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Clay shrinkage calculator

Converts between wet, dry and fired dimensions for a known shrinkage rate, and works out your clay’s actual shrinkage from a test bar.

The question this answers: “I need a finished dish exactly 220mm across to fit a shelf. How big do I make it wet?”

From your clay supplier, or measured from your own test bar. Usually 10–15%.

Result

Make it this big when wet
250 mm
That is an increase of
13.6%
Note that this is larger than the shrinkage percentage — the commonest hand-calculation error is adding the shrinkage back on instead of dividing.
Show the working
  1. Shrinkage factor1 − (12 ÷ 100)0.88
  2. Required wet size220 mm ÷ 0.88250 mm
  3. Uplift needed(250 ÷ 220 − 1) × 10013.6%

Make it this big when wet: 250 mm. That is an increase of: 13.6%

How it works

Fired size = wet size × (1 − shrinkage). Required wet size = target fired size ÷ (1 − shrinkage). Measured shrinkage from a test bar = (wet length − fired length) ÷ wet length. Note that the reverse calculation is division rather than adding the percentage back on, which is the error most people make by hand: a 12% shrinkage requires making the piece about 13.6% larger, not 12%.

What it assumes

  • Shrinkage is treated as linear and uniform in every direction, which is a good approximation for most clay bodies and forms.
  • Total shrinkage combines drying shrinkage and firing shrinkage. Figures quoted by clay suppliers are usually total shrinkage to a stated cone — check which.
  • Typical total shrinkage ranges from around 10% for a heavily grogged stoneware to 15% or more for a fine porcelain, but this varies by body and by firing temperature.

Where it stops being reliable

  • Thrown forms shrink slightly more across the diameter than vertically, because of the alignment of clay particles during throwing. For precision work, measure in the direction that matters.
  • Shrinkage differs between firing temperatures for the same clay. A body fired to a lower cone shrinks less.
  • Does not account for warping, which changes dimensions in ways no percentage can predict.
  • The only reliable figure is one measured from your own clay in your own kiln. Supplier figures are a starting point.

Sources

Sources for this section

Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-13. More on Pottery.