Clay Shrinkage Calculator

For any potter or ceramicist, understanding and accounting for clay shrinkage is fundamental. Clay shrinks as it dries and shrinks further when it is fired. If you don't account for this, your final piece could be significantly smaller than intended. The Clay Shrinkage Calculator is an indispensable tool that helps you manage this process precisely.

Why Does Clay Shrink?

Clay consists of tiny flat particles (platelets) surrounded by water. This water acts as a lubricant, allowing the clay to be plastic and workable.

• Drying Shrinkage: As the piece dries, the water between the platelets evaporates. The platelets move closer together, causing the entire piece to shrink. This happens until the piece is "leather-hard" and then "bone-dry."
• Firing Shrinkage: During firing, chemical and physical changes occur. The remaining water is burned off, organic matter is eliminated, and the clay particles fuse together in a process called vitrification. This fusing process pulls the particles even closer, resulting in additional shrinkage.

How to Use the Calculator

Our calculator has two main functions to help you master shrinkage:

Part 1: Calculate Your Clay's Shrinkage Rate

Every clay body has a different shrinkage rate. You must test your specific clay to get an accurate number. The best way is to create a "shrinkage ruler."

1. Create a flat, rectangular test tile of your clay.
2. When it's wet, precisely mark two lines on it, exactly 10 cm (or 10 inches) apart. This is your Wet Size.
3. Let the tile dry completely and then fire it to the temperature you typically use.
4. Measure the distance between the two marks on the fired tile. This is your Fired Size.
5. Enter these two numbers into the calculator to find your clay's total shrinkage rate.

Part 2: Use the Rate for Your Projects

Once you know your shrinkage rate, you can use it to plan your pieces:

• To Predict Final Size: Enter the size of your wet, freshly-made piece. The calculator will tell you how big it will be after firing. This is great for making sure a mug isn't too small or a plate isn't too large.
• To Calculate Needed Wet Size: This is the most powerful function. If you need to make a piece that fits something else, like a lid for a jar or a coaster for a specific mug, you enter your desired final (fired) size. The calculator will tell you how big you need to make the piece when it's wet to achieve that exact final dimension.

Key Formulas

The calculator uses these simple but crucial formulas:

• Shrinkage Rate (%) = ((Wet Size - Fired Size) / Wet Size) × 100
• Fired Size = Wet Size × (1 - (Shrinkage Rate / 100))
• Wet Size = Fired Size / (1 - (Shrinkage Rate / 100))

Frequently Asked Questions (FAQ)

Does the shrinkage rate change?

Yes. The shrinkage rate is specific to a clay body AND the temperature it is fired to. If you fire the same clay to a higher temperature, it will likely shrink more. Always test for the specific conditions you use.

Why can't I just add the percentage back on?

This is a common mistake. If a piece shrinks by 10%, you can't just make it 10% bigger to start with. For example, if you want a 10cm final piece with 10% shrinkage, making it 11cm wet is incorrect (11 × 0.90 = 9.9cm). You must use the correct formula: Wet Size = Fired Size / (1 - Shrinkage Rate), which gives 10 / (1 - 0.10) = 11.11 cm. This is what our calculator does for you.

What's a typical shrinkage rate?

Shrinkage rates can vary widely, but they often fall between 8% and 15%. Earthenware clays generally shrink less than stoneware or porcelain clays.