Pottery Shrinkage Scale Calculator

Work backwards from a target fired size to the wet size to throw, and forwards from a thrown size to what it will fire to.

Pottery & Clay

The clay shrinkage calculator converts a measurement you have into a fired size. This one is built the other way round: you have the size the finished piece has to be, an order for a 20 centimetre plate or a lid that must fit a jar, and you need the size to make it. The scale factor is the key number, and it multiplies or divides by exactly the same amount in either direction. Both directions are reported so the two can be checked against each other.

At a glance

What you enter: Which direction are you working?, Target fired length (the long side), Target fired width or height (the short side), Wet length you are making, Wet width or height, Measured fired length, Total linear shrinkage from wet to fired, Wall thickness, measured fired
What you get: Wet length to work to, Wet width to work to, Fired length, Fired width, Scale multiplier for every dimension, Percentage larger the wet form has to be, Scale factor, wet to fired, Volume factor, Share of the footprint area lost, Fired interior length, Fired interior width, Shrinkage your measurement implies, Your measurement against the figure used here
Formula shown: Yes, every step, right below your results.
Cost and privacy: Free, no account needed, and nothing you type is stored.

See worked examples ↓ Read the FAQ ↓ Share this calculator ↓

Which direction are you working?

The key difference from the clay shrinkage calculator: this tool starts from the size the piece has to finish at.

Target fired length (the long side) mm

The finished size the piece has to be. Used by the first option.

Target fired width or height (the short side) mm

For a round form, enter the same figure as the length.

Wet length you are making mm

The size on the wheel or the bench, before drying and firing. Used by the second and third options.

Wet width or height mm

Measured fired length mm

Only used by the third option, to work out the shrinkage you actually got.

Total linear shrinkage from wet to fired %

Drying plus firing. Five to twelve percent is typical, and porcelain can be higher.

Wall thickness, measured fired mm

Used for the interior capacity of the target form.

Reset

Results are estimates for general information only, always verify with a professional or official source before acting on them.

How to use the Pottery Shrinkage Scale Calculator

1 Enter Which direction are you working?, The key difference from the clay shrinkage calculator: this tool starts from the size the piece has to finish at.
2 Enter Target fired length (the long side) (mm), The finished size the piece has to be. Used by the first option.
3 Enter Target fired width or height (the short side) (mm), For a round form, enter the same figure as the length.
4 Enter Wet length you are making (mm), The size on the wheel or the bench, before drying and firing. Used by the second and third options.
5 Enter Wet width or height (mm)
6 Enter Measured fired length (mm), Only used by the third option, to work out the shrinkage you actually got.
7 Enter Total linear shrinkage from wet to fired (%), Drying plus firing. Five to twelve percent is typical, and porcelain can be higher.
8 Enter Wall thickness, measured fired (mm), Used for the interior capacity of the target form.
9 Press Calculate. You get Wet length to work to, Wet width to work to, Fired length, Fired width, Scale multiplier for every dimension, Percentage larger the wet form has to be, Scale factor, wet to fired, Volume factor, Share of the footprint area lost, Fired interior length, Fired interior width, Shrinkage your measurement implies, Your measurement against the figure used here, and every intermediate step stays visible under "How is this calculated?"
10 Change one number at a time to see what moves the result, that is usually the most useful thing a calculator can tell you.

Worked examples

Real numbers run through this calculator. Open any example to load it into the form above and change it.

A 200 mm plate to a finished size

A stoneware body shrinking 8 percent in total.

You enter

Which direction are you working?: 1
Target fired length (the long side): 200 mm
Target fired width or height (the short side): 200 mm
Wet length you are making: 217 mm
Wet width or height: 217 mm
Measured fired length: 200 mm
Total linear shrinkage from wet to fired: 8%
Wall thickness, measured fired: 7.40 mm

You get

Wet length to work to: 217.39 mm
Wet width to work to: 217.39 mm
Fired length: 200.00 mm
Fired width: 200.00 mm
Scale multiplier for every dimension: 1.09
Percentage larger the wet form has to be: 8.7%
Scale factor, wet to fired: 0.92
Volume factor: 0.78
Share of the footprint area lost: 15.4%
Fired interior length: 185.20 mm
Fired interior width: 185.20 mm
Shrinkage your measurement implies: 7.8%
Your measurement against the figure used here: -0.2%

Throw it at 217.4 millimetres, which is 8.7 percent larger than the target. Multiply any other finished dimension by 1.087, and remember that the fired piece holds about 22 percent less than its wet dimensions suggest.

Open this example in the calculator →

A mug body checked against the wheel

A wet diameter of 90 millimetres in a body that is expected to shrink 7 percent.

You enter

Which direction are you working?: 2
Target fired length (the long side): 200 mm
Target fired width or height (the short side): 200 mm
Wet length you are making: 90 mm
Wet width or height: 90 mm
Measured fired length: 200 mm
Total linear shrinkage from wet to fired: 7%
Wall thickness, measured fired: 6 mm

You get

Wet length to work to: 90.00 mm
Wet width to work to: 90.00 mm
Fired length: 83.70 mm
Fired width: 83.70 mm
Scale multiplier for every dimension: 1.08
Percentage larger the wet form has to be: 7.5%
Scale factor, wet to fired: 0.93
Volume factor: 0.80
Share of the footprint area lost: 13.5%
Fired interior length: 71.70 mm
Fired interior width: 71.70 mm
Shrinkage your measurement implies: -122.2%
Your measurement against the figure used here: -129.2%

The mug fires to 83.7 millimetres across. If it is meant to match a 20 millimetre fired lid it also needs the height checked at the same factor, because every dimension scales by 0.93.

Open this example in the calculator →

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Frequently asked questions

How do I work out the wet size for a fired size?

Divide the finished size by one minus the shrinkage. A 200 millimetre fired plate in a body that shrinks 8 percent has to be made at 200 divided by 0.92, which is 217.4 millimetres. That is 8.7 percent larger than the target rather than 8 percent, because the shrinkage is taken off the wet size rather than added to the fired one. Multiplying the target by 0.92 gives 184 millimetres, which is the wrong direction and produces a plate that fires undersized.

How is this different from the clay shrinkage calculator?

That tool starts from a measurement you have and tells you what it will fire to. This one starts from the size the piece has to finish at and tells you what to make, which is the direction a commission or a fitted lid needs. It also reports the scale multiplier for every dimension, so a form can be scaled as a whole rather than dimension by dimension, and it keeps both directions visible so the two can be checked against each other.

Does a lid need the same shrinkage allowance as the pot?

Only if it is the same clay and the same firing. A lid in a different body, or one fired to a different temperature, will shrink by a different amount and can end up too tight or too loose even though both were measured from the same pot. Make and fire a test lid with the pot it belongs to wherever possible, and remember that a lid which fits at the end of the glaze firing is the one that matters, not the one that fits after the bisque.

Why is the volume loss so much bigger than the length loss?

Because shrinkage happens in all three directions at once. An 8 percent linear shrink leaves 92 percent in each direction, and 0.92 cubed is 0.778, so the fired piece holds about 22 percent less than its wet dimensions suggested. At 12 percent linear the volume factor is 0.68, so nearly a third of the capacity is gone. That is why a form sized by eye on the wheel so often comes out of the kiln smaller than intended.

How do I measure my own shrinkage accurately?

Make a bar or a small cylinder from the body you use, mark two points exactly 100 millimetres apart with a needle while the clay is still soft, and fire it the way you fire your work, including the same top temperature and soak. Measure the marks again and the difference in millimetres is your shrinkage percentage. Use the third option here to check a measurement against the figure you have been working with, and repeat the test whenever you change body or supplier.