The clay shrinkage calculator above handles the three questions a potter actually asks about shrinkage. How big will this come out of the kiln? How big do I have to make it so that it comes out right? And what is my clay body actually doing, measured from a test bar rather than assumed from a catalogue?
Arb Digital builds free calculators that state their assumptions and let you replace every constant with your own measurement. That matters more than usual here, because clay shrinkage is not a property of “clay” in general. It is a property of one body, mixed to one water content, dried in one studio and fired to one temperature, and any figure taken from anywhere else is a starting guess.
What This Clay Shrinkage Calculator Does
Shrinkage happens in two separate stages that are measured separately and behave differently. Drying shrinkage occurs as water between the clay platelets leaves and the platelets close up; it is largely complete once the piece is bone dry. Firing shrinkage occurs in the kiln as the body vitrifies and densifies, and it continues as temperature rises, which is why the same body fired higher shrinks more.
The calculator keeps those stages apart. In forward mode it takes a wet measurement, applies drying shrinkage to get the dry size, then applies firing shrinkage to the dry size to get the fired size. In reverse mode it runs the same chain backwards from a fired target. In test bar mode it derives both percentages from three measurements of one bar, which is the only way to get numbers you can trust for your own material.
Every result is linear. A shrinkage percentage in ceramics describes one dimension, not area and not volume, and the calculator reports the volume consequence separately because that is where people get caught out.
How to Use It
- Pick a mode. If you are trying to hit a finished size — a lid that must fit a jar, a tile that must fit a grid, a bowl that must sit in a fixture — use reverse mode.
- Enter your measurement in whatever unit you like. The calculator never converts units, so the answer comes back in the same one.
- Enter your two shrinkage percentages. If you have never tested the body, load an approximate dry-to-fired figure from the clay body list and treat the result as provisional.
- To get real numbers, roll a test bar, scribe two marks exactly 100 units apart, and measure between those marks wet, dry and fired. Put the three figures into test bar mode.
- Read the wet-to-fired factor. That single multiplier is the number worth writing on the side of your clay bin.
The Formula: How It Is Calculated
Write the drying shrinkage as a fraction Sd and the dry-to-fired shrinkage as Sf. Then:
dry = wet × (1 − Sd), fired = dry × (1 − Sf), and therefore fired = wet × (1 − Sd) × (1 − Sf).
Work through the defaults. A wet bar of 100 with 5% drying shrinkage dries to 100 × 0.95 = 95. Firing that at 7% gives 95 × 0.93 = 88.35. The total wet-to-fired shrinkage is 100 − 88.35 = 11.65%, and the wet-to-fired factor is 0.8835.
Reverse mode inverts that: wet = fired ÷ ((1 − Sd) × (1 − Sf)). To finish at 88.35 you need 88.35 ÷ 0.8835 = 100 wet, which closes the loop exactly.
Test bar mode uses the standard definitions. Drying shrinkage is (wet − dry) ÷ wet, and firing shrinkage is (dry − fired) ÷ dry. With 100, 95 and 88.35 that returns 5% and 7%, which is the same calculation run in the other direction.
Why 5% Plus 7% Is Not 12%
This is the mistake that costs work. Shrinkage percentages are applied in sequence to a shrinking object, so they compound rather than add. The second percentage is taken from the already-reduced dry size, not from the original wet size.
With 5% and 7% the difference is small: 11.65% against 12%, or 0.35 of a unit on a 100 unit piece. On a mug that is invisible. On a 400 mm platter it is 1.4 mm, and on a lid that must seat inside a gallery it can be the difference between a fit and a rattle. The bigger the individual percentages, the wider the gap: 10% and 12% add to 22% but compound to 20.8%.
The habit that prevents this entirely is to stop thinking in percentages and think in factors. Multiply 0.95 by 0.93 once, get 0.8835, and use that one number forever. Our percentage change calculator makes the same point in a general setting, and the percentage calculator is useful if you want to check any single step by hand.
Volume Shrinkage Is Much Larger Than It Sounds
A body that shrinks 11.65% linearly loses far more than 11.65% of its volume, because the reduction applies in all three dimensions at once. Volume shrinkage is 1 − (1 − S)³, so 0.8835³ = 0.6898 and the piece loses about 31% of its volume.
This is why a jug thrown to hold a litre wet holds noticeably less once fired, and why capacity is one of the few things worth testing physically rather than calculating. It is also why weight of clay is a poor proxy for finished capacity. If capacity is the specification you have to hit, work out the wet dimensions you need using reverse mode and then check the fired volume with our cylinder volume calculator, which will get you close for a straight-sided form.
Surface area falls by the square rather than the cube: 0.8835² = 0.781, so about 22% less area. That matters for glaze quantity, because you mix glaze for the fired surface and apply it to the bisque surface, which is bigger.
What the Published Clay Body Figures Do and Do Not Tell You
The four options in the clay body list come from Digitalfire’s glossary entry on firing shrinkage, which gives approximate dry-to-fired ranges of 3–4% or less for earthenware, 5–6% for stoneware, 7–8% for whitewares and over 10% for vitreous porcelains, while stressing that “fired shrinkages are relative within a system; there is no absolute of how much a clay should shrink”.
Notice what is missing from that list: drying shrinkage. There is no equivalent published figure this page can load for you, and that is deliberate rather than an omission. As the same source explains in its entry on drying shrinkage, the measurement is only meaningful when sample preparation and water content are controlled, because a wetter bar has more water to lose and therefore shrinks more. Two potters using the same bagged clay will get different drying shrinkage figures if one throws wetter than the other.
So the honest position is this. The firing figure loaded from the list is a reasonable provisional value for a body of that class. The drying figure is yours and nobody else’s, and until you have measured it the total is an estimate with an error bar you cannot see.
Why Your Own Bar Beats Any Table
Four things move a body’s shrinkage enough to notice, and none of them are on any datasheet. The first is water content at forming: throwing wetter than you wedge means more drying shrinkage. The second is firing temperature, and this one is large — a stoneware taken two cones higher will shrink measurably more because vitrification has progressed further. The third is kiln position, since real kilns are not isothermal and the same load can differ top to bottom. The fourth is reclaim, because reprocessed clay carries a different water history and often behaves slightly differently from fresh.
The procedure that fixes all of it is cheap. Roll three bars from the same batch, scribe marks 100 mm apart with a ruler and a needle tool, measure wet, dry them slowly and measure again, fire them in the load and position you actually use, and measure a third time. Average the three. Repeat whenever you change body, supplier or firing schedule. Our percent error calculator is handy for checking how far apart your three bars landed, which tells you how much confidence the average deserves.
One practical warning: mark bars so you can identify them after firing, and record the cone and schedule with the result. A shrinkage figure without a temperature attached is not a figure.
Common Mistakes to Avoid
- Adding the two percentages together. They compound. Multiply the factors, do not sum the percentages.
- Applying a linear percentage to volume or capacity. A 12% linear shrinkage is roughly a 32% volume loss. The two are not interchangeable.
- Using a supplier figure for drying shrinkage. That number depends on how wet you work, so it has to come from your own bar.
- Ignoring the firing temperature. The same body fired higher shrinks more. A shrinkage figure only applies to the schedule it was measured at.
- Measuring the whole bar instead of between scribed marks. Bar ends round and slump. Two marks well inside the ends give a far more repeatable measurement.
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For the arithmetic underneath this page, the percentage calculator and the percentage change calculator handle single steps, and the percent error calculator compares a measured result against a target. The ratio calculator is useful for scaling a whole set of dimensions at once, and the cylinder volume calculator gets you a capacity estimate for a straight-sided form. If you are curious about how other materials change size with heat rather than with water loss, the thermal expansion calculator covers the reversible case, which is a different phenomenon from the permanent shrinkage modelled here. Everything else is on the free online tools hub.
Frequently Asked Questions
Multiply the wet measurement by one minus the drying shrinkage to get the dry size, then multiply that by one minus the firing shrinkage to get the fired size. With 5% drying and 7% firing, 100 wet becomes 95 dry and 88.35 fired, a total wet-to-fired shrinkage of 11.65%.
Because the firing percentage is applied to the already-shrunk dry size rather than to the original wet size, so the two compound instead of adding. Adding 5% and 7% gives 12%, but the true figure is 11.65%, and the gap widens as the individual percentages get larger.
Divide the fired size by the product of the two shrinkage factors. To finish at 88.35 with 5% drying and 7% firing shrinkage you divide by 0.95 times 0.93, which is 0.8835, giving 100 as the wet size. The reverse mode on this page does exactly that.
Digitalfire gives approximate dry-to-fired ranges of 3 to 4% or less for earthenware, 5 to 6% for stoneware, 7 to 8% for whitewares and over 10% for vitreous porcelains. Those are provisional starting values only, because shrinkage depends on the specific body and the temperature it is fired to.
Linear. It describes one dimension. Volume shrinkage is one minus the linear factor cubed, so an 11.65% linear shrinkage is about a 31% volume loss, and surface area falls by about 22%.
Roll a flat bar, scribe two marks a known distance apart such as 100 mm, and measure between the same two marks when wet, when bone dry and again after firing in the position and schedule you normally use. Average three bars, and record the cone alongside the result.
Mostly water content at forming and firing temperature. Someone who throws wetter loses more water during drying and gets a higher drying shrinkage, and a body taken to a higher cone vitrifies further and shrinks more in the kiln.
This page is an estimating tool. It applies the shrinkage percentages you supply to the measurement you supply; the accuracy of the result depends entirely on how well those percentages match your own clay body, water content and firing schedule, so test before committing to work that must hit a size.
