# Metric and imperial clay shrinkage rulers: how to read either scale

Studio notes · Measuring clay

Read centimetres, millimetres and inches without mixing units, and understand why a shrinkage scale is not an ordinary ruler.

By Zoë · The Poodle Potter

Published 2026-09-20 · 5 min read

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![Close-up of wet hands shaping the rim of a terracotta-coloured clay pot](https://potteryruler.com/images/guides/forming-clay.webp)

A note that says “make it 4” leaves rather a lot to guess. Four inches or four centimetres? Measured wet, or wanted after firing? The number needs a unit and a stage before it can help at the making table.

Metric and imperial shrinkage rulers do the same job. The unit changes how you describe a length; it does not change the clay’s shrinkage percentage. Choose the unit you can read comfortably and keep it beside every dimension.

## Read the unit before the number

On a metric scale, **1 centimetre (cm) = 10 millimetres (mm)**. A reading of **8.6 cm means 86 mm**, or 8 cm and another 6 mm. It does not mean 8 cm and 0.6 mm.

If a centimetre interval is divided into ten equal spaces, each space represents 0.1 cm, or 1 mm, in the scale’s labelled units. Count the spaces between the numbered marks, not the marks themselves. Do not assume every metric ruler has those subdivisions: check the actual scale and its instructions.

Inches may be divided into fractions or decimals. For example:

- **4 1/2 in = 4.5 in**.
- **4 3/8 in = 4.375 in**, because 3 divided by 8 is 0.375.
- **4.3 in is not 4 3/8 in**. A decimal point does not stand in for a fraction bar.

If the interval from 4 to 5 in has eight equal spaces, count three spaces beyond 4 to read **4 3/8 in**. Halfway across is 4 4/8 in, usually written **4 1/2 in**. Another ruler may use different divisions, so check a whole numbered interval before reading a fraction.

These are ways to interpret numbers and subdivisions, not a description of the markings on the PoodlePotter ruler. Its detailed tick layout is not confirmed.

## The unit stays the same; the spacing changes

A **compensated scale** has its marks spaced to allow for shrinkage. On a scale designed to show the expected fired size when laid against wet clay, the labelled distances are larger physically than the same numbers on an ordinary ruler. A small division labelled 1 mm still means 1 mm of estimated fired length, not 1 mm of the wet clay beneath it.

For example, suppose a scale allows for **12% linear shrinkage from wet clay to glaze-fired clay**. Linear means a change in length, not volume. A span labelled 10 cm on that compensated scale needs to cover:

**10 cm ÷ 0.88 ≈ 11.36 cm of actual wet clay.**

That wet length is expected to become 10 cm after firing. The same principle applies to inches. A compensated scale’s “10 cm” mark therefore should not line up with an ordinary ruler’s 10 cm mark when their zeros are aligned.

Identify the ruler’s zero, unit, percentage and intended direction from its labels and instructions. Do not assume the physical end is zero, or that an unlabelled edge is an ordinary scale. Our [calibration guide](https://potteryruler.com/guides/calibrating-shrinkage-rulers) explains how to check the physical span against the compensation it is meant to provide.

## The same wet piece in three units

[NIST defines one inch as exactly 25.4 mm](https://www.nist.gov/pml/owm/si-units-length), so **1 in = 2.54 cm = 25.4 mm**. To convert inches to millimetres, multiply by 25.4; to convert millimetres to inches, divide by 25.4.

Imagine a wet slab measuring 5 in along one edge with an ordinary ruler. That same edge is 12.7 cm or 127 mm long. For this hypothetical example, suppose a matching clay test, made at the same starting moisture state and taken through the intended drying and glaze-firing schedule, gave 12% wet-to-glaze-fired linear shrinkage. That leaves 88% of the wet length, so multiply it by 0.88 to estimate the length after the glaze firing has cooled.

| Working unit | Actual wet length | Estimated glaze-fired length at 12% shrinkage |
| --- | --- | --- |
| Centimetres | 12.7 cm | 11.176 cm |
| Millimetres | 127 mm | 111.76 mm |
| Inches | 5 in | 4.4 in |

All three rows describe the same change. The extra decimal places make the conversion visible; they are not a promise that a ruler can resolve them or clay will reproduce them.

The imagined 12% result is not a rule for all clay. Wet, leather-hard, bone-dry, bisque and glaze-fired measurements belong to different points in the process. [Digitalfire’s SHAB test](https://digitalfire.com/test/shab) records dry and fired lengths separately, along with the firing conditions. [Measuring your own clay shrinkage](https://potteryruler.com/guides/measuring-clay-shrinkage) gives you a rate with a defined start and end, rather than just a percentage to copy.

## Convert once, then leave the units alone

If a drawing arrives in inches but your tools are metric, convert the target before making the template. A glaze-fired target of **4 3/8 in becomes 111.125 mm**. Keep that value through the calculation, then round the final working dimension to a precision your tools and process can support. Do not turn 4 3/8 into 4.3 first: 4.3 in is only 109.22 mm.

Conversion does not compensate for shrinkage. A target converted from inches to millimetres is still a target at the same stage. Equally, if you have already read the expected fired size from a suitable compensated scale, do not subtract shrinkage from that reading again. For the distinction between prediction and planning a starting size, see [stage-specific sizing](https://potteryruler.com/guides/stage-specific-pottery-sizing).

I’d keep the original instruction and the working note together: “Glaze-fired target: 4 3/8 in = 111.125 mm; working in mm.” Give the calculated wet dimension its own line. That makes it much harder to build the wet piece to the finished target by accident.

## Choosing a format, or sharing a ruler photograph

Use centimetres if your templates and ordinary measuring tools are metric; use inches if that is how you naturally work. Neither unit is inherently more accurate. Readable subdivisions, a suitable rate and consistent measuring matter more than the unit name.

PoodlePotter's ruler has **centimetre or inch options**. That does not imply a dual-unit layout, millimetre subdivisions or particular inch fractions; those details are not confirmed.

When checking any listing or sending someone a ruler photo, include:

- The unit label, percentage and zero, not just a close-up of a number.
- Enough of an interval to see and count its subdivisions.
- A note saying whether the reading is an actual length or a compensated estimate, and which clay stages it connects.

Keep that same unit and stage note on your template, too. It should still make sense when you pick it up next time.

A little testing now. Fewer surprises after the kiln.

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[View this page on https://potteryruler.com/guides/metric-imperial-shrinkage-rulers](https://potteryruler.com/guides/metric-imperial-shrinkage-rulers)