Clay Shrinkage Calculator for Wet, Dry and Fired Sizes

How the clay shrinkage calculator works

The clay shrinkage calculator uses two short formulas. Shrinkage is the length you lost divided by the length you started with, so total shrinkage is (wet length minus fired length) divided by wet length, times 100. A bar marked at 100 mm that fires to 88.5 mm has shrunk 11.5 mm out of 100, which is 11.5%.

Going the other way, the wet size you need is the fired size divided by one minus the shrinkage as a fraction. For a 100 mm fired tile in a clay that shrinks 12%, that’s 100 divided by 0.88, which comes to 113.6 mm. The tool also shows the scale factor, here 1.136, so you can multiply any other measurement on the same piece by it.

If you enter a bone-dry length too, the tool splits the total into two parts. Drying shrinkage is (wet minus dry) divided by wet. Firing shrinkage is (dry minus fired) divided by dry, because by the time a pot goes into the kiln the dry size is the one you’re working from. With our default bar, 100 mm wet, 96.5 mm dry and 88.5 mm fired, that’s 3.5% in drying and 8.3% in firing. The two don’t simply add up to the total, because each is measured from a different starting length. That’s normal, not an error.

How much does clay shrink from wet to fired?

Clay shrinkage from wet to fired is usually 6 to 8% for earthenware, 11 to 14% for mid-fire stoneware and 14 to 16% for porcelain, and the hotter you fire a body the more it shrinks. Those are typical ranges, though, and your own clay is the only figure that really counts.

Australian data sheets line up well with those ranges. Walker’s No.10 stoneware lists 3.5% drying and 11.5% total shrinkage at 1280 °C, its SWP porcelain 2.7% and 11.5%, and its SES school body 7.1% at 1060 °C but 11.6% at 1280 °C. That last pair is a useful reminder that the same clay shrinks more as it vitrifies, so a body you bisque at cone 06 and glaze at cone 6 will keep shrinking in the glaze firing.

Fine, wet, plastic clays tend to shrink more in drying than coarse, grogged ones, because there’s more water between the particles to leave. Grog, sand and other non-plastic materials open the body up and cut drying shrinkage, which is one reason sculptural and large-ware clays feel gritty. Porcelain shrinks the most overall because so much of it melts into glass at high temperatures and pulls the particles together.

How do you make a clay shrinkage test bar?

Roll a slab of your clay about 6 to 8 mm thick, cut a strip roughly 140 mm long and 30 mm wide, and scratch two fine marks exactly 100 mm apart while it’s still plastic. A round 100 makes the sums easy, because every millimetre lost is one percent of shrinkage.

Write the clay name and date on the back with a needle tool, then let the bar dry slowly and flat, turning it now and then so it doesn’t curl. Measure between the marks again when it’s bone dry, which gives you the drying figure. Then put it through the same bisque and glaze firings as your real work and measure one last time. A steel ruler or calipers and good light help, since half a millimetre on a 100 mm bar is half a percent.

Make two or three bars rather than one. Clay is never perfectly even, and an average of three readings is far more trustworthy than a single bar that curled or got knocked. If you fire to more than one temperature, keep a separate bar for each, because the figure at cone 04 won’t match the one at cone 6.

Drying shrinkage vs firing shrinkage: why measure both?

Measuring both tells you where the shrinkage, and therefore the risk, happens. Drying shrinkage drives most cracking, warping and handles pulling away, while firing shrinkage drives fit, such as lids, galleries and anything that has to match a fixed size after the kiln.

A clay with high drying shrinkage needs slow, even drying. Cover rims and handles with plastic, dry pieces on a board that lets them move, and avoid joining wet and drier clay, since each part shrinks by a different amount. Lakeside Pottery puts mid-fire stoneware at roughly half its shrinkage in drying and half in firing, which matches what many studio potters see.

Firing shrinkage is the part that catches people out with fitted pieces. A lid and its pot need to be made from the same clay, at the same moisture, and fired together, so they shrink as a pair. Knowing your firing figure also helps you judge whether a glaze firing reached maturity: a test bar that shrinks noticeably less than usual may have been underfired.

Worked examples: mugs, tiles and lids

Say you want a mug that’s 9 cm tall once fired, in a stoneware that shrinks 12% overall. Divide 90 mm by 0.88 and you need to throw it about 102 mm tall, after trimming. If the finished mug should be 8 cm across the outside, throw it to about 91 mm. The same scale factor of 1.136 works for every measurement on the piece.

Tiles are where shrinkage really matters, because a set has to fit a fixed space. A 150 mm fired tile in a 7% earthenware needs to be cut at 150 divided by 0.93, which is about 161.3 mm. Cut a paper or card template at that size and you’ll get the same answer every time.

For lids, measure the gallery of the pot when both pieces are leather hard, rather than working from fired sizes, so the two pieces shrink together. Where the calculator really helps is replacing a lost or broken lid: measure the fired opening, enter your clay’s shrinkage, and make the new lid to the wet size it gives you. Expect to make two and keep the better fit.

Volume shrinks faster than length, since it shrinks in all three directions at once. A 12% linear shrinkage leaves 0.88 x 0.88 x 0.88 of the volume, about 68%, so a mug loses roughly a third of its capacity between the wheel and the cupboard.

Common clay shrinkage mistakes

The most common mistake is multiplying the fired size by one plus the shrinkage instead of dividing by one minus it. At 12%, 100 mm times 1.12 gives 112 mm rather than the correct 113.6 mm, so pieces made that way finish a little small, and the gap grows with bigger work and higher shrinkage.

The second is trusting a data sheet figure for work that has to fit. Published shrinkage is measured under the maker’s conditions, and your clay batch, wedging, drying and kiln can all move it a percent or so. Use the table as a starting point and switch to your own bar as soon as you can.

The last is measuring different things. Measure the same marks at each stage, along the same edge, and never compare a wet ruler measurement with a fired caliper reading taken somewhere else. When you print the shrink ruler, print at actual size with any page scaling turned off, then check the bottom scale against a real ruler before trusting it.