Pottery Clay Shrinkage Calculator

A pottery clay shrinkage calculator tells you how big to make a piece wet so it fires to the size you want: divide the target fired size by (1 − shrinkage). It also predicts fired size from wet size and works out your clay’s real shrinkage percentage from a simple test bar.

How to use the pottery clay shrinkage calculator

1. Pick a mode. Choose “Target fired size → wet size” when you are planning a piece, “Wet size → fired size” when you want to know how a piece you already made will turn out, or “Measure my clay’s shrinkage” if you have fired a test bar.
2. Choose a unit. Inches, centimeters or millimeters. Results come back in the same unit, so no conversion is needed.
3. Enter the shrinkage. Type the total (wet-to-fired) shrinkage from your clay’s spec sheet, or switch to separate drying and firing figures if that is how your manufacturer lists them. You can also load one of the example clays.
4. Enter one or more dimensions. Height, diameter/width and length are all optional; fill in whichever you need.
5. Press Calculate. You get the wet or fired dimensions, the total shrinkage used, the linear factor, the approximate volume change and the formula, so you can check the math yourself.

For test-bar mode, roll out a slab, scribe a line of a known length (100 mm is convenient) while the clay is wet, measure it again when bone dry (optional) and once more after the final firing, then enter the lengths. The calculator accepts total shrinkage below 50%; that is this calculator’s supported limit, not a statement about what clay can physically do.

The clay shrinkage formula, explained

Total wet-to-fired shrinkage is measured against the wet length; drying shrinkage also uses the wet length, while dry-to-fired firing shrinkage uses the dry length. The formulas follow Digitalfire’s definitions:

• Shrinkage % = (wet length − fired length) ÷ wet length × 100
• Wet size = fired size ÷ (1 − shrinkage %/100)
• Fired size = wet size × (1 − shrinkage %/100)
• Volume factor = (1 − shrinkage %/100)³, assuming uniform shrinkage in all three dimensions

A common mistake is to add the shrinkage to the fired size (4 in + 11% = 4.44 in). Because shrinkage is a fraction of the larger wet size, you have to divide instead, which gives 4.49 in. The difference grows with higher-shrinkage porcelains and bigger pieces.

Many clays list drying and firing shrinkage separately. Digitalfire notes that firing percentages are “not total shrinkage wet-to-fired, but dry-to-fired,” so the two figures combine by multiplication: total = 1 − (1 − drying) × (1 − firing). The table below uses the manufacturers’ own published product text (checked November 9, 2026).

How much does pottery clay shrink?

It depends heavily on the clay body and how hot you fire it. Digitalfire describes a typical plastic pottery clay as shrinking about 6% in drying, and gives firing-shrinkage ranges of roughly 7–8% for whitewares, more than 10% for vitreous porcelains, about 5–6% for stonewares and 3–4% or less for earthenwares. Lakeside Pottery’s clay chart assumes a total shrinkage of 11–14%, with about half happening in drying and half in firing at cone 6.

Those ranges are useful for orientation, but they are not a substitute for your own clay’s figure. The same body can shrink more if you fire it hotter, less if you underfire it, and differently again from batch to batch. That is why the calculator asks you to enter the number rather than guessing it for you. Some product descriptions only give part of the picture; for example, SIO-2 PF Red Low Fire (Cone 07-04) is described with 5.5% drying shrinkage but no firing figure, so you would need a test bar to know its total.

Shrinkage matters even more for capacity. Assuming the clay shrinks uniformly in all three dimensions, volume shrinks with the cube of the linear factor, so an 11% linear shrinkage leaves only about 70.5% of the wet volume. Real pieces can shrink a little unevenly, so treat the volume figure as an approximation. If you want a mug to hold a certain amount after firing, size the wet form generously, and remember that walls and the base also take up space.

Methodology & sources

The calculator uses only standard geometry: the wet-length definition for total and drying shrinkage, the dry-length basis for firing shrinkage, their multiplicative combination, and the cube of the linear factor for volume (assuming uniform shrinkage in all three dimensions). Example clay figures are the manufacturers’ own product text for Laguna Frost WC437, Laguna #550, Laguna Dave’s WC384, SIO-2 PRAI, SIO-2 PA and SIO-2 PF (checked November 9, 2026). Results are rounded to two decimals.

• Digitalfire Reference Library – Drying shrinkage: formula and typical drying shrinkage
• Digitalfire Reference Library – Firing shrinkage: dry-to-fired basis and typical ranges by clay type
• Lakeside Pottery – Clay Poundage Chart: typical 11–14% total shrinkage
• Manufacturer product text (Laguna, SIO-2): average and drying/firing shrinkage figures shown in the table above, checked 2026-11-09

Last updated: November 2026

FAQ

How do I calculate clay shrinkage percentage?

Mark a line on a wet test bar, let it dry and fire it, then measure the line again. Shrinkage % = (wet length − fired length) ÷ wet length × 100, so a 100 mm mark that fires to 88 mm means 12% shrinkage.

How big should I make a pot so it ends up a certain size?

Divide the fired size you want by (1 − shrinkage). For a clay with 11% total shrinkage, a 4.00 in fired mug needs to be 4.00 ÷ 0.89 = 4.49 in wet.

Can I just add drying and firing shrinkage together?

Not exactly. Firing shrinkage is usually quoted on the dry length, so total shrinkage = 1 − (1 − drying) × (1 − firing). For 5.9% drying and 7.0% firing that gives 12.49%, slightly less than the simple sum of 12.9%.

How much does shrinkage reduce a mug’s capacity?

Assuming the clay shrinks uniformly in all three dimensions, volume shrinks by the cube of the linear factor. With 11% linear shrinkage the fired piece keeps 0.89³ ≈ 70.5% of its wet volume, a loss of about 29.5%.

Why doesn’t my result match the clay’s spec sheet exactly?

Manufacturer figures are averages, often with a stated tolerance such as ±2%, and actual shrinkage changes with firing temperature, water content and how the clay is worked. A test bar fired in your own kiln gives the most reliable number.

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