Clay Shrinkage Calculator

Find your clay's shrinkage percentage from wet, dry, and fired measurements, and calculate the wet size needed to hit a target fired dimension.

Quick Facts

Formula
Shrinkage % = (Start length − End length) ÷ Start length × 100
Works with any consistent unit — inches, cm, or mm.
Typical range
Stoneware and earthenware bodies commonly shrink about 10-15% wet-to-fired
Porcelain bodies often run higher, commonly 12-20%.

Your Results

Calculated
Total shrinkage
-
Wet to fired, percent
Wet-to-dry shrinkage
-
Drying stage, percent
Dry-to-fired shrinkage
-
Firing stage, percent
Required wet size
-
To reach your desired fired size (mm)

Ready

Enter your wet, dry, and fired test-bar measurements, then press Calculate.

About the Clay Shrinkage Calculator

Clay shrinks in two stages between the wheel or slab and the finished piece: first as water leaves the clay during drying (wet to bone-dry), then again as the clay body vitrifies in the kiln (bone-dry to fired). This calculator uses length measurements from those three stages to find each shrinkage percentage, plus the wet size you need to build to hit a target fired dimension.

The formula

Shrinkage between any two stages is calculated the same way:

Shrinkage % = (Starting length − Ending length) ÷ Starting length × 100

Applying this formula stage by stage gives wet-to-dry shrinkage, dry-to-fired (firing) shrinkage, and total wet-to-fired shrinkage. Because the result is a percentage, any consistent length unit works — inches, centimeters, or millimeters — as long as you use the same unit for every measurement in one calculation.

Sizing a piece for a target fired size

To finish at a specific size, work backward from the total shrinkage percentage: Wet size = Desired fired size ÷ (1 − Total shrinkage ÷ 100). For example, a clay body with 12% total shrinkage and a 150 mm target fired height needs to be built to about 170.5 mm while wet.

Measuring shrinkage yourself

  • Roll out a flat test bar and mark two points a fixed distance apart (10 cm is a common standard) while the clay is still wet.
  • Measure the same span again once the bar is bone-dry, and a third time after it has been fired to your target temperature.
  • Different clay bodies — and even different batches of the same clay — can shrink at slightly different rates, so always test the specific clay and firing schedule you plan to use.

Frequently Asked Questions

How do I calculate clay shrinkage percentage?
Mark two points a known distance apart on a wet clay test bar (10 cm is a common standard), then measure that same span again once the bar is bone-dry and again after firing. Total shrinkage percentage equals (wet length minus fired length) divided by wet length, multiplied by 100.
What is a normal shrinkage rate for clay?
Most stoneware and earthenware clay bodies shrink roughly 10% to 15% from wet to fired. Porcelain bodies typically shrink more, often 12% to 20%, because of their finer particle size and higher firing temperature. Always check the manufacturer's spec sheet for your specific clay.
How do I size a piece so it comes out the right size after firing?
Divide your desired fired size by (1 minus the total shrinkage rate as a decimal). For example, with 12% total shrinkage and a 150 mm target, build the piece 150 / (1 - 0.12), or about 170.5 mm, while wet.
Does the unit of measurement matter?
No. Shrinkage is calculated as a ratio, so you can measure in inches, centimeters, or millimeters as long as you use the same unit for every measurement in a single calculation.

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