How this calculator works
A clay body contains water between its particles and chemically bound water in its clay minerals. As the piece dries and then fires, that water leaves and the particles pack closer, so every dimension shrinks by roughly the same proportion. The calculator applies that proportion to length, height and width. Because all three dimensions shrink, the volume falls by the cube of the linear factor: a 12 percent linear shrinkage removes 32 percent of the volume. A clay that shrinks unevenly, or a piece with thick and thin sections, can still warp, so the percentage is a planning guide rather than a guarantee.
Formula
Fired dimension = wet dimension x (1 - shrinkage). Wet dimension = fired dimension / (1 - shrinkage). Shrinkage is a fraction, so 12 percent is 0.12. Volume change = (1 - shrinkage)^3 - 1, and inverting the direction gives 1 / (1 - shrinkage)^3 - 1.
Worked example
A wet slab 20 cm long loses 12 percent of that length, so the fired length is 20 x 0.88 = 17.6 cm. A 10 cm height becomes 8.8 cm and an 8 cm width becomes 7.04 cm. The volume change is 0.88 cubed minus one, or -31.85 percent, which is why a piece looks much smaller after firing than the linear change alone suggests.
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Frequently asked questions
Is shrinkage measured on wet clay or on leather-hard clay?
Suppliers usually quote total shrinkage from the wet or plastic state through to the fired state, but some quote drying shrinkage and firing shrinkage separately. Add the two only if they are measured from different starting points; otherwise use the total figure supplied for the body.
Why is volume loss so much larger than length loss?
Every dimension shrinks, so the volume is scaled three times over. A 12 percent length change multiplies to about 32 percent in volume. This is why thick pieces can crack: the outside shrinks before the inside has dried enough to follow.
Can I use this to size a piece that must fit a mould?
Yes, use the fired to wet direction: divide the required final measurement by one minus the shrinkage. Allow for the mould material and any joins, and make a test tile from the same body and firing schedule before committing to the final piece.
What if my shrinkage is above 100 percent?
That is not physical, because it would mean the piece vanishes. The calculator rejects 100 percent and anything higher, and also rejects a negative shrinkage for the forward direction, which would be an expansion.