Pottery Shrinkage Calculator: How to Use It

This calculator works out the fired shrinkage of a clay body: you enter the wet (leather-hard or bone-dry) length of a test piece and the fired length, and it returns the percentage of linear shrinkage. It also gives you the wet length you need to start with to hit a target fired dimension, based on a shrinkage percentage you enter. You get both the shrinkage percentage and the required starting size, so you can plan your pieces accurately.

Formula

Linear shrinkage is the difference between the wet length and the fired length, divided by the wet length, expressed as a percentage. The formula is:

Shrinkage (%) = ((Wet length – Fired length) / Wet length) × 100

Where:

• Wet length is the length of the piece before firing (usually measured at bone dry, in inches, cm, or mm).
• Fired length is the length after the final firing (in the same unit).

To find the starting length needed to end at a target size, use:

Wet length = Target fired length / (1 – (Shrinkage % / 100))

This second formula is the one you use when you want a mug to be 4 inches tall after firing, and you know your clay shrinks 12%.

Worked Example

Using the default values: wet length = 10 in, fired length = 8.5 in.

1. Subtract fired from wet: 10 – 8.5 = 1.5 in (the amount of shrinkage).
2. Divide by wet length: 1.5 / 10 = 0.15.
3. Multiply by 100: 0.15 × 100 = 15%.

So the shrinkage is 15%. Now, if you want a target fired length of 8 in and you assume 15% shrinkage, the wet length needed is:

1. Convert 15% to a decimal: 0.15.
2. Subtract from 1: 1 – 0.15 = 0.85.
3. Divide target by that: 8 / 0.85 = 9.41 in.

You would need to throw or hand-build the piece to 9.41 in wet to end up at 8 in after firing. The calculator shows 9.41 in for the default values.

Why Shrinkage Matters in Pottery

Every clay body shrinks as it dries and again during firing. The total linear shrinkage typically ranges from 4% to 15%, depending on the clay. Porcelain shrinks more than stoneware or earthenware, and highly plastic clays shrink more than groggy clays. If you don’t account for shrinkage, your finished pot will be smaller than intended, and lids won’t fit, handles may crack, and tiles won’t align. Knowing your clay’s shrinkage rate is essential for making functional ware with consistent dimensions.

How to Measure Your Clay’s Shrinkage

To get an accurate number, make a simple test bar. Roll out a slab of clay about 1 cm thick, cut a rectangular bar, and press two small marks into the surface exactly 10 cm apart (or 4 inches). Use a ruler and a needle tool. Let the bar dry completely, then measure the distance between the marks again. That gives you dry shrinkage. Fire the bar to your normal cone temperature, then measure the marks again. The difference from the wet length to the fired length is your total linear shrinkage. Most potters use the fired measurement only, because the dry shrinkage is small compared to the firing shrinkage.

Common Mistakes and Misconceptions

• Using the fired length as the base: Shrinkage is always calculated from the wet length, not the fired length. If you divide by the fired length, you get a different, larger percentage.
• Confusing linear shrinkage with volumetric shrinkage: Clay shrinks in three dimensions, so the volume change is much larger than the linear change. A 15% linear shrinkage means the volume shrinks by about 39% (1 – 0.85^3).
• Measuring at the wrong dryness: Leather-hard clay still contains water, so it will shrink more as it dries. Always measure at bone dry for consistent results.
• Assuming shrinkage is the same in all directions: Thrown pots can shrink differently vertically than horizontally due to the alignment of clay particles. Test bars measure only one direction.

How to Use the Calculator for Real Projects

For a mug that should be 4 inches tall after firing, and your clay shrinks 12%, enter 4 as the target and 12 as the shrinkage. The calculator tells you to start with 4.55 inches. For a dinner plate that should be 10 inches in diameter, start with 10 / (1 – 0.12) = 11.36 inches. Remember that the shrinkage percentage you use should be based on your own test bar, not the manufacturer’s number, because your process (how much you compress the clay, how you dry it) affects the result. If you are making a lid, you must add a small allowance for the lid to fit loosely – the calculator gives you the exact size, but you may want to add 1–2 mm for clearance.

Alternatives and Related Calculations

Some potters use a shrinkage ruler – a special ruler that has a scale expanded by the shrinkage percentage, so you can measure wet clay and read the fired size directly. That works well for a single known shrinkage, but a calculator is more flexible if you use multiple clays. You can also calculate dry shrinkage separately by measuring the wet length and the dry length before firing. The calculator above gives total shrinkage from wet to fired, which is what most people need. If you want to know how much larger to make a mold for slip casting, you would use the same formula in reverse – enter the target fired size and the shrinkage percentage to get the mold size.

Frequently Asked Questions

Q: What is a typical pottery shrinkage percentage?
Most commercial stoneware clays shrink between 10% and 15% from wet to fired. Earthenware often shrinks less, around 6–10%, and porcelain can shrink 12–15% or more. Always test your specific clay.

Q: Do I measure shrinkage from wet or from bone dry?
You can measure either, but the most useful number for planning is the total linear shrinkage from wet (as you throw or hand-build) to fired. That includes both drying and firing shrinkage. Some potters also track dry shrinkage separately to diagnose cracking issues.

Q: Why does my clay shrink more than the manufacturer says?
Manufacturer shrinkage is measured under controlled conditions. Your actual shrinkage can be higher if you add more water, throw thinner, or dry more slowly. It can be lower if you add grog or sand. The only reliable number is your own test bar.

Q: Can I use this calculator for glaze shrinkage?
No, glazes shrink differently and are usually applied to already-fired bisque. Glaze fit is about thermal expansion, not linear shrinkage. This calculator is for clay bodies only.

Q: How do I make a test bar for shrinkage?
Roll a slab about 1 cm thick, cut a rectangle about 10 cm long, and make two marks exactly 10 cm apart. Dry it fully, fire it to your normal cone, then measure the distance between the marks. Subtract the fired distance from 10, divide by 10, and multiply by 100 to get the percentage.

Q: What if my fired piece is larger than the wet piece?
That would mean negative shrinkage, which only happens if you measured incorrectly or if the clay has a bloating defect. Clay does not expand on firing. Re-measure and re-check your marks.

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in wet | Wet length | in | 10 | 0 | 1000
in fired | Fired length | in | 8.5 | 0 | 1000
out shrink | Shrinkage | % | (wet - fired) / wet * 100 | 2
in target | Target fired length | in | 8 | 0 | 1000
in pct | Shrinkage to use | % | 15 | 0 | 100
out start | Wet length needed | in | target / (1 - pct / 100) | 2