Fire the bar to the same cone you use for real work, because shrinkage keeps increasing with temperature.
Why clay shrinks
Wet clay is a mix of fine particles and water. As it dries, water leaves the spaces between particles and they pull closer together, so the piece shrinks, typically by 4–7%. In the kiln, the particles start to fuse and the clay shrinks again. The hotter the firing, the more it vitrifies and the more it shrinks. Add the two together and many stoneware and porcelain bodies lose 10–14% of every dimension between the wheel and the finished pot. ClayShrink turns that percentage into practical numbers.
Measuring your own clay
Published shrinkage figures are a guide, but your clay, your water content and your kiln all change the result. Make a few test bars from the clay you actually use. Scratch a line exactly 10 cm long on each while wet, let them dry slowly, then measure the line when bone dry and again after the glaze firing. Enter the three lengths and the calculator gives the drying, firing and total shrinkage. Average several bars for a reliable figure.
Sizing work before firing
To end at a finished size, divide by one minus the shrinkage. With 12% shrinkage, multiply by about 1.136. List the measurements you care about, such as height, rim and foot diameter or the size of a lid seat, and the table shows the wet size for each. Capacity is affected far more, because shrinkage happens in three dimensions: a 12% linear shrink loses nearly a third of the volume. If a mug must hold 350 ml when finished, it needs to hold about 514 ml freshly thrown.
The shrinkage ruler
A shrink rule makes the maths disappear. Each mark is stretched by your shrinkage, so you measure the wet piece and read the fired size directly. Print it at actual size, check the calibration line with an ordinary ruler, and laminate it or stick it to card.
Batching glazes
Most glaze recipes list a base that totals 100, plus additives such as colourants and opacifiers as extra percentages. Mark additives, set the batch size and weigh the grams shown. The water estimate for a target specific gravity assumes typical material density; always confirm with a hydrometer or by weighing 100 ml of slurry. Everything you enter is saved only in this browser.
Frequently asked questions
How do I calculate shrinkage?
Shrinkage % = (wet length − fired length) ÷ wet length × 100. A 10 cm mark that fires to 8.8 cm is 12%.
Why make several test bars?
Averaging three bars evens out measuring errors and warping.
Does glaze firing temperature matter?
Yes. A higher cone usually means more shrinkage.
Is ClayShrink free?
Yes, with no sign-up or ads.
Are my numbers saved?
Yes, in this browser only.