How this instrument works
Every clay body shrinks as water leaves it during drying, then shrinks again as the kiln densifies the clay matrix during firing. The combined effect means a piece made 10 inches wide when wet (greenware) might come out of the kiln closer to 8.7 inches — and that ratio, expressed as a percentage, is the shrinkage rate potters use to plan wet dimensions against a target fired size. It's measured directly: make a test tile at a known wet size, fire it in the same clay body and firing schedule you'll use for real work, measure the fired result, and compute the percentage drop.
This is deliberately the total shrinkage from wet to fired, not the drying shrinkage and firing shrinkage measured as two separate stages — those two percentages can't simply be added together to get the total, because each is measured against a different starting size (drying shrinkage against the wet size, firing shrinkage against the already-shrunk dry size). Measuring wet size directly against fired size sidesteps that arithmetic trap and gives potters the single figure they actually need.
Shrinkage varies by clay body: earthenware typically shrinks the least, stoneware more, and porcelain the most, reflecting how much the clay vitrifies (turns glassy) at its firing temperature. A commonly cited range for stoneware's total wet-to-fired shrinkage is roughly 10–15%, which is why a shrinkage test tile — not a manufacturer's average figure — is the reliable way to know your specific clay's number, since batch, moisture content and firing schedule all shift the result somewhat.
- Make a test tile or test piece from your clay and measure it precisely while still wet (greenware) — many potters score a straight 10 cm or 10 in reference line into the tile for easy remeasuring later.
- Fire the tile through your normal bisque and/or glaze firing schedule, the same one you'll use for real pieces.
- Enter Wet (greenware) size (in) — the measurement you took before firing.
- Enter Fired size (in) — the same measurement taken on the fired tile.
- Read Shrinkage (%) — the total wet-to-fired shrinkage rate for that clay body and firing schedule.
- Reuse this percentage to size future wet pieces: divide your target fired size by (1 − shrinkage% ÷ 100) to find the wet size that will shrink down to it.
Worked example — a test tile, 10 in wet to 8.7 in fired
Enter 10 into Wet (greenware) size (in) and 8.7 into Fired size (in) — a test tile scored at exactly 10 inches wide before firing, measuring 8.7 inches after. Shrinkage (%) reads (10 − 8.7) ÷ 10 × 100 = 13.0%.
That 13% figure sits within the commonly cited 10–15% total shrinkage range for stoneware, and it's now a reusable number for this specific clay and kiln schedule: to make a piece that fires out to exactly 10 in, you'd need to build it roughly 10 ÷ (1 − 0.13) ≈ 11.49 in wet.
Questions
Why does clay shrink twice — once drying, once firing?
Drying shrinkage happens as water trapped between clay particles evaporates and the particles pull closer together; firing shrinkage happens afterward as kiln heat vitrifies the clay body, melting some material into glass that draws the structure tighter still. Both stages reduce size, which is why the total wet-to-fired shrinkage this calculator reports is larger than either stage measured alone.
What's a typical shrinkage rate for different clay types?
As a rough guide, earthenware tends to shrink the least (it vitrifies least at its lower firing temperature), stoneware sits in a commonly cited 10–15% wet-to-fired range, and porcelain — which vitrifies the most — typically shrinks the most of the three. These are general tendencies, not guarantees: always verify with your own test tile, since specific clay bodies, moisture content and firing schedules all shift the actual number.
How do I size a wet piece to hit a specific fired dimension?
Rearrange the formula: wet size = fired size ÷ (1 − shrinkage% ÷ 100). With a known 13% shrinkage rate, a target fired size of 8.7 in needs a wet size of 8.7 ÷ 0.87 = 10 in — which is just this calculator's own worked example run in reverse, and the calculation potters actually use when a piece has to fit a mold, lid, or another fired part exactly.
Why can't I just add my drying shrinkage percentage to my firing shrinkage percentage?
Because the two percentages are each measured against a different starting size — drying shrinkage against the original wet size, firing shrinkage against the already-smaller dry size — so simply summing them overstates the true total. Measuring wet size directly against fired size, as this calculator does, avoids that error entirely by comparing both ends of the process to the same starting measurement.
Does glaze affect the fired size I should measure?
Measure the bare clay body's shrinkage on an unglazed or bisque-fired test tile where possible — glaze adds a thin surface layer that can slightly affect a caliper measurement without reflecting the clay body's actual dimensional shrinkage, so it's cleaner to keep the shrinkage test and the glaze test separate.