How to calculate pottery size before firing
The calculator divides your desired fired size by the shrinkage factor—(1 − shrinkage ÷ 100)—to give the wet size you need to achieve. This is the correct approach; adding the percentage to the fired size gives a slightly wrong answer that accumulates when measuring multiple dimensions.
A mug example
For a mug that should finish at 10 cm tall in clay with 12.5% shrinkage, make the wet form approximately 11.43 cm tall. Apply the same linear conversion independently to height, width, and diameter.
For a mug that holds 350 ml when fired, account for shrinkage in all three dimensions—a volumetric change of roughly 33% in total volume for 12.5% linear shrinkage. Build forms larger by the linear factor and verify with fired test pieces.
Allow for process variation
The answer is a calculated target, not a guarantee. Handles, rims, uneven walls, firing position, and clay memory can alter a form differently in different directions. Use the calculated dimension as your starting point, then refine with experience.
If you don't know your clay body's shrinkage rate, measure it with a test bar. Supplier figures vary from batch to batch. You can also print a full-scale shrinkage ruler to measure directly on your wet work.
Lid and rim sizing
When sizing a lid to fit a specific opening, both lid and gallery shrink together if they're the same clay body. The fit should be close to correct, but slight differences in wall thickness and forming technique can affect final fit. Always test-fire a lid before making a full production run.