Clay Shrinkage Calculator
Work out total shrinkage from the drying and firing stages, and how large to throw a piece so it finishes on size.
Shrinkage varies by clay body, by how wet you threw it and by how hot you fire, so a test bar from your own clay at your own cone beats any published figure. Nothing is uploaded.
Two stages of shrinkage do not add up
Clay loses size twice — once as the water leaves between wet and bone dry, and again in the kiln. The second stage applies to what is left after the first, not to the original, so 6% and 6% comes to 11.64% rather than 12%. The gap is small at studio percentages and widens as they climb.
| Drying | Firing | Actual total | Adding them gives | Gap |
|---|---|---|---|---|
| 5% | 5% | 9.75% | 10% | 0.25 |
| 6% | 6% | 11.64% | 12% | 0.36 |
| 7% | 5% | 11.65% | 12% | 0.35 |
| 8% | 8% | 15.36% | 16% | 0.64 |
| 12% | 10% | 20.80% | 22% | 1.20 |
Which order the two stages come in makes no difference to the total — 7% then 5% is the same as 5% then 7% — because multiplying the two survivals together does not care about sequence. A test bar measured wet and again after firing gives the whole thing in one number, and skips the question entirely.
Marking up by the shrinkage always leaves you short
The natural correction is to add the shrinkage to the size you want. It does not work, because the percentage is taken off the larger wet figure rather than the smaller fired one. To finish at 100 mm at 11.64% you have to throw 113.17 mm; marking up gives 111.64 and lands 1.53 mm small.
| To finish at | Throw at | Marking up gives | Short by |
|---|---|---|---|
| 100 mm | 113.17 mm | 111.64 mm | 1.53 mm |
| 150 mm | 169.76 mm | 167.46 mm | 2.30 mm |
| 200 mm | 226.35 mm | 223.28 mm | 3.07 mm |
| 300 mm | 339.52 mm | 334.92 mm | 4.60 mm |
The error scales with the piece: on a 300 mm platter it is 4.60 mm, which is the difference between a lid that fits and one that does not. The correction is a division — the finished size over the fraction that survives — and the two only agree when there is no shrinkage at all.
How to use
- Enter your drying and firing shrinkage percentages.
- Or measure a test bar wet and again after firing.
- Enter the finished size you want.
- Read how large to throw it.
Frequently asked questions
Do drying and firing shrinkage add together?
No, they compound. The firing stage acts on what drying left behind, not on the original, so 6% and 6% comes to 11.64% rather than 12%. The gap is small at studio percentages and widens as they climb — 12% and 10% is 20.80%, not 22%.
How much larger should I throw a piece?
Divide the finished size by the fraction that survives, do not add the shrinkage to it. At 11.64% total, a 100 mm finished piece needs throwing at 113.17 mm; marking up by 11.64% gives 111.64 and fires 1.53 mm small.
Why does marking up not work?
Because the percentage is taken off the larger wet figure rather than the smaller fired one. Adding it back to the fired size applies it to the wrong base. The error scales with the piece, so a 300 mm platter comes out 4.6 mm under — the difference between a lid that fits and one that does not.
Does it matter which stage comes first?
Not to the total. Multiplying the two survival fractions together gives the same answer either way, so 7% then 5% is identical to 5% then 7%. It matters to when the piece is vulnerable, but not to the arithmetic.
How do I measure my own shrinkage?
Draw a line of known length on a flat test bar while the clay is still wet, dry it, fire it at your usual cone and measure the line again. That gives you the total in one number, from your clay at your temperature, which beats any published figure.
Why do published figures vary so much?
Because shrinkage depends on the clay body, on how wet you worked it, and on how hot you fire. A porcelain fired to cone 10 can lose well over 12%; a low-fire earthenware much less. The same bag of clay at two cones gives two answers.
What is a shrink ruler?
A ruler with the scale stretched by your shrinkage rate, so you can measure the wet piece directly in finished units. It does the same division this calculator does — and it is worth checking that yours matches your clay, since they are sold in fixed percentages.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.