# What percentage shrinkage ruler do I need for my clay?

Studio notes · Choosing a ruler

Choose a shrinkage percentage from your clay supplier's data, check which stages it covers, and decide when a studio test should guide your ruler choice.

By Zoë · The Poodle Potter

Published 2026-09-20 · 5 min read

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Choose the **linear shrinkage percentage for your particular clay, between the stages you actually measure**. Linear means change in length, not volume. A supplier's figure is a useful starting point; a repeatable test through your own making and firing process is more useful when a dimension really matters.

I wouldn't choose a ruler just because the bag says stoneware or porcelain. Those names don't tell you enough about the clay or what will happen in your kiln.

## Start with the exact clay, not the clay family

Find the supplier's technical sheet for the body name or code on your bag. Before treating its percentage as your ruler setting, check three things:

- **What is being measured?** Look for linear shrinkage. Water absorption is a different property, even though it may appear beside shrinkage as another percentage.
- **Where does the measurement start and finish?** Is it wet-to-bone-dry, bone-dry-to-fired, or total wet-to-fired shrinkage?
- **Which firing does it describe?** Match the stated firing conditions to yours, including the kiln atmosphere (the gases around the clay). If the sheet lists several temperatures or cones, use the entry for your intended firing. A [pyrometric cone](https://www.ortonceramic.com/pyrometric-cones) indicates heatwork, the combined effect of time and temperature; matching the peak temperature alone isn't enough.

If the sheet only says “shrinkage: 12%”, ask the supplier what that includes. A useful question is: “Is this total linear shrinkage from wet clay to the final firing, or firing shrinkage measured from bone-dry clay, and under which firing conditions?”

If the stages are clear and the firing is comparable, use that figure provisionally for planning. If either is unclear, ask the supplier or test before choosing a ruler for a fitted project. A generic stoneware or porcelain average won't fill the gap.

## Make the percentage follow the measurement

A wet-to-glaze-fired rate belongs with a measurement taken while the clay is wet and a target after the final firing and cooling. If you measure at leather-hard instead, part of the shrinkage has already happened. Using the full wet-to-fired rate would count that part again.

Even “wet” needs a little care. Freshly formed clay and clay left to stiffen on the bench need not have the same shrinkage left. Test at the same working consistency you normally use when measuring your pots.

Likewise, bone-dry, bisque-fired and glaze-fired are different destinations. A figure suitable for predicting a bone-dry dimension is not automatically suitable for a finished pot. The [stage-specific sizing guide](https://potteryruler.com/guides/stage-specific-pottery-sizing) helps you match both ends of the measurement.

[Digitalfire's shrinkage test](https://digitalfire.com/test/shab) records drying and firing shrinkage separately, describing firing shrinkage as the change from dry to fired. Don't assume that a supplier using the words “firing shrinkage” has included drying. Nor should you simply add two separately calculated stage percentages: they can use different starting lengths.

For a hypothetical check, suppose the same marked distance measures **100 mm wet and 88 mm after the final glaze firing and cooling** in your recorded firing conditions. It has lost 12 mm out of its original 100 mm, so that test gives **12% wet-to-glaze-fired linear shrinkage**. It does not tell you the leather-hard-to-fired percentage without a leather-hard measurement as well.

## What if your studio result disagrees with the supplier?

Don't average the two numbers to settle the argument. First check whether they describe the same journey.

Check the starting moisture condition, the measurement endpoints and the firing record before blaming the clay. Use an ordinary ruler or calipers to check the actual lengths, not a shrinkage-compensated scale: its labels account for expected shrinkage rather than showing ordinary distances.

Then look for repeatability. [Ceramic Arts Network's clay-testing guide](https://ceramicartsnetwork.org/daily/article/Testing-1-2-3-How-to-Test-Clay-Bodies-to-Find-the-Right-Sculpture-or-Pottery-Clay-for-Your-Work) stresses consistent procedures and testing in the kiln used for your work. One awkward mark or distorted bar isn't a good reason to abandon the supplier's figure. But if several sound tests made and fired like your work consistently give a different result, use that studio evidence for that process. Keep both values in your notes, with their conditions, so you can still see what each describes.

The [test-bar guide](https://potteryruler.com/guides/measuring-clay-shrinkage) covers how to get those measurements. For choosing a ruler, the important result is a rate with a named clay body, starting stage, finishing stage and firing record. If the results vary enough to affect the fit you need, investigate the variation before choosing a more precise-looking percentage. A ruler cannot remove it.

## Choose the scale only after choosing the rate

A repeatable 12% wet-to-glaze-fired result makes a 12% ruler a reasonable choice for that same workflow. It doesn't make 12% your permanent setting for every clay or every stage.

If the evidence gives a decimal percentage, don't round it automatically just to match a product menu. Whether rounding is acceptable depends on the size of the piece and how much dimensional difference you can tolerate. The [decimal-rate guide](https://potteryruler.com/guides/decimal-shrinkage-rates) works through that decision.

The PoodlePotter preview lists whole-number rates from **10% to 18%**; decimal and out-of-range rates are not confirmed. If your tested rate isn't supported, use the calculation with an ordinary ruler, or look for a tool that supports it. You don't need to change your measured rate to use a different tool.

## Keep the reason for your choice beside the ruler

I'd label the ruler with the clay name and something like **“12% wet-to-glaze-fired; studio test”**, then keep the test date and firing record beside it. If you're using supplier data instead, say so on the label. “12%” on its own is easy to carry over to the wrong job.

Revisit the choice when you change clay or firing conditions, or when finished dimensions stop agreeing with your notes. For a critical fit, try a representative piece before committing to a batch. The percentage is a useful estimate of size change, not a promise against warping or a guarantee that two parts will fit.

A little testing now. Fewer surprises after the kiln.

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