What size should I throw a mug to get an 80 mm rim?

Divide by one minus the shrinkage. In a stoneware that shrinks 12 percent: 80 divided by 0.88 is 91 mm on the wheel. The free Jordsmide Pottery Studio works this out from a drawing of the finished pot.

Why can't I just add 12% to the size?

Because the percentage comes off the bigger wet measurement, not the smaller fired one. Adding 12 percent to 80 mm gives 89.6 mm, which fires down to 78.8 mm, about a millimetre short. Dividing gets it right.

Is there a free clay shrinkage calculator?

Yes. The Jordsmide Pottery Studio at jordsmide.com is free and handles shrinkage as part of designing the pot. You draw the finished size, pick your clay body, and it shows the thrown dimensions, capacity and clay weight, and prints a 1:1 template.

Jordsmide › Guides › Clay shrinkage

Clay shrinkage: what size do I throw?

Q: How much does clay shrink?

Usually 8% to 14% in total, from wet clay to fired pot. Stoneware is around 12%, porcelain around 14%, earthenware around 8%. It depends on the clay body and how hot you fire, so a test tile is the only way to know your own rate.

Divide the size you want by one minus your shrinkage rate. For an 80 mm rim in a clay that shrinks 12%, throw it at 91 mm.

Jordsmide · updated 2026

The formula

thrown size = fired size ÷ (1 − shrinkage) 80 ÷ 0.88 = 91 mm on the wheel

Works for your measurements, rim, base, height. You can use the same shrinkage number for all of them!

Don't add the percentage

This is a mistake that is easy to do. Shrinkage is a percentage of the wet size, not the fired size. Add it on and you end up to small.

Method	You throw	It fires to	Off by
Add 12% to 80 mm	89.6 mm	78.8 mm	−1.2 mm
Divide 80 by 0.88	90.9 mm	80.0 mm	right

On a single mug, a millimetre doesn't matter much. It does matters when the pot has to fit something, a purchased lid, a light fixture, or bowls that nest. The error also grows with size: 1.5% off on a 300 mm platter is 4-5 mm.

Skip the calculations

Instead of making calculations by yourself, you can use our tool!

Draw the mug at the size you want it to finish, pick your clay, and the Studio shows the thrown numbers next to the fired ones.

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Typical rates

Total shrinkage, wet clay to fired pot. These are starting points only, it is possible that your clay will be different, and your supplier's datasheet is a better guess than this table. In the tool you can add a manual shrinkage if you have more accurate information.

Clay	Shrinkage	Throw a 100 mm feature at
Earthenware	~8%	108.7 mm
Stoneware	~12%	113.6 mm
Porcelain	~14%	116.3 mm

Hotter firing also means more shrinkage. The same stoneware shrinks more at cone 10 than at cone 6, because it keeps tightening as it vitrifies. If you change your firing schedule, test again rather than reusing the old number.

Testing your own clay

Takes a few minutes plus one firing, and it replaces the guesswork.

Roll a flat slab of the clay you actually throw with.
Mark two lines exactly 100 mm apart while it's wet. Using 100 makes the maths easy.
Dry it slowly, then bisque and glaze fire it as you normally would.
Measure between the marks again. 88 mm means 12% shrinkage.

Three tiles averaged is better than one. Scratch the clay name and cone into each before it dries, or you won't know which is which later.

Datasheets can be confusing. Clay shrinks twice, once drying, once firing, and some datasheets list those separately. They don't simply add together. Measuring wet-to-fired yourself avoids the question.

The other direction

To find where something already on the wheel will end up, multiply:

fired size = thrown size × (1 − shrinkage) 95 × 0.86 = 81.7 mm fired

This is the useful one for callipers. Set them to the thrown target before you start.

What this doesn't cover

The formula gets the size right. A few other things still move:

Warping. A rim can go oval regardless of how it was sized.
Trimming. Clay taken off the foot isn't in the calculation.
Glaze. A few tenths of a millimetre. Doesn't matter on a mug; can matter on a lid seat.
Slumping. Tall thin forms can lose height and gain width.

Letting the tool do it

You can do this with a calculator. But shrinkage is usually not the only number you need, you also want capacity, clay weight, and something to throw against.

The Jordsmide Pottery Studio handles all of it together. Draw the mug at its finished size, choose your clay, and you get:

thrown diameters and height with shrinkage applied
capacity in ml
how much clay to weigh out
a 1:1 PDF to print and throw against
a throwing rib cut to your profile at thrown size

Free, no signup

Runs in the browser. Nothing to install, no account, no ads. Designs stay on your own machine.

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Common questions

How much does clay shrink?

Usually 8-14% total, wet to fired. Stoneware around 12%, porcelain around 14%, earthenware around 8%.

What size do I throw for an 80 mm rim?

About 91 mm in a 12% stoneware. Or draw it at 80 mm in the Studio and read the thrown number off the screen.

Does clay shrink the same in height and width?

Close enough that one number for every dimension works for most pots. Wide flat forms may pull in slightly more than tall narrow ones, but not by much.

Do thicker walls shrink less?

No, shrinkage comes from the clay, not the wall thickness. Thick walls dry and fire unevenly though, which shows up as warping or cracks rather than a different size.

What if I haven't tested my clay?

Start with 12% for stoneware, 14% porcelain, 8% earthenware, and treat the first batch as your test. Measure a finished pot against what you threw and adjust.