Clay Shrinkage & Fired Size Calculator

How it works

Shrinkage % = (wet − fired) ÷ wet × 100; fired = wet × (1 − s); wet needed = fired target ÷ (1 − s); area shrinkage = 1 − (1 − s)²; volume = 1 − (1 − s)³

Clay shrinks twice: as water leaves during drying the particles pull together (roughly 5–7%), and in the glaze firing the body vitrifies and densifies further, for a total of 10–15% for most stoneware and porcelain and 5–8% for low-fire and sculpture bodies. Because it is a linear percentage, dimensions must be divided by the shrinkage factor, not simply increased by the percentage — a 12% body needs a 284 mm wet piece for a 250 mm fired one, not 280. Area and volume shrink far more (about 23% and 32% at 12%), which is why a generous-looking wet bowl fires small. The only trustworthy number is your own: score a 100 mm line on a tile of the actual clay, fire it with your usual glaze schedule and measure. Uneven drying or thickness makes parts shrink differently, so lids and pots are made together.

Worked example

Cone 6 stoneware at 12% shrinkage (factor 0.88): a bowl that should fire to 250 mm across is thrown at 284 mm; a 300 mm wet platter fires to 264 mm. Its area drops 23% and volume 32%, so a litre of wet capacity becomes about 680 mL.

Frequently asked questions