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Pottery Shrinkage Calculator

Enter your target fired size and shrinkage to find the wet size to throw.

How it is calculated

Shrinkage is measured against the wet size: shrink% = (wet − fired) / wet × 100. Turning that around gives wet = fired / (1 − shrink/100), and the size lost is wet − fired. The fired size is the finished dimension you want, in centimetres, and the shrinkage is the total percentage from wet clay to finished piece, drying and firing together. The shrinkage must be at least 0 and below 100, and the fired size must be above zero. Both results are rounded to two decimals. The tool uses one number for the whole piece, so it cannot allow for different shrinkage on the glazed and unglazed parts, or for warping.

Worked example

You want a mug that is 10 cm tall after the final firing, and your clay's total shrinkage is 12%. The share of the wet size that remains is 1 − 0.12 = 0.88. Then 10 / 0.88 ≈ 11.36, so you should throw the mug 11.36 cm tall. The piece loses 11.36 − 10, about 1.36 cm, during drying and firing.

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Common mistakes

  • Adding the shrinkage on top of the target. Multiplying 10 cm by 1.12 gives 11.20 cm, which is too small; you must divide by 0.88 to get 11.36 cm.
  • Using only the drying or only the firing shrinkage from the clay data. The field needs the total from wet to fully fired.
  • Entering 0.12 for a 12% shrinkage. The field takes percent, so type 12.
  • Measuring a dry or bisque piece as if it were wet. Shrinkage is defined from the wet size, so measure the freshly thrown piece.
  • Trusting a single percentage for every firing. Shrinkage varies with the clay body and the firing temperature, so check it on a test tile of your own clay.

About this calculator

Clay shrinks as it dries and again as it fires, so a potter has to throw or build a piece larger than the target finished size. The math inverts the standard shrinkage formula — shrinkage% = (wet size − fired size) ÷ wet size — to solve for wet size = fired size ÷ (1 − shrinkage%). Typical shrinkage runs about 10-13% for stoneware clay bodies and can reach 15%+ for some porcelains, and every clay's specific rate is usually listed on the bag or test-tile data from the supplier.

Studio potters and ceramicists use this before throwing a functional piece like a mug or bowl that needs to fit a specific lid, stack with a set, or hit a precise finished diameter, while production potters use it to keep a whole batch consistent across the wet-to-bisque-to-glaze-fire shrink cycle.

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