What is Clay Shrinkage — and Why Does It Matter?
You open the kiln and the mug looks great — but it's smaller than you expected. Not dramatically, just noticeably. The lid you made to fit a specific jar no longer quite reaches. The tiles you cut for a wall project are a centimetre too short. If this has happened to you, you've met clay shrinkage.
It happens to every piece of clay ever fired, and it will happen to every piece you ever make. The good news: once you understand what's going on, you can plan for it. A single test tile is all it takes.
How shrinkage actually works
Clay is made of very fine particles suspended in water. When the clay is freshly made and still soft, those particles float in a water film that holds them slightly apart. As the piece dries out over the following hours or days, the water escapes, the particles pack closer together, and the whole piece contracts. This is drying shrinkage.
But drying is only half the story. When the piece goes into the kiln, the heat finishes the job — burning away the last traces of water, driving out other materials, and beginning to fuse the clay particles into a harder, more permanent structure. This process is called vitrification (from the Latin for glass, because the clay starts to become slightly glassy), and it causes a second round of shrinkage called firing shrinkage.
Add the two together and you have total shrinkage. A clay body might lose 5% of its size drying and another 8% firing — around 13% total. That sounds like a small number until you're trying to make a lid fit a jar.
Why it matters for your work
For most decorative pieces — a freeform vase, a sculptural figure, a decorative bowl — shrinkage is barely worth thinking about. Your piece comes out a bit smaller than you made it, and that's fine.
It starts to matter the moment two measurements need to relate to each other. Lids must fit their jars. A set of mugs should match in height. Tiles must fit a fixed space. A tall, elegant form that looks graceful when wet can look shorter and squatter after firing if the proportions shift more than expected.
The reassuring flip side: for beginners making freeform work, shrinkage is mostly something to be aware of rather than something to stress about. The goal right now is just knowing it happens — and knowing how to account for it when precision matters.
How much does clay shrink?
Not all clay bodies shrink the same amount. The rate depends on the clay's mineral composition, particle size, and how hot you fire it.
- Porcelain tends to shrink the most — often 12–15% total. Its very fine particles pack tightly during firing.
- Stoneware typically sits in the 10–13% range.
- Earthenware usually shrinks less, around 5–8%, because it fires at lower temperatures and vitrifies less completely.
Manufacturers publish a shrinkage figure for each clay body, and it's a useful starting point. But the real number for your specific clay, in your kiln, at your firing temperature, can differ from the data sheet. Testing your own clay is always more reliable — and it only takes one test bar to get a number you can actually trust.
To put a real number on it: a 200mm slab made from a clay body with 12% total shrinkage will measure around 176mm after firing.
How to measure your clay body's shrinkage
The standard method uses a test bar — a straight strip of clay you measure at each stage.
- Roll out a slab at a consistent thickness (about 1cm works well).
- Cut a bar about 100mm or 200mm long. Mark the exact measurement points with a pin tool or pencil — mark specific points on the bar rather than just measuring end-to-end, since the bar can warp during drying and the ends may crumble slightly.
- Let it dry completely. Bone dry means completely dry all the way through, not just on the surface — in a warm room with good airflow, allow at least 48 hours. A bone-dry piece feels room temperature against your cheek; a damp one feels noticeably cool.
- Measure between your marked points. That gives you the drying shrinkage.
- Fire the bar to your usual temperature.
- Measure once more. That gives you the total shrinkage from wet to fired.
To calculate the shrinkage percentage: subtract the smaller measurement from the larger, divide by the original wet measurement, multiply by 100. If your bar was 200mm wet and 174mm after firing, that's (200 − 174) ÷ 200 × 100 = 13% total shrinkage.
The Shrinkage Calculator does this arithmetic for you — enter your wet and fired measurements and it shows the rate immediately.
Using your shrinkage rate in practice
Once you know your clay body's shrinkage rate, you can work backwards from any target size.
Say you want a mug to stand 100mm tall after firing, and your clay shrinks 12% total. Divide 100 by 0.88 (which is 1 minus 0.12). You get 113.6mm — so you'd throw or build the piece to about 114mm tall while the clay is wet.
The formula in plain English: divide your target size by (1 minus the shrinkage rate).
The Shrinkage Calculator's Plan a piece tab handles this automatically, and it works in millimetres, centimetres, or inches.
Once you've done a few test tiles and planned a few pieces this way, the adjustment starts to feel instinctive. Experienced potters often just know that "I want it this big, so I'll make it a bit bigger than that" — but they built that instinct by doing the maths first.
A few common questions
My piece came out slightly smaller — do I need to worry? For freeform decorative work, usually not. It matters most when pieces need to fit together — lids, sets, tiles, handles that need to reach a fixed width. If you're making something purely decorative and not matching it to anything, shrinkage is just part of the process.
Can I reduce shrinkage? Not significantly without changing the clay body. Adding grog (fired, crushed clay particles) can help reduce warping and cracking, but it doesn't eliminate shrinkage. The rate is set by the clay's minerals and your firing temperature.
Does glaze shrink too? Yes — glaze has its own contraction rate as it cools, and if that rate doesn't match the clay body's, you can end up with crazing (a network of fine cracks in the surface) or crawling (glaze that pulls away from the clay). That topic — called glaze fit — is separate from body shrinkage and goes into glaze chemistry. It's worth exploring once you're past the basics.
Ready to put this into practice? Use the Shrinkage Calculator to find your clay body's shrinkage rate from a test tile, or to calculate how big to make your next piece.