Pottery Shrinkage Converter
Convert between wet, leather-hard, bisque, and fired pottery sizes.
Stoneware shrinks 10-14% — size your wheel-thrown piece from a fired target.
Enter a measurement at any stage — others compute from a typical 12% total shrinkage. Adjust shrinkage rates for your clay body below.
Pottery shrinks twice on its way from wheel to finished piece. Once during drying (water leaving the clay body), and once during firing (silica fusing). Together, total linear shrinkage for typical stoneware runs 10 to 14% — a 30 cm bowl on the wheel becomes about 26 cm after firing. Knowing your exact shrinkage rate is the difference between making 350 ml cups and 300 ml cups.
The three shrinkage stages:
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Wet to leather-hard (about 5-8% linear): Water in the pore spaces leaves first. The piece becomes firm enough to trim and handle but is still cool to the touch and has visible moisture.
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Leather-hard to bone-dry / bisque (additional 0-3% linear): Remaining water evaporates. Bisque firing (cone 06-04, around 1830-1940°F / 1000-1060°C) adds minimal additional shrinkage — maybe 1-2% — because vitrification hasn’t started yet.
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Bisque to glaze-fired (3-7% additional): Glaze firing at maturity temperature (cone 6 to cone 10 for stoneware) drives the major firing shrinkage as silica fuses and the clay body vitrifies.
Total typical shrinkage by clay body:
| Clay body | Total linear shrinkage | Fire temperature |
|---|---|---|
| Low-fire earthenware | 7-10% | Cone 06-04 |
| Mid-fire stoneware | 10-12% | Cone 5-6 |
| High-fire stoneware | 12-14% | Cone 9-10 |
| Porcelain | 12-16% | Cone 9-10 |
| Translucent porcelain | 14-18% | Cone 10-12 |
| Bone china | 13-15% | Cone 6-8 (biscuit then glaze) |
These are linear (1-D) shrinkage values. Volume shrinkage is roughly 3× the linear percentage — a 12% linear shrinkage means about a 32% volume reduction.
The math (sizing up).
To make a 25 cm fired bowl from a clay body with 12% total shrinkage:
Wheel-thrown size = Target size / (1 − shrinkage)
= 25 cm / (1 − 0.12) = 25 / 0.88 = 28.4 cm wet
Sizing intermediate stages.
Working backwards from a 25 cm fired piece with 12% total shrinkage and 6% wet-to-leather-hard:
- Wet: 25 / 0.88 = 28.4 cm
- Leather-hard: 28.4 × (1 − 0.06) = 26.7 cm
- Bisque: 26.7 × (1 − 0.02) ≈ 26.2 cm
- Fired: 26.2 × (1 − 0.04) ≈ 25.1 cm ✓
Volume shrinkage.
Linear shrinkage is what you see; volume shrinkage is what determines if your finished cup holds 250 ml or 280 ml. For a target 250 ml fired volume with 32% total volume shrinkage:
Wheel-thrown volume = Target volume / (1 − volume shrinkage)
= 250 / (1 − 0.32) = 250 / 0.68 = 368 ml
You’d throw a 370 ml wet pot to fire down to 250 ml.
Measuring your own clay’s shrinkage.
Don’t trust the manufacturer’s spec sheet alone — your studio conditions, firing schedule, and trimming style all affect actual shrinkage. The standard test:
- Make a test bar 10 cm long, 1 cm wide, 1 cm thick
- Mark exactly 10 cm with two pin holes while wet
- Measure between the pin holes at each stage: leather-hard, bone-dry, bisque, glaze-fired
- Calculate shrinkage at each step: (initial − current) / initial × 100%
Run three or four test bars from the same batch to average out variation. Repeat for each new bag of clay if you want accuracy down to 1%.
Forms that resist shrinkage prediction.
Tall narrow forms (vases, bottles, goblets) shrink more vertically than horizontally because gravity stretches the clay during drying — sometimes 2-3% more in the vertical direction. Wide flat forms (plates, platters) can warp inward at the rim because of differential drying. Plan for these — make the wet form slightly taller or wider than the simple shrinkage formula predicts.
Why this matters for production work.
A potter producing standardized mugs needs to know exact final volume. The first 50 mugs of a new clay body should be measured and the wheel-throwing sizes adjusted before settling on a production size. Variance of more than 5% in finished volume on a “matching set” is noticeable; customers see it and feel like the pieces don’t match.
Glaze-only shrinkage is not pottery shrinkage.
Glaze itself shrinks during firing, but separately from the clay body. Glaze shrinkage that doesn’t match clay shrinkage causes crazing (glaze cracks) or shivering (glaze flakes off). Match the coefficient of thermal expansion of your glaze to your clay body — this is a separate calculation from the dimensional shrinkage handled here.
Drying speed affects shrinkage.
Fast-dried clay shrinks slightly less than slow-dried clay because the structure locks before all moisture migrates out. Don’t change drying speed mid-batch — the inconsistency causes warping. Even drying produces even shrinkage.