# Custom clay shrinkage ruler for your measured percentage Studio notes · Choosing a ruler What to measure and record before asking for a custom shrinkage scale, when extra precision helps, and which ruler options are confirmed here. By Zoë · The Poodle Potter Published 2026-09-20 · 5 min read [All guides](https://potteryruler.com/guides) ![Stacks of glazed ceramic plates and bowls on white shelves](https://potteryruler.com/images/guides/ceramic-forms.webp) Before asking someone to make a shrinkage scale, I'd put the clay notes and a sketch of the intended piece side by side. “My clay shrinks 12%” is a start, but a useful request also says between which stages, under which firing conditions, and what needs to fit. Here, choosing a percentage means choosing a whole-number rate from **10% to 18%**. Decimal rates, percentages outside that range and custom lengths are not confirmed. See the [buy page](https://potteryruler.com/buy) for current options and checkout availability; the brief below is for discussing a scale with a maker who accepts custom enquiries. ## Give your percentage a beginning and an end A shrinkage ruler uses **linear shrinkage**: the reduction in a length, not in volume. A wet-to-glaze-fired measurement is different from a bone-dry-to-glaze-fired measurement. Leather-hard and bisque are other starting points again; a percentage needs to name both ends of the journey. For a hypothetical test, a line measuring 100 mm in freshly formed wet clay and 88 mm after the intended glaze firing and cooling has lost 12% of its original length. Measure these actual lengths with an ordinary ruler or calipers, not a shrinkage scale: **(100 − 88) ÷ 100 × 100 = 12% wet-to-glaze-fired shrinkage.** That is evidence for that clay and process, not a promise about every future pot. Keep the raw lengths as well as the answer. If you later discover the starting measurement was taken at leather-hard, the notes will explain why it doesn't describe the whole wet-to-fired change. [Digitalfire's shrinkage test method](https://digitalfire.com/test/shab) records dry and fired lengths separately and stresses consistent sample preparation. For a practical starting point, follow the [test-bar guide](https://potteryruler.com/guides/measuring-clay-shrinkage), then repeat the test under the conditions you actually use. A supplier's figure is useful too, provided you can identify its measurement stages and firing conditions; label it as supplier data rather than your own test. ## The brief to keep or send to a ruler maker If a maker accepts custom-scale enquiries, these are the details I'd gather before asking. They are also useful notes for choosing an existing percentage. - **Clay identity:** manufacturer, exact body name or code, and batch if known. Note whether the sample was fresh clay or reclaim. - **Evidence for the rate:** supplier data or your test results, with original and later lengths, units, number of samples and the spread of results. Don't send only an average that hides inconsistent tests. - **Measurement stages and process:** for example, freshly formed wet clay to cooled glaze-fired clay. Describe the starting moisture condition, forming method and measurement direction, drying conditions, bisque and glaze schedules, firing cone and atmosphere where known. - **Requested scale:** the percentage and units you want. Keep the measured result separate from any rounded percentage you're considering. Ask how the scale should be read and whether its rate can be labelled on the tool. - **Actual measuring span:** the largest starting-stage dimension you need to span in one go. A compensated scale has marks spaced to allow for shrinkage; its largest finished-size label need not equal the tool's physical length. - **The job and acceptable variation:** sketch the target dimension at its named stage, showing whether it is an inside diameter, outside diameter, height or other length. Give acceptable lower and upper limits. Record clearance, the working gap between fitted parts, separately; say whether a quoted gap is on each side or across the whole diameter. A reusable message could be: > I'm using [clay body]. Across [number] samples, my [starting stage] to [later stage] linear shrinkage was [range]%, using [forming and firing conditions]. The measured lengths are [results]. I want a [percentage]% scale in [units] to plan [project and target dimension]. My largest starting-stage span is [length], and the acceptable finished variation is [amount]. Can you confirm whether you can make that scale and whether its usable span is sufficient? That asks a maker to confirm a specification. It doesn't assume they offer the percentage, length or marking arrangement you have in mind. ## Is the extra specificity useful? A closely matched scale is most useful when you return to the same clay and process, and the dimensional difference matters to the work. It is less persuasive when the percentage comes from one awkward-to-read mark or the next firing will use a different schedule. If your repeated results vary more than the difference between the rates you're considering, another decimal place will not make that variation disappear. I'd check the measurements and process before commissioning a more specific scale. The [custom-versus-fixed guide](https://potteryruler.com/guides/custom-versus-fixed-shrinkage-rulers) compares the resulting size differences with what a project can accept. For an occasional dimension, arithmetic and an ordinary ruler may be enough. Using the hypothetical 12% wet-to-glaze-fired rate above, a 100 mm glaze-fired target needs **100 ÷ 0.88 ≈ 113.64 mm wet**. That is a calculated starting size, not a claim that wet clay can be made or fired to hundredths of a millimetre. You divide by the fraction remaining, rather than adding 12%; [Old Forge Creations explains why](https://www.oldforgecreations.co.uk/blog/clay-shrinkage). Before agreeing to a scale, ask the maker to confirm its reading direction and usable span. The [calibration guide](https://potteryruler.com/guides/calibrating-shrinkage-rulers) shows how to check the spacing once you know what the labels represent. Keep the maker's proposed specification beside your original measurements. If you settle on a different rate, record that as a tool choice, not a revised test result. A closer-matched scale can save arithmetic; it still cannot prevent warping or guarantee the fit in your sketch. A little testing now. Fewer surprises after the kiln. [Explore the ruler options](https://potteryruler.com/buy) [View this page on https://potteryruler.com/guides/custom-clay-shrinkage-ruler](https://potteryruler.com/guides/custom-clay-shrinkage-ruler)