Two shrinkages, one number that matters
Clay shrinks twice and for different reasons. Drying shrinkage happens as the water that separates the clay platelets evaporates and the platelets pack closer together; it is finished by the time the piece is bone dry and it accounts for most of the movement in a plastic body. Firing shrinkage happens in the kiln, as fluxes melt and the body vitrifies, closing the remaining pore space. It rises with temperature, which is why the same clay shrinks more at cone 10 than at cone 6.
What you actually need to plan a pot is neither of those on its own but the total, wet to fired. That is the number that turns a finished size into a thrown size, and it is the one this calculator leads with. The two components are still worth knowing, because they tell you where the risk lies: heavy drying shrinkage means warping and cracking if the piece dries unevenly, while heavy firing shrinkage tends to show up as slumping and warping in the kiln.
Measure it on your own body, at your own cone. Published figures for a clay describe the manufacturer's test conditions, and shrinkage varies with how wet you work, how much you compress, and where in the kiln a piece sits.
Why the two shrinkages do not add up
Following ASTM C326, drying shrinkage is measured against the wet length and firing shrinkage against the dry length. Those are different denominators, so the two percentages describe reductions of different things and cannot be added.
Take the default bar: 100 mm wet, 94 mm dry, 88 mm fired. Drying is (100 − 94) ÷ 100 = 6.000%. Firing is (94 − 88) ÷ 94 = 6.383%. Add them and you get 12.383%, but the bar plainly went from 100 to 88, which is 12.000%. The correct combination multiplies what survives each stage: 0.94 × 0.93617 = 0.88, so the total is 1 − 0.88 = 12.000%. The gap of 0.383 percentage points is small on a test bar and is 400 × 0.00383 = 1.5 mm on a 400 mm platter.
The allowance is the same trap in reverse. To finish at 200 mm with 12% shrinkage, do not make it 200 × 1.12 = 224 mm — that fires to 224 × 0.88 = 197.1 mm, three millimetres short. Divide by what survives: 200 ÷ 0.88 = 227.27 mm, which fires to 227.27 × 0.88 = 200.00 mm exactly. The multiplier 1 ÷ (1 − S) is what the calculator reports as the wet-size multiplier, and it is worth writing on the wall above the wheel.
Volume behaves differently again. Every dimension shrinks by the same fraction, so volume shrinks by that fraction cubed: 1 − (1 − S)³. At 12% linear, volume falls by 1 − 0.88³ = 1 − 0.681 = 31.85%. That is the number that matters when a piece is specified by capacity rather than by size — a mug thrown to hold 350 ml wet holds about 238 ml fired.
Worked example: throwing a 200 mm fired bowl
You roll a test bar, score two marks exactly 100 mm apart while it is still soft, dry it slowly to bone dry, bisque it and glaze fire it to cone 6. It measures 94 mm bone dry and 88 mm out of the glaze firing. You want a bowl whose fired rim is 200 mm across.
- Drying shrinkage. (100 − 94) ÷ 100 = 6.000%.
- Firing shrinkage. (94 − 88) ÷ 94 = 6.383%, measured against the dry bar.
- Total shrinkage. (100 − 88) ÷ 100 = 12.000%. Check it the other way: 1 − (0.94)(0.93617) = 1 − 0.88 = 12.000%. Agreement confirms the readings are consistent.
- What survives. 1 − 0.12 = 0.88.
- Wet-size multiplier. 1 ÷ 0.88 = 1.13636.
- Throw the rim at. 200 × 1.13636 = 227.27 mm, so set the callipers at 227 mm and expect the fired rim within a millimetre of target.
- Check a piece you already threw. A 250 mm wet rim fires to 250 × 0.88 = 220 mm.
- Volume loss. 1 − 0.88³ = 1 − 0.681472 = 31.85%.
The volume figure is the one that surprises people. A bowl thrown to hold a litre of water at the leather-hard stage holds only 681 ml fired. If the piece has to hold a specific quantity — a 350 ml mug, a 2 litre casserole — size it from the volume relationship, which means multiplying the target capacity by 1 ÷ 0.88³ = 1.4674 to get the wet capacity.
What your number tells you about the body
Most plastic throwing bodies land between about 10% and 14% total shrinkage, and where yours sits inside that range tells you what to expect on the bench.
Low, under about 10%. Usually a grogged or sculptural body. It holds size and detail well, dries with less risk of cracking, and tolerates uneven drying. The trade-off is often porosity: a body that moves little in the firing has often not vitrified much, so check absorption before using it for functional ware.
Middle, 10% to 14%. The ordinary range for stoneware and most porcelains. Predictable enough that a single test bar per batch of clay is sufficient.
High, above about 15%. Fine, plastic, low-grog bodies — some porcelains reach this. They reward careful, slow, even drying and punish anything that dries at different rates. Flat forms warp, handles pull at the join, and thick-to-thin transitions crack.
Watch the split between the two stages as well as the total. A body with high drying shrinkage and modest firing shrinkage is telling you the risk is on the shelf, before the kiln: dry under plastic, dry flat things between boards, and dry rims more slowly than bases. A body with modest drying and high firing shrinkage puts the risk in the kiln, where slumping and warping happen and where a piece sitting too close to a hot element sees a different temperature from its neighbours.
One thing the calculator cannot capture: shrinkage is not perfectly isotropic on a thrown pot. Clay aligned by the throwing action tends to shrink slightly more across the direction of alignment than along it, which is part of why thrown rims go faintly oval. Treat the calculated size as accurate to about a percent on a real pot, not to the two decimal places the arithmetic offers.
Wet size and volume loss by total shrinkage
| Total shrinkage | Multiplier 1/(1−S) | Wrong multiplier 1+S | Wet size for 200 mm | Volume shrinkage |
|---|---|---|---|---|
| 8% | 1.08696 | 1.08000 | 217.39 mm | 22.13% |
| 10% | 1.11111 | 1.10000 | 222.22 mm | 27.10% |
| 11% | 1.12360 | 1.11000 | 224.72 mm | 29.50% |
| 12% | 1.13636 | 1.12000 | 227.27 mm | 31.85% |
| 13% | 1.14943 | 1.13000 | 229.89 mm | 34.15% |
| 14% | 1.16279 | 1.14000 | 232.56 mm | 36.39% |
| 15% | 1.17647 | 1.15000 | 235.29 mm | 38.59% |
| 16% | 1.19048 | 1.16000 | 238.10 mm | 40.73% |
The third column is included only to be avoided: at 12% it undersizes the wet piece by 3.2 mm in 227, and the fired result comes out 2.9 mm small. Volume shrinkage is 1 − (1 − S)³ and reaches a third of capacity by 12% linear.
How to make a test bar that gives an honest number
Roll a flat bar of the clay about 120 mm long and 15 mm thick, from the same batch and at the same working consistency you actually throw with. Score two clean marks exactly 100 mm apart while it is soft, using a ruler and a needle tool, and mark the clay body and cone on the bar itself. Dry it slowly and flat, turning it so it does not curl, and weigh it daily until the weight stops falling — that, not how it feels, is bone dry. Measure it, then bisque and glaze fire it in a normal load at your usual cone, and measure again. ASTM C326 formalises this procedure for whiteware clays, and the two things it insists on are the ones amateurs skip: a benchmark long enough that a half-millimetre reading error is small, and drying to constant weight rather than to the touch.
What changes a clay's shrinkage
- Firing temperature. The single largest factor. The same body fired to cone 10 shrinks appreciably more than at cone 6, because more of the pore space closes. Test at the cone you actually fire.
- Water content when you work. Wetter clay has more water to lose, so it shrinks more in drying. A body thrown very wet and one wedged stiff give different numbers from the same bag.
- Grog and sand. Non-plastic additions do not shrink, so they reduce the body's total shrinkage roughly in proportion to how much of it they make up.
- Compression and forming method. Slip-cast, pressed and thrown pieces made from the same body do not shrink identically, because they start at different densities.
- Position in the kiln. Temperature varies through a kiln, and where a body is close to full vitrification, a small temperature difference produces a visible size difference.
- Recycled clay. Reclaimed clay that has picked up water, grog dust or another body is no longer the material you tested. Re-test a bar from a reclaim batch before using it for anything dimensional.
When shrinkage becomes the whole problem
For most pots, shrinkage is a background number you apply once. It becomes the central problem in three situations. Lidded forms, where lid and pot must shrink together — make and dry them together, from the same batch, and fire them together, because a difference of half a percent between two clays is a lid that no longer fits. Tiles and flat work, where dimensional accuracy is the product and where uneven drying shows as curl. And anything that must fit something else: a teapot into a box, a sink into a countertop, a set of nesting bowls.
The same arithmetic — divide by what survives, never multiply by what is lost — appears throughout materials work. Sewists apply it to prewashed cloth with the fabric shrinkage calculator, and woodworkers apply the reverse to seasonal movement with the wood movement calculator. Mould makers avoid it altogether by casting rigid material, which is why silicone and resin work uses volume arithmetic instead — see the silicone mould material calculator for how a displaced-volume calculation replaces a shrinkage one.
For studio costing, the firing itself is usually the larger variable. Two firings per piece — bisque and glaze — at studio electricity rates add up, and the kiln firing cost calculator turns a kiln's rating and schedule into a cost per load and per pot. Shrinkage affects that indirectly: a body that shrinks more packs more finished pieces into the same kiln shelf.