What this calculator does
Clay shrinks as it dries and again when fired, and the combined figure is typically 10 to 15% depending on the body. A piece thrown at 20 cm with 12% shrinkage comes out of the kiln at 17.6 cm.
Working backwards is the more useful direction, and it is not simply adding the percentage back. To finish at 20 cm with 12% shrinkage you need to make it at 22.73 cm, not 22.4, because the shrinkage applies to the larger wet dimension rather than to the fired one. That difference grows with the shrinkage rate.
The formula
The fired dimension is the wet dimension multiplied by one minus the shrinkage percentage, applied independently to each dimension. The reverse calculation divides the target fired dimension by one minus the shrinkage, which is why it is not the same as adding the percentage back on. Volume shrinks by roughly the cube of the linear factor, so a 12% linear shrinkage is about 32% by volume.
| Term | Meaning |
|---|---|
| Total shrinkage | Drying plus firing combined, typically 10 to 15% linear for most clay bodies. |
| Drying shrinkage | Loss as water leaves between the clay particles, usually 4 to 8%. |
| Firing shrinkage | Further loss as the body vitrifies at temperature, usually 5 to 10%. |
| Test tile | A marked strip fired alongside the work to measure the actual shrinkage of a specific body at a specific temperature. |
The inputs explained
| Field | What to enter |
|---|---|
| Wet length (cm) | Wet length in centimetres. |
| Wet width (cm) | Wet width in centimetres. |
| Total shrinkage rate (clay body spec, wet-to-fired) (%) | Total shrinkage percentage, drying and firing combined. Measure it with a test tile rather than trusting a published figure. |
When to use it
Making a piece to fit something
A lid for an existing jar, a tile for a specific gap or a piece for a frame all have to finish at a fixed size, which means starting larger by the right amount.
Working to a commission size
A commissioned piece specified at a finished dimension has to be made oversize, and guessing the allowance is how pieces come out wrong.
Comparing clay bodies
Different bodies shrink by different amounts, and switching between them changes the wet size needed for the same result.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much does shrinkage change the fired size?
A fixed wet size across a range of shrinkage rates.
| Total shrinkage | Fired size | Shrinkage per dimension | Wet size needed for a target fired length |
|---|---|---|---|
| 8% | 18.40 × 9.20 cm | 1.60 × 0.80 cm | 21.74 cm (to fire at 20.00 cm) |
| 10% | 18.00 × 9.00 cm | 2.00 × 1.00 cm | 22.22 cm (to fire at 20.00 cm) |
| 12% | 17.60 × 8.80 cm | 2.40 × 1.20 cm | 22.73 cm (to fire at 20.00 cm) |
| 15% | 17.00 × 8.50 cm | 3.00 × 1.50 cm | 23.53 cm (to fire at 20.00 cm) |
Questions
How much does clay shrink?
Typically 10 to 15% linear from wet to fired, combining drying and firing. Porcelain tends toward the higher end and heavily grogged stoneware toward the lower. The only reliable figure is the one you measure for your own body at your own firing temperature.
How do I measure shrinkage?
Make a test tile, mark exactly 10 cm on it while wet, then measure the same marks after firing. If they measure 8.8 cm the shrinkage is 12%. Doing this for each clay body and firing temperature you use is a small job that prevents repeated mistakes.
Why is working backwards not just adding the percentage?
Because shrinkage is a percentage of the wet size, not of the fired size. To finish at 20 cm with 12% shrinkage, divide 20 by 0.88, which gives 22.73. Adding 12% to 20 gives 22.4, which would fire to 19.7 and come out short.
Does shrinkage affect volume the same way?
No, far more. Volume shrinks by roughly the cube of the linear factor, so 12% linear shrinkage is about 32% by volume. A vessel made to hold a specific quantity needs the volume calculated rather than the dimensions.
For cross-stitch fabric sizing, see the cross-stitch fabric calculator. For bean bag filling, see the bean bag filling calculator.