Pottery Shrinkage Calculator
Enter the measured wet, bone-dry, bisque-fired, and final-fired gauge lengths from a clay test bar. The calculator returns each shrinkage stage and the wet making size needed for a chosen fired dimension.
Required wet making size (mm)113.64
- Drying shrinkage
- 6%
- Dry-to-bisque shrinkage
- 1.06%
- Bisque-to-final shrinkage
- 5.38%
- Total firing shrinkage
- 6.38%
- Total wet-to-fired shrinkage
- 12%
- Wet oversize allowance (mm)
- 13.64
- Predicted bone-dry size (mm)
- 106.82
- Predicted bisque size (mm)
- 105.68
- Wet-to-fired sizing multiplier
- 1.14
- Estimated volume shrinkage
- 31.85%
Uses measured gauge-length ratios; staged percentages are not added together.
Assumes the tested bar represents the same clay batch, forming method, drying process, cone, and firing schedule.
How to use this calculator
- Choose millimeters or inches and use that unit for every measurement.
- Enter the gauge length between the same permanent marks at the wet, bone-dry, bisque, and final-fired stages.
- Enter the fired size you want the finished piece to reach.
- Use the required wet making size and oversize allowance for throwing, handbuilding, molds, or a shrinkage ruler.
How the shrinkage test is calculated
This calculator follows the same linear idea used in ASTM C326 shrinkage testing: mark a test bar, measure the distance between the marks at each stage, and compare each later length with the correct starting length for that stage. It does not ask for a guessed shrinkage rate. It uses your measured wet, dry, bisque, and final-fired lengths.
Let W be the wet gauge length, D the bone-dry length, B the bisque-fired length, F the final-fired length, and T the desired final-fired dimension.
drying shrinkage = 100 x (W - D) / W
dry-to-bisque shrinkage = 100 x (D - B) / D
bisque-to-final shrinkage = 100 x (B - F) / B
total firing shrinkage = 100 x (D - F) / D
total wet-to-fired shrinkage = 100 x (W - F) / W
The sizing step reverses the measured wet-to-fired ratio. The wet-to-fired multiplier is K = W / F, so the wet making size is M = T x W / F. The wet oversize allowance is M - T. The predicted bone-dry size is T x D / F, and the predicted bisque size is T x B / F.
Why the stages are not added
Each stage percentage uses a different starting length. Drying shrinkage is based on the wet length, dry-to-bisque shrinkage is based on the dry length, and bisque-to-final shrinkage is based on the bisque length. Adding those percentages treats them as if they all came from the same starting length, which overstates the total. The combined shrinkage must come from the actual length ratio, such as 100 x (W - F) / W.
What changes the result most
The final-fired gauge length has the largest effect on sizing because it sits in the denominator of T x W / F. A small measurement error after final firing can move the wet making size noticeably, especially when the bar is short. Longer test bars, careful gauge marks, and measuring several bars from the same clay batch give a steadier multiplier.
What this calculator leaves out
The volume result is an isotropic estimate: 100 x [1 - (F / W)^3]. It assumes the clay shrinks equally in all directions. Real ware can shrink differently across thrown walls, slabs, handles, rims, pressed molds, or grogged bodies. The calculation also leaves out warping, bloating, glaze effects, kiln gradients, trimming loss, lid fit, and the extra production tolerance needed for tiles, molds, and other dimension-critical work.
Worked example
With the default test bar, the wet gauge length is 100 mm, bone-dry length is 94 mm, bisque length is 93 mm, and final-fired length is 88 mm. Drying shrinkage is 6%, dry-to-bisque shrinkage is 1.06%, bisque-to-final shrinkage is 5.38%, and total wet-to-fired shrinkage is 12%.
For a 100 mm fired target, the wet-to-fired multiplier is 100 ÷ 88 = 1.1364. The required wet making size is 113.64 mm, the wet oversize allowance is 13.64 mm, the predicted bone-dry size is 106.82 mm, and the predicted bisque size is 105.68 mm. The estimated volume shrinkage is 31.85%.
Common questions
How do I make and mark a clay shrinkage test bar?
Make a straight bar from the same clay, moisture condition, and forming method you plan to use. Put two permanent gauge marks a known distance apart while the bar is wet, then measure between the same marks after drying, bisque firing, and final firing.
Why are drying shrinkage and firing shrinkage calculated from different starting lengths?
Each percentage describes loss from the stage that came immediately before it. Drying shrinkage starts from the wet length, while firing shrinkage starts from the dry or bisque length. That is why the stage percentages should be combined through length ratios rather than added.
How large should I throw a mug or plate for a specific fired size?
Use the required wet making size for the dimension you care about, such as rim diameter or finished height. For the default 100 mm to 88 mm test bar, a 100 mm fired target needs a wet size of 113.64 mm before drying and firing.
Can I use this to make a pottery shrinkage ruler?
Yes. The wet-to-fired sizing multiplier is the scale factor for a shrinkage ruler. Multiply each fired mark by that factor to lay out the corresponding wet mark for the tested clay and firing schedule.
Why does the same clay shrink differently at different cones?
Final firing temperature and heat work change how much the body vitrifies. A hotter or longer firing can pull particles closer together, while bloating or overfiring can produce expansion, so a test bar only applies to the firing schedule used for that test.
Should glazed and unglazed test bars be measured separately?
Yes when the glaze or firing setup may affect size. Glaze thickness, fit, surface tension, and kiln placement can change the fired measurement, so dimension-critical ware should be tested under the same conditions as production pieces.