clay shrinks equally in every direction.

Worked example

Default inputs: test bar 100 mm wet, 94 mm dry, 88 mm fired; target finished size 20 cm.

Drying shrinkage = (100 − 94) ÷ 100 = 6%.
Total shrinkage = (100 − 88) ÷ 100 = 12%.
Firing shrinkage = (94 − 88) ÷ 94 = 6.4% of the dry length.
Wet size = 20 ÷ (1 − 0.12) = 20 ÷ 0.88 = 22.73 cm.
Bone-dry size = 20 × 94 ÷ 88 = 21.36 cm; volume shrinkage = 1 − 0.88³ = 31.9%.

How to read the result

The wet size is the size to throw or roll; the bone-dry size lets you check a piece before bisque firing. The large volume figure explains why thick pieces crack: they lose a third of their volume unevenly. If lids must fit, make lid and pot from the same clay batch and fire them together.

Common mistakes

Multiplying by 1.12 instead of dividing by 0.88, which gives a piece that ends up too small.
Using the shrinkage printed on the bag, which was measured at a different temperature.
Measuring the bar edge-to-edge instead of between scored marks.
Testing at bisque temperature and applying it to a glaze firing.

Keep in mind: Shrinkage changes with the clay body, water content, how hard the clay is worked and the firing temperature. Re-test when any of these change.

Frequently asked questions

My clay shrinks 12%. How big should I make a 20 cm bowl?

Divide by 0.88: 20 ÷ 0.88 = 22.7 cm. Multiplying by 1.12 gives only 22.4 cm.

How do I make a test bar?

Roll a slab, cut a bar, and score two marks exactly 100 mm apart while wet. Measure the marks again when bone dry and after the final firing.

Why is volume shrinkage so much bigger?

The piece shrinks in all three directions, so 12% linear shrinkage removes about 32% of the volume.

Sources: ASTM C326, Standard Test Method for Drying and Firing Shrinkages of Ceramic Whiteware Clays

Checked 9 Nov 2026. ASTM C326, Standard Test Method for Drying and Firing Shrinkages of Ceramic Whiteware Clays (C326-03), Section 5.1 Eq. 1: linear drying shrinkage as a percentage of the plastic length; total shrinkage measured on the fired specimen on the same basis. The test-bar readings are an example.

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