Pottery Clay Shrinkage Calculator

The Pottery Clay Shrinkage Calculator takes the guesswork out of sizing pottery. Clay shrinks as it dries and again as it is fired — typically 8% to 15% in total — so a mug thrown at 5 inches tall will not come out of the glaze kiln at 5 inches. This tool converts wet (greenware) sizes to fired sizes, works backwards from a target fired size to the wet size you should throw or hand-build, and can measure your clay body's actual shrinkage rate from a simple test tile — with clay body presets, an animated before/after visual, and a step-by-step formula breakdown.

Why Does Clay Shrink?

Shrinkage happens in two stages. During drying, the water that separates the clay platelets evaporates and the particles draw closer together — this accounts for roughly half of the total shrinkage. During firing, chemically bound water burns off and, at higher temperatures, the body vitrifies: glassy phases form and pull the particles even tighter. Hotter firings densify the clay more, which is why the same clay shrinks more at cone 10 than at cone 06, and why porcelain (fired to full vitrification) shrinks more than earthenware.

Clay Shrinkage Formulas

The classic mistake: to compensate for 12% shrinkage, you cannot simply make the piece 12% bigger. Shrinkage is measured from the larger wet size, so the reverse calculation must divide by 0.88 (a 13.64% enlargement), not multiply by 1.12. For a 10-inch fired plate: 10 ÷ 0.88 = 11.36 in wet, while 10 × 1.12 = 11.2 in would fire to only 9.86 in — visibly short of the target.

Typical Shrinkage Rates by Clay Body

Total wet-to-glaze-fired linear shrinkage varies by clay body and firing temperature. These ranges are typical manufacturer figures — always confirm with your clay's spec sheet or your own test tile:

How to Run a Shrinkage Test Tile

1. Roll a slab of your clay about 1 cm (3/8 in) thick.
2. Scribe a line exactly 10 cm (or 10 in) long on the wet slab with a ruler and a needle tool. Marking exactly 10 makes the math instant: every 0.1 lost = 1% shrinkage.
3. Dry and fire the tile through your normal full cycle — bisque and glaze firing at your usual cone.
4. Measure the line again and enter both numbers in the Measure Shrinkage Rate mode of this calculator.

Tips for Working With Shrinkage

How to Use This Calculator

1. Choose a mode: Wet → Fired to predict a final size, Target Fired → Wet to work backwards from the size you want, or Measure Shrinkage Rate to find your clay's rate from a test tile.
2. Set the shrinkage rate: pick a clay body preset (earthenware, stoneware, porcelain…) or type the rate from your clay's spec sheet or test tile.
3. Enter your dimensions — up to three at once (height, width/diameter, depth) in inches or centimeters.
4. Click Calculate to see the converted sizes, the animated wet-vs-fired visual, and the full formula breakdown.

Frequently Asked Questions

How much does pottery clay shrink?

Most clay bodies shrink between 8% and 15% in total from wet to glaze-fired. Earthenware is typically around 5–8%, mid-fire stoneware around 11–13%, and porcelain around 12–16%. Roughly half the shrinkage happens during drying and the rest during firing. Always check your specific clay's spec sheet or run a test tile.

How do I calculate the wet size for a target fired size?

Divide the target fired size by (1 − shrinkage rate ÷ 100). For example, for a 10 inch fired plate with 12% shrinkage: 10 ÷ 0.88 = 11.36 inches wet. Simply adding 12% (10 × 1.12 = 11.2) gives a piece that fires too small, because shrinkage is measured from the larger wet size.

How do I measure my clay's shrinkage rate?

Roll a slab and scribe a line exactly 10 cm (or 10 inches) long on it. Dry and glaze-fire the tile as you normally would, then measure the line again. Shrinkage rate = (wet length − fired length) ÷ wet length × 100. A 10 cm line that fires to 8.8 cm means 12% shrinkage.

Does clay shrink the same amount in all directions?

Approximately, yes — linear shrinkage applies to every dimension, so a 12% rate shrinks height, width, and depth each by about 12%. In practice thrown pieces can shrink slightly more in height than width because of particle alignment from throwing, and thick or unevenly dried pieces can warp. Volume shrinks much more than length: at 12% linear shrinkage, volume drops by about 32%.

Why did my piece shrink more than the bag said?

Published rates are measured at a specific cone; firing hotter increases shrinkage. Wetter, softer clay also starts larger, so it loses more in drying. Recycled clay with more water, a different cone than the spec sheet, or measuring before the clay was fully wet all shift the result. A test tile fired in your own kiln at your own cone is the only reliable number.

Does glaze affect shrinkage?

Glaze itself adds only a negligible layer of thickness and does not change the clay's shrinkage. What matters is the glaze firing temperature: the body continues to densify and shrink during the glaze firing, which is why total wet-to-glaze-fired shrinkage is larger than wet-to-bisque shrinkage.

Additional Resources

• Pottery - Wikipedia
• Vitrification - Wikipedia