Clay Shrinkage Calculator
Work out what wet size to throw or hand-build so a piece fires to the size you actually want, using your clay's shrinkage rate.
Scale factor at 12% shrinkage: 1.14× — multiply every target dimension by this while you work.
Every clay body shrinks a different amount; check your clay's spec sheet, or better, test with a shrinkage bar from your own bag of clay. Measure rims and openings with calipers, not a ruler by eye. Lids need to be made from the same clay as their pot and expect the same shrinkage, or they won't fit once both pieces are fired. Tall forms shrink in height too, not just width, so apply the scale factor to every dimension.
How it works
Clay shrinks twice: once as it dries from wet to leather-hard to bone dry, and again in the kiln as the particles fuse together. Stoneware typically loses 10 to 13% of its size total by the time it comes out of the glaze firing, and the number is different for every clay body, so your clay's spec sheet (or a test bar you make and measure yourself) is the real source of truth. This calculator just does the arithmetic once you have that number: enter a shrinkage percentage and it tells you either what wet size to aim for so a finished piece comes out at the size you want, or what a piece you already threw will end up at once it's fired.
Worked example: say your clay shrinks 12% and you want a mug that's 10 cm tall once it's fired. Divide 10 by (1 − 0.12) and you get 11.36 cm — that's how tall to throw it wet. Going the other direction, if you already threw a bowl at 11 cm across and your clay shrinks 12%, it will fire down to 9.68 cm. The scale factor for 12% shrinkage works out to 1.14, meaning every wet dimension should be about 1.14 times the fired size you're aiming for, which is handy when you're eyeballing a whole form rather than one measurement at a time.
FAQ
Why isn't 12% just "12% smaller" in a straight line?
Because shrinkage is measured against the fired size, not the wet size. If you started wet and simply cut 12% off, you'd undershoot the target, since that 12% should be a share of the larger starting number. Dividing by (1 − shrinkage) instead of multiplying by (1 − shrinkage) corrects for that, which is why the wet size always comes out a bit bigger than a quick mental estimate.
Does shrinkage apply the same way to height and width?
Yes. A tall cylinder shrinks in height the same percentage it shrinks in diameter, so if you only scale the width when planning a tall form, it will come out shorter than you expect relative to how wide it is. Apply the scale factor to every dimension, not just the one you're watching most closely.
Where do I find my clay's actual shrinkage number?
The manufacturer's spec sheet lists it, usually as a combined wet-to-fired percentage at a specific cone. If you don't have the sheet, make a simple shrinkage bar: roll a strip, mark it exactly 10 cm wet, let it dry, fire it, then measure the gap between the marks. That measured number is more accurate for your exact clay and firing than any general chart.
Do lids need special treatment?
Yes. A lid and its pot need to be thrown from the same clay body and fired in the same kiln load, since even a small difference in shrinkage between two clays (or two different firing temperatures) is enough to make a lid too tight or too loose to seat properly.
For more on picking a clay and getting pieces safely to the kiln, see the differences between earthenware, stoneware, and porcelain, how to dry pottery before firing so it doesn't crack, and why pottery cracks or explodes in the kiln.