Pottery Clay Shrinkage Calculator
Clay shrinks as it dries and fires, so a thrown piece comes out smaller than it went in. Measure a test tile before and after firing to get your shrinkage rate, then use it to predict the fired size of any wet piece, or to work out how large to throw for a target fired size.
Clay shrinkage formula
Shrinkage rate = (wet - fired) / wet
Shrink factor = fired / wet = 1 - shrinkage rate
Fired size = wet piece * shrink factor
Wet size to throw = target fired / shrink factor
Measure the test tile in whatever unit you like; the rate is dimensionless, so the predictions come out in the unit you enter for the wet piece and target.
Worked example
A test tile marked at 100 mm wet measures 88 mm fired, so shrinkage = (100 minus 88) / 100 = 12.00 percent and the shrink factor is 0.88. A 150 mm wet piece will fire to 150 times 0.88 = 132.00 mm. To hit a 120 mm fired size, throw 120 / 0.88 = 136.36 mm wet.
Clay shrinkage: frequently asked questions
How is clay shrinkage rate measured?
Mark a known length on a wet test tile, fire it, then measure again. Shrinkage rate is (wet length minus fired length) divided by wet length, as a percentage. A common range is roughly 10 to 15 percent total from wet to fired, but it varies by clay body and firing temperature.
How big will my fired piece be?
Multiply the wet measurement by one minus the shrinkage rate. A 100 mm wet piece at 12 percent shrinkage fires to 100 times (1 minus 0.12) = 88 mm. This calculator does this for you in any consistent unit.
How big should I throw to hit a target fired size?
Divide the target fired size by one minus the shrinkage rate. For an 88 mm fired result at 12 percent shrinkage, throw 88 / (1 minus 0.12) = 100 mm wet. This is the inverse of the shrink calculation.
Why does shrinkage vary between clays?
Shrinkage depends on the clay body's particle size, water content and how vitrified it becomes at your firing temperature. Always test your own clay at your own cone with a test tile rather than relying on a generic percentage.
Sources and notes
- The method is the standard ceramic shrinkage definition: rate equals (wet minus fired) divided by wet length, measured on a test tile.
- Shrinkage is a measured property of your clay at your firing temperature; this tool assumes no fixed percentage.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.