Wet-to-Fired Dimension Calculator
Make it fit the first time. Enter the finished size you want and your clay's shrinkage to get the wet dimension to build — or go the other way and predict what a wet piece will measure once it comes out of the kiln.
Use your own test-bar figure — published rates differ from what your kiln actually does.
Formula & how it works
To size wet work, wet = fired ÷ (1 − shrinkage ÷ 100). To predict a fired size, fired = wet × (1 − shrinkage ÷ 100).
Note that dividing is not the same as adding the percentage: at 12% shrinkage the wet piece must be about 13.6% larger, not 12%.
Worked example
For a mug meant to finish 4 inches tall from a clay that shrinks 12%, build it 4 ÷ 0.88 = 4.55 inches wet. Going the other way, a 4.55 inch wet piece fires back to 4.00 inches. Adding 12% instead would give 4.48 wet and a short 3.94 inch mug.
Designing around shrinkage
Divide, don't add
This is the mistake that costs pieces. Shrinkage is expressed as a fraction of the wet size, so recovering it means dividing by what remains rather than adding the percentage back. The gap is small on a mug and significant on a large platter or a lidded jar.
Everything scales together
Height, diameter, wall thickness, and the gap between a lid and its gallery all shrink by the same linear proportion. Volume falls faster still, roughly with the cube, which is why a vessel thrown to a generous-looking wet size can finish holding noticeably less than you pictured.
Leave tolerance where it matters
Calculated sizes assume perfect drying and firing, and real work warps a little, trims unevenly, and settles. For pieces that must fit something — a lid, a shelf, a shipping box — aim slightly generous and plan to adjust at the trimming stage rather than hoping the math lands exactly.