Calculator

Clay shrinkage calculator

Use lengths you measured, or a shrinkage percentage whose stage you can name. This page does not know your clay. It does not judge firing maturity, glaze fit, strength, food use, or a kiln program.

Worked example, without the buttons

Measured shrinkage is 100 × (initial − final) / initial. A bar at 100 mm that finishes at 88 mm shrank 12%.

The starting length for a desired finish is desired / (1 − shrinkage / 100). For 100 mm after 12% shrinkage, start at 100 / 0.88 = 113.636… mm. For 10 cm after 12%, start at about 11.36 cm (11.3636… cm). Multiplying 10 cm by 1.12 gives 11.2 cm, and that shortcut is wrong.

Ten inches down to nine inches is 10%. The same lengths in centimetres, 25.4 cm to 22.86 cm, are also 10%. At 0% shrinkage the starting length equals the desired finish.

Two stages combine as 1 − (1 − first) × (1 − second). Five percent followed by 7% is 11.65%, not 12%. Do not add the percentages, and do not treat the linear percentage as a volume change.

A final length longer than the initial length is expansion or a swapped measurement. It is not a negative shrinkage target. An initial length of zero, a negative length, a missing number, or a shrinkage of 100% or more is rejected.

A manufacturer’s absorption percentage is a different test. The same clay can shrink differently when the process changes. Keep a process log beside the numbers. The measurement guide is not on the public site until a person approves it. A percentage from someone else’s sheet is not your measurement.