Studio notes · Measuring clay
Preset percentage chart: what 6%, 10%, 12%, 14%, and 16% mean
A quick linear shrinkage chart: predict glaze-fired size from wet clay, or find the wet size for a fired target, with clearly labelled millimetre examples.
A 12% shrinkage rate means 88% of the starting length remains. I find that remaining number easier to work with: multiply by it to look ahead, or divide by it to work backwards.
These are linear measurements: a length, width or diameter, not volume. The percentages below are examples, not recommendations for particular clay bodies. 6% is illustrative, not a shop preset. Our ruler's confirmed rates are whole numbers from 10% to 18%; this chart shows a selection, not the full range.
The quick chart: wet clay to glaze-fired clay
For each row, assume the stated percentage is your measured wet-to-glaze-fired shrinkage for your clay and process, with the fired measurement taken after cooling. The two size columns answer different questions: “What will 100 mm wet become?” and “How large should I make it wet to finish at 100 mm?”
| Wet-to-glaze-fired shrinkage (%) | Fraction of wet length remaining at glaze-fired stage | Glaze-fired length from 100 mm wet (mm) | Wet length for a 100 mm glaze-fired target (mm) |
|---|---|---|---|
| 6% (illustrative) | 0.94 = 94% | 94 | 106.38 |
| 10% | 0.90 = 90% | 90 | 111.11 |
| 12% | 0.88 = 88% | 88 | 113.64 |
| 14% | 0.86 = 86% | 86 | 116.28 |
| 16% | 0.84 = 84% | 84 | 119.05 |
Reverse sizes are rounded to two decimal places to make the arithmetic checkable, not to promise that level of precision in clay. These are ordinary physical dimensions, not instructions to read a particular shrinkage-ruler marking.
Use the remaining fraction for any size
Remaining fraction = 1 − (shrinkage percentage ÷ 100). With a rate measured between the stages you need:
- Later length = starting length × remaining fraction.
- Starting length = target later length ÷ remaining fraction.
For a different size, use the same fraction. At 12%, a 150 mm wet length becomes 150 × 0.88 = 132 mm glaze-fired. To aim for 150 mm glaze-fired instead, start at 150 ÷ 0.88 ≈ 170.45 mm wet.
Back at the chart's 100 mm target, simply adding 12% gives 112 mm wet, which would shrink to 98.56 mm, not 100 mm. The percentage loss is taken from the larger wet length. Old Forge Creations explains this reverse-sizing trap with further examples.
The fraction works in centimetres or inches too; keep the same unit on both sides of the calculation. The metric and imperial guide covers changing units.
Choose the row from evidence, not the clay's name
“Stoneware” or “porcelain” is not enough to choose a row. Use a rate with a named starting stage, ending stage and firing conditions. A wet-to-bone-dry figure cannot stand in for wet-to-glaze-fired shrinkage. A supplier's “firing shrinkage” may start at bone-dry, not wet; check its definition. Leather-hard and bisque measurements have their own starting points too. See the stage-specific sizing guide if your stages differ from this chart.
Digitalfire's shrinkage test method distinguishes drying shrinkage from firing shrinkage. For a useful studio record, keep the clay body, starting moisture state, forming method, drying conditions and firing schedule alongside your measured lengths. Our test-bar guide walks through taking those measurements.
If you're making something to fit, settle the target dimension before using the chart. Clearance, the space between fitted parts, is a separate allowance, including any room needed for glaze. The calculation won't choose that gap or prevent warping; check an important fit with a trial piece.