Clay Shrinkage Calculator

Understanding Clay Shrinkage

When clay dries or is fired in a kiln, water evaporates and the material densifies, causing the piece to shrink. The amount of shrinkage depends on the type of clay body, its water content, and the firing temperature. This calculator estimates the final size of a clay piece after drying or firing, helping you plan dimensions accurately before you start working.

How the Calculation Works

The calculator uses a simple linear shrinkage formula:

Final Size = Original Size × (1 - Shrinkage Percentage / 100)

For example, if you start with a slab that is 100 mm wide and your clay shrinks by 12%, the final width will be 100 × (1 - 0.12) = 88 mm.

Shrinkage percentages vary by clay body. Common ranges include:

• Earthenware: 6%–10% total shrinkage
• Stoneware: 10%–15% total shrinkage
• Porcelain: 12%–18% total shrinkage
• Raku: 4%–8% total shrinkage

These values include both drying shrinkage and firing shrinkage combined. Some clays shrink more during drying, others more during firing. Always check the manufacturer's specifications for the most accurate percentage.

How to Use the Calculator

Enter the original dimension of your clay piece (length, width, or height) and the expected shrinkage percentage for your specific clay body. The calculator will return the estimated final dimension after drying or firing.

For best results:

• Use the same unit of measurement throughout (millimeters, centimeters, or inches).
• Test your clay by making a small shrinkage test tile before starting a large project.
• Account for shrinkage in all three dimensions if your piece is not uniform.

Example

You are making a porcelain mug and want the finished height to be 100 mm. Your porcelain clay shrinks 15% total. To find the starting height:

Starting Size = Final Size / (1 - Shrinkage Percentage / 100)

Starting Size = 100 / (1 - 0.15) = 100 / 0.85 ≈ 117.6 mm

You should build the mug to approximately 118 mm tall to achieve a final height of 100 mm after firing.

Understanding Your Results

The result is an estimate based on the shrinkage percentage you provide. Actual shrinkage can vary due to:

• Moisture content: Wetter clay shrinks more during drying.
• Firing temperature: Higher temperatures cause more vitrification and shrinkage.
• Clay conditioning: Wedging and recycling clay can alter shrinkage behavior.
• Thickness: Thicker sections may dry and shrink unevenly.

Always make test tiles under the same conditions as your final piece to confirm the shrinkage rate.

Common Mistakes

• Using the wrong shrinkage percentage: Confusing drying shrinkage with total shrinkage (drying + firing) is a frequent error. Always use the total shrinkage unless you are calculating for only one stage.
• Ignoring uneven shrinkage: Clay does not always shrink uniformly, especially in complex shapes or pieces with varying thickness.
• Forgetting to account for glaze: Glaze application does not significantly change clay body shrinkage, but glaze fit can affect the final piece if the clay and glaze shrink at different rates.

Practical Use Cases

• Pottery: Plan the size of mugs, bowls, and vases so they meet functional requirements after firing.
• Tile making: Ensure tiles fit a specific layout or pattern by accounting for shrinkage in each dimension.
• Sculpture: Calculate armature and base sizes so the finished piece fits its intended space.
• Slab building: Determine the size of slabs needed for boxes, plates, or platters.