Pottery Shrinkage Calculator

Measure the same reference marks three times

Mark a known span in freshly formed clay, then measure that exact span after drying and after the final firing. End-to-end bar measurements can be distorted by trimming, so stable reference marks are useful. Record the clay batch, forming direction, and firing schedule beside the measurements. A tile intended to fit a frame or a lid intended to fit a vessel needs dimensional planning, not just a material purchase estimate.

Stage formulas and why percentages do not simply add

For wet span W, dry span D, and fired span F, drying shrinkage = 100 × (W - D) / W. Firing shrinkage = 100 × (D - F) / D. Total shrinkage = 100 × (W - F) / W. Retained linear fraction q = F / W. Wet target dimension = desired fired dimension / q; enlargement factor = 1 / q.

Firing shrinkage uses the smaller dry denominator. Adding the two stage percentages therefore exaggerates total shrinkage. The combined retained fraction is (D / W) × (F / D). If shrinkage is uniform in all directions, area retained = q² and volume retained = q³. These geometry outputs are not clay mass loss, porosity, or water absorption measurements.

Worked example: a 100 mm reference span

A span that changes from 100 mm wet to 94 mm dry to 88 mm fired has 6% drying shrinkage, 6.383% firing shrinkage, and 12% total linear shrinkage. A 200 mm fired target needs a 227.2727 mm wet template. For a finished tolerance of ±2 mm, the modeled wet range is 225 to 229.5455 mm. Under uniform shrinkage, fired area retains 77.44% and fired volume 68.1472%.

Interpret the tolerance range cautiously

The lower and upper dimensions translate your chosen design tolerance using one fixed test result. They are not confidence intervals. If repeated bars differ, measure that variability separately and trial a prototype before making a fitted set. Rounded display dimensions can consume some tolerance; keep unrounded values for template design and measure with an appropriate instrument. For different directional shrinkage, run the tool separately for each direction.

Sources and limitations

Digitalfire's DSHR drying shrinkage test describes shrinkage relative to the plastic measurement; its SHAB shrinkage/absorption procedure gives further test-bar context (live technical references, no numbered edition used). This is a studio arithmetic aid, not an ASTM-compliance test. It supports nonincreasing spans and at most 50% total linear shrinkage. Expansion, warping, glaze thickness, anisotropy, and kiln variability are not predicted.

Related tools

• Picture frame mat calculator for framing a finished flat ceramic piece.
• Inches to centimeters for converting a design before entering millimeters (centimeters × 10).

Last updated: 2026-10-09. Use measurements from the process you intend to repeat.