Clay Shrinkage Calculator

Total shrinkage is (wet length − fired length) ÷ wet length; to hit a target fired size, divide that size by (1 − shrinkage), don't add the percentage on top.

The Shrinkage Formulas Potters Use

Clay loses size twice: once as water evaporates while it dries, and again as particles fuse and pull together in the kiln. The standard way to measure it is a test bar — roll out a slab, scratch a line exactly 100 mm (or 10 cm) long while it's still plastic, then re-measure the line when bone dry and again after firing. From those three measurements:

• Drying shrinkage = (wet − dry) ÷ wet × 100
• Firing shrinkage = (dry − fired) ÷ dry × 100 — measured against the dry length, since that's the size that went into the kiln
• Total shrinkage = (wet − fired) ÷ wet × 100

Because firing shrinkage uses a smaller starting length, the drying and firing percentages don't simply add up to the total — a point the ceramic reference library Digitalfire makes explicitly. To plan a piece, the calculator reverses the total: wet size = fired size ÷ (1 − total shrinkage ÷ 100).

Worked Example

A test bar marked at 100 mm measures 93.75 mm bone dry and 87.5 mm after a cone 6 glaze firing. Drying shrinkage is 6.25/100 = 6.25%. Firing shrinkage is 6.25/93.75 = 6.67%. Total shrinkage is 12.5/100 = 12.5% — not the 12.92% you'd get by adding the first two.

Now say you want a plate that is 10 inches across after firing, in a clay that shrinks 12% overall. The wet diameter is 10 ÷ 0.88 = 11.36 inches. The tempting shortcut — adding 12% to get 11.2 inches — fires down to about 9.86 inches, a noticeable miss on anything that needs to fit a lid, a frame, or a matching set.

Frequently Asked Questions

How much does clay usually shrink? Most bodies land somewhere between about 8% and 15% total from wet to final firing. Low-fire earthenware often sits toward the lower end, mid-fire stoneware commonly around 11–13%, and porcelain can reach 14–16%. Your clay supplier's data sheet usually lists a figure, but a test bar fired in your own kiln is more trustworthy.

Should I measure length or volume? Length. Shrinkage percentages for clay are linear figures. Volume drops much more — a 12% linear shrink leaves roughly 68% of the original volume (0.88³), which is why a mug can hold noticeably less than you expected.

Do the units matter? No. Use millimeters, centimeters, or inches — just use the same unit for every measurement. Results come back in that same unit.

Limitations

Shrinkage changes with firing temperature, how long the kiln soaks at peak, how wet the clay was when you shaped it, and even direction — thrown pieces can shrink a little more in height than in width. Treat a single test bar as a good estimate, measure several if precision matters, and re-test whenever you change clay batch or firing schedule.