Choosing a ruler for a decimal shrinkage rate such as 9.5% or 12.5%

A shrinkage result of 12.5% doesn't automatically mean you need a 12.5% ruler. The useful question is how much difference rounding would make to the piece you're planning. Half a percentage point can be an unimportant fraction of a millimetre on one project and an awkward couple of millimetres on another.

I'd keep the decimal in the calculation first, then decide whether a whole-number scale is close enough. There's no need to round your clay's measured rate simply because a ruler has fewer options.

What does that decimal actually tell you?

Suppose a line measures 100 mm in freshly formed wet clay and 90.5 mm after glaze firing and cooling. Its measured linear shrinkage is:

(100 − 90.5) ÷ 100 × 100 = 9.5%

If it finishes at 87.5 mm instead, the result is 12.5%. These are changes in length, not volume.

Those tidy answers describe the measurements, not a promise that every pot will behave identically. Before choosing a decimal scale, repeat the test with the same clay body, comparable starting moisture and forming method, and the drying and firing conditions you intend to use. Keep the spread of results as well as their average. A decimal average from widely scattered bars isn't evidence of a dependable decimal rate.

Check whether you can read the marks closely enough, too. With a fixed 100 mm wet starting span, a 0.5 mm difference in the final reading changes the calculated shrinkage by half a percentage point: enough to turn 12.5% into 12% or 13%. If you can't reliably distinguish those readings, the decimal isn't a sound reason to choose a more specific scale.

Digitalfire's shrinkage test method records dry and fired measurements separately. That distinction matters: a bone-dry-to-fired result cannot stand in for a wet-to-glaze-fired rate. Our test-bar guide covers the practical measuring and record-keeping.

For all the examples below, assume the stated rate represents wet-to-cooled-glaze-fired shrinkage, under conditions matching the planned project. The rates are hypothetical, not recommendations for particular clay bodies.

Two decimals, one 200 mm fired target

To plan a wet dimension, divide the glaze-fired target by the fraction remaining:

Wet size = glaze-fired target ÷ (1 − shrinkage percentage ÷ 100)

At 9.5%, the remaining fraction is 0.905. A 200 mm glaze-fired target therefore needs a calculated wet size of 200 ÷ 0.905 ≈ 220.99 mm.

Using a 10% scale for that same target instead means planning a wet size of 200 ÷ 0.90 ≈ 222.22 mm. If the clay actually shrinks by 9.5%, it finishes at approximately 201.11 mm: 1.11 mm larger than intended.

At 12.5%, the correct calculation is 200 ÷ 0.875 ≈ 228.57 mm wet. The two neighbouring whole-number rates pull in opposite directions:

• Planning with 12% gives about 227.27 mm wet, which becomes 198.86 mm glaze-fired at the actual 12.5% rate: 1.14 mm too small.
• Planning with 13% gives about 229.89 mm wet, which becomes 201.15 mm glaze-fired: 1.15 mm too large.

So rounding the rate upwards makes the planned wet piece larger; rounding downwards makes it smaller. Neither is automatically the safer choice for a fitted piece.

How the rounding cost grows with the project

This table uses the same reverse-sizing method: choose a glaze-fired target, calculate its wet size using the rounded rate, then predict the result using the actual decimal rate. A plus sign means larger than the target; a minus sign means smaller.

The hundredths of a millimetre make the comparison visible; they are not a claim about achievable studio accuracy. Calculations use unrounded intermediate values and assume uniform linear shrinkage. They exclude measuring, making and firing variation, and any distortion.

Compare these differences with the amount your project can reasonably be too large or too small. A 2 mm discrepancy might be acceptable in a free-standing decorative slab but not in a piece intended for a fixed opening. Even a small piece can need a close fit. Any necessary clearance, including room for glaze, is a separate decision, not something to obtain by rounding the shrinkage rate.

Choosing a tool without losing the decimal

If rounding leaves too little room for the other variations in your work, keep the decimal calculation and mark the resulting wet dimension with an ordinary ruler or make a full-size template. For the 12.5% example, that means marking about 228.57 ordinary millimetres, not finding 228.57 on a shrinkage scale and compensating twice. You don't need a decimal shrinkage ruler to use a decimal rate.

A dedicated decimal scale may be convenient for repeated work, provided your test results justify it and its maker confirms the exact rate. If you use a printable scale, check its compensated distances before relying on it. On a scale labelled with expected glaze-fired sizes for measuring wet clay, the intervals must be enlarged: a 200 mm target at 12.5% needs about 228.57 mm of physical span. Check the maker's reading instructions rather than assuming every scale works in that direction.

The PoodlePotter preview lists whole-number rates from 10% to 18%. Neither 9.5% nor 12.5% is a confirmed option. The 400 mm examples also exceed the ruler's 9.45 in physical length; see the large-piece measuring options for ways to lay those out.

I'd spend the extra effort on a trial piece before buying a more specific scale for a critical fit. A decimal can remove a known rounding difference. It can't remove the variation that showed up in your tests.