Why 12% linear shrinkage is not 12% less capacity
A vessel shrinks in height, width and depth. If each dimension keeps 88% of its starting size, its volume keeps 0.88 × 0.88 × 0.88 = 0.681472 of its starting volume. That is about 31.85% volume loss, not 12%.
Let s be your measured linear shrinkage percentage between the selected starting stage and the final-fired stage:
Volume multiplier = (1 − s/100)³
Fired capacity = starting capacity × (1 − s/100)³
Required starting capacity = target fired capacity ÷ (1 − s/100)³
The same relationship works in mL, litres, US fluid ounces or Imperial fluid ounces when the input and main result use the same unit. For conversion, 1 US fl oz = 29.5735295625 mL and 1 Imperial fl oz = 28.4130625 mL; these are different units.
Capacity changes in all three dimensions
500 mL × 0.681472 ≈ 340.74 mL
Worked examples—not clay presets
| Question | Calculation | Estimate |
|---|---|---|
| A 500 mL starting capacity | 500 × 0.88³ | 340.74 mL fired |
| A 350 mL fired target | 350 ÷ 0.88³ | 513.59 mL starting capacity |
| A 16 US fl oz fired target | 16 ÷ 0.88³ | 23.48 US fl oz starting capacity |
Which shrinkage rate should I enter?
Match the rate to the starting stage. Wet-to-final-fired shrinkage includes drying and the complete firing sequence. For a bone-dry or bisque-fired vessel, enter only its measured remaining linear shrinkage to the final firing. Do not apply the full wet-to-fired percentage again.
Do not assume that half the total shrinkage is still to come after bisque. Different clays and firing schedules behave differently. When you change the starting stage above, the calculator clears the rate so you can enter a matching measurement. Read about stage baselines or measure a shrinkage test bar.
Capacity is not the amount of clay you need
This tool estimates the empty space inside a vessel. It does not calculate clay weight, wall thickness, the volume occupied by the clay itself, absorption, glaze fit or food safety. Use interior measurements: a roughly cylindrical cavity has volume π × (inside diameter ÷ 2)² × inside height. With dimensions in centimetres, the result is in cubic centimetres, equal to mL. Outside dimensions overestimate capacity.
For a curved or tapered form, a simple cylinder is only a rough approximation. Do not pour water into unfired clay to measure capacity. After firing and cooling, a water-capacity check on an appropriate test vessel is more useful than extra decimal places in the calculator.
Leave room above the drink
A brim-full 350 mL mug is not necessarily comfortable to use with 350 mL of coffee. Choose whether your target means brim-full capacity or a lower fill line, and compare the same definition at both stages. This calculator does not impose a universal headspace percentage or infer the depth of a fill line.
When the estimate is less reliable
- Thrown or formed clay may shrink differently along different directions. One test-bar rate cannot predict all interior dimensions.
- Warping, slumping, changing wall thickness, a foot or gallery, and glaze on the interior can change the cavity independently of uniform scaling.
- Very high percentages produce large reverse-sizing estimates and magnify measurement error. Expansion or bloating is outside this tool’s ordinary-shrinkage model.
Test a representative vessel using your actual clay and firing sequence. No rate here is a recommended percentage for a named clay.
References and related tools
The three-dimensional capacity relationship and interior-measurement approach are explained in Ceramic Arts Network: Math for Potters, Part 2—Volume. Its example percentages describe the author’s clay, not universal presets. For measuring shrinkage, see Digitalfire’s SHAB test.
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Inputs are calculated in this page without being saved or submitted. There is no account, analytics script, or data-collection form in this tool.