Clay tools · Methodology
How our clay calculations work
Linear shrinkage compares the same marked distance at different stages. Each percentage needs the right starting length.
Measure the same distance
Use the same two reference marks on one specimen, measured wet, bone-dry, and after the final firing has cooled. Keep all lengths in the same unit: millimetres, centimetres, or inches. A 100 mm wet reference makes the arithmetic convenient, but the formulas work for any positive starting length.
The calculator uses wet and final-fired lengths. The test log can also use a bone-dry length to separate drying from firing shrinkage. See how to run a shrinkage test for the measurement workflow.
Three percentages, two baselines
Drying: wet to bone-dry
Drying shrinkage (%) = (wet length − dry length) ÷ wet length × 100
The reference is the original wet length.
Firing: bone-dry to final-fired
Firing shrinkage (%) = (dry length − fired length) ÷ dry length × 100
The reference is the bone-dry length, not the wet length. This includes the change through the firing sequence to the final measurement; it is not just bisque-to-glaze shrinkage.
Total: wet to final-fired
Total shrinkage (%) = (wet length − fired length) ÷ wet length × 100
The reference is the original wet length. This is the rate used to plan a wet starting size for a desired final-fired size.
Do not simply add stage percentages
For an illustrative test with 100 mm wet, 94 mm dry, and 88 mm fired, drying shrinkage is 6%, firing shrinkage is about 6.383%, and total shrinkage is 12% — not 12.383%. The firing percentage starts from the shorter dry length.
If only the two stage rates are available, let D be the drying percentage and F the dry-to-fired percentage:
Total (%) = [1 − (1 − D/100) × (1 − F/100)] × 100
Use rates from the same specimen and process, with these baselines. Rounded stage rates may produce a slightly different total; calculate directly from the measured lengths when available. Read more about drying versus firing shrinkage.
Work backwards from a finished size
Required wet size = desired final-fired size ÷ (1 − total shrinkage/100)
At 12% total shrinkage, a 150 mm final-fired target needs about 170.45 mm wet. Adding 12% to the target would not undo 12% shrinkage. Known-rate mode needs a wet-to-final-fired rate for the process you intend to use, not a dry-to-fired rate alone.
What these results can — and cannot — tell you
Shrinkage belongs to the tested clay and process, not to a universal clay percentage. Clay body and batch, water content, forming method, drying conditions, and firing schedule or atmosphere can change the result. Record those conditions and repeat tests before relying on a dimension for a close fit.
These tools estimate a linear dimension, not volume, weight, absorption, glaze fit, or food safety. Warping and uneven shrinkage are not modelled. Displayed results are rounded; extra digits do not improve your measurements.
The calculator and test log accept positive lengths, zero shrinkage, and shrinkage below 100%; they reject expansion (a later length larger than the earlier reference). The printable card can display a negative percentage as a prompt to investigate the measurements or expansion. A rejected value does not prove that clay cannot expand.
Method references
The measurement approach and stage distinction are described in Digitalfire’s SHAB shrinkage/absorption test, Drying shrinkage, and Firing shrinkage. Our tools cover the length calculations, not the full SHAB absorption procedure.