FREE TOOL · FIRING SHRINKAGE
Clay shrinkage calculator
Work out your clay’s shrinkage from two measurements, then the size to build so your piece comes out of the kiln the size you wanted. Free, no sign-up.
Which clay are you using?
Every clay shrinks a little differently. Pick yours to start from an average rate.
What size do you want after firing?
The dimensions of the finished piece, out of the kiln.
Why clay shrinks
Wet clay is made of tiny flat clay particles held apart by water. It shrinks in two stages: as it dries, then as it fires.
01
Drying: the water leaves
The water between the particles evaporates and they pack closer together, so the piece shrinks before it ever reaches the kiln. That’s also why it needs to dry slowly and evenly: a part that dries faster shrinks ahead of the rest, and the piece warps or cracks.
02
Firing: the clay gets denser
During the bisque, the water bound to the clay and the organic matter burn off, but the piece barely moves. At high temperature, part of the clay melts: that glass fills the gaps between the grains and the piece tightens a second time. The more the clay vitrifies, the more it shrinks, which is why porcelain shrinks more than earthenware.
03
What changes the rate
The clay body itself, how fine it is (a grogged clay, full of already-fired grains, shrinks less), the water you add while working and your kiln temperature. The number on the bag is an average, which is why it’s worth measuring your own clay.
How to measure your clay’s shrinkage
Your supplier’s rate is a good start. A test tile gives you yours, in your own kiln.
01
Mark 100 mm
Roll out a slab of the clay you want to test, at your usual thickness. Cut three strips about 15 × 3 cm and score two thin lines exactly 100 mm apart on each one.
02
Fire it like your pots
Let them dry, then bisque and glaze fire them like your pots. Measure the distance between the lines again and average the three: that is the fired length to enter in the calculator.
03
Measure again
The line now measures 88 mm? Your shrinkage is 12%. Enter both lengths under “Measure my clay”.
A shrinkage rate belongs to one clay and one firing temperature. If you change either, test again.
The full method, with the mistakes to avoid: how much does clay shrink?
Average shrinkage rates by clay body
Earthenware (low fire): 5–8%
Stoneware (high fire): 10–13%
Porcelain: 12–16%
Raku and heavily grogged clays: 6–10%
Black and coloured clays: 10–14%
Ballpark figures: two stonewares from two suppliers can be 3 points apart. Nothing replaces your own measurement.
Make bigger pots, replacement lids
A
Replacement lids
Measure the opening of the fired pot and enter it as the finished size. For a 3 in opening in stoneware at 12%: 3 ÷ 0.88 = 3.41 in to build, about 3 7/16″ on a ruler. Leave a little play, and fire a test tile if the lid has to fit snugly.
B
Hitting an exact size
A cup that has to sit on an existing saucer, a box sized to an object, a set that stacks: enter the height, diameter and depth you want, and the calculator gives all three wet sizes at once.
C
Predicting the fired size
Switch to “I know the wet size”: a piece built at 5 in in stoneware at 12% comes out at 5 × 0.88 = 4.4 in.
Frequently asked questions
Should I measure after the bisque or after the glaze firing?
After the glaze firing. Measuring after the bisque only underestimates shrinkage by 3 to 6 points, because most of the vitrification happens at the highest temperature.
Is my supplier’s shrinkage rate good enough?
It’s a good starting point, but it’s measured in a lab at a specific temperature. Your kiln and firing change the result: measure your own rate once.
Why divide instead of adding the percentage?
Because shrinkage is calculated on the wet size. For an 80 mm finished piece at 12% shrinkage, build 80 ÷ 0.88 = 90.9 mm. Adding 12% gives 89.6 mm, and the piece comes out too small.
Does clay shrink the same in every direction?
Roughly. A slab-built piece can shrink a little more in the direction the slab was stretched, one more reason to fire a test tile.
From shrinkage to template
Enter your rate in the Gabarits Céramique tool: it applies it to every dimension of the piece, accounts for slab thickness and prints the template at actual size.