About the Clay Shrinkage Calculator
This clay shrinkage calculator helps potters and ceramic artists hit a finished size. Clay shrinks as it dries and shrinks again in the kiln, typically 10–15% in total for stoneware and porcelain. Use it three ways: measure a test bar to find the shrinkage rate of your clay body, enter a target fired size to see how big to make the piece while wet, or enter a wet size to predict the fired size.
It is useful for throwing sets of mugs or bowls that must match, making lids and galleries that fit, sizing tiles and plaques, and fitting ceramic parts to hardware such as lamp fittings, clock movements or cabinet knobs. The volume shrinkage figure tells you how much a pot’s capacity will drop between wet and fired.
Shrinkage depends on the clay body, how wet it was, and the firing temperature, so the most reliable number comes from your own test bar fired in your kiln to your usual cone. Manufacturers’ data sheets give a good starting point.
How to use the clay shrinkage calculator
- 1Choose whether you need a wet size, a fired size, or your shrinkage rate.
- 2For a rate, measure a line on a wet test bar and again after the glaze firing.
- 3Otherwise enter your clay’s total shrinkage percentage.
- 4Enter the target fired size or the wet size in any unit.
- 5Use the shrinkage ruler table to size a whole set of pieces at once.
Formula and method
Potters express shrinkage as a percentage of the wet (plastic) size: a 10 cm line that fires to 8.8 cm has shrunk 12%. To predict a fired size, multiply the wet size by (1 − s). To work backwards from a target, divide by (1 − s) rather than adding the percentage — adding 12% to 10 cm gives 11.2 cm, but the correct wet size is 10 ÷ 0.88 = 11.36 cm.
Clay shrinks in all three dimensions, so a vessel’s volume falls by 1 − (1 − s)³; 12% linear shrinkage cuts capacity by about 32%. The figures assume even shrinkage; tall forms and flat tiles may shrink slightly differently across and along the grain of the clay.
- s
- Total linear shrinkage, wet to fired (decimal)
- wet
- Plastic (freshly made) size
- fired
- Size after the final firing
Worked examples
10 cm fired mug at 12% shrinkage
Dividing the 10 cm target by (1 − 0.12) = 0.88 gives 11.36 cm, so throw the mug about 11.4 cm tall. Its volume will shrink by 1 − 0.88³ = 31.9%, so make it roughly 1.47 times the capacity you want.
Test bar: 10 cm line fires to 8.75 cm
The line lost 1.25 cm of its original 10 cm, so the clay shrinks (10 − 8.75) ÷ 10 = 12.5%. To scale designs, multiply fired dimensions by 1 ÷ 0.875 = 1.143.
25 cm wet platter, 13% porcelain
A 25 cm platter loses 13% of its size: 25 × 0.87 = 21.75 cm after the final firing — more than 3 cm smaller than it left the wheel.
Frequently asked questions
How much does clay shrink when fired?+
Most stoneware and porcelain bodies shrink about 10–15% in total from wet to fully fired, and earthenware usually 5–10%. Porcelain fired to high temperatures sits at the upper end.
How do I measure clay shrinkage?+
Roll a test bar, mark a 10 cm (or 100 mm) line on it while wet, and remeasure after drying and after the final firing. Shrinkage % = (wet length − fired length) ÷ wet length × 100.
Why not just add the shrinkage percentage to the size?+
Shrinkage is a percentage of the wet size, not the fired size. Adding 12% to 10 cm gives 11.2 cm, which fires to 9.86 cm. Dividing 10 by 0.88 gives the correct 11.36 cm.
Does clay shrink more in drying or firing?+
Both contribute. Drying shrinkage is often 4–7% as water leaves the clay, and firing adds the rest as particles fuse; the split varies by body, so use total shrinkage for sizing.
How much smaller will my pot’s volume be?+
Volume shrinks with the cube of the linear change. At 12% linear shrinkage a pot keeps only 0.88³ ≈ 68% of its wet capacity, so a 500 ml wet mug holds about 340 ml fired.