Clay Firing Shrinkage Calculator
Clay shrinks as it dries and fires, so a pot built to a given size comes out smaller, and a piece that must fit a finished dimension has to be built larger. The relationship is a fixed linear percentage for a given clay body and firing temperature. This calculator works both ways: enter a wet size and it gives the fired size, or enter a target fired size and it gives the wet size you must build. Critically, the reverse direction divides rather than subtracts, which is where many makers go wrong. Your shrinkage rate, measured from a test tile, is the input.
Clay shrinkage formula
s = shrinkage rate / 100
Fired size = wet size * (1 - s)
Size lost = wet size - fired size
Wet size for target = target fired / (1 - s)
Shrinkage factor = 1 - s
Wet to fired multiplies by (1 - s). To hit a target fired size you divide by (1 - s); adding the percentage back overshoots and is the common mistake.
Pottery shrinkage context
- Total shrinkage is body and firing specific; read it from the clay data sheet or measure a test tile.
- Mark a 100 mm line on wet clay, fire, and remeasure to read shrinkage as a direct percentage.
- Linear shrinkage is roughly uniform in all directions for most clay bodies.
- Higher firing temperatures generally increase shrinkage as the clay matures and vitrifies.
- The millimetre is an SI unit defined through the NIST weights and measures programme.
Clay shrinkage: frequently asked questions
How much does clay shrink when fired?
Total shrinkage from wet to fired depends on the clay body and firing temperature, typically in a range from a few percent to over 12 percent, and it is stated on the clay's data sheet. Because it is body and temperature specific, this calculator takes your measured shrinkage rate as an input rather than assuming one.
How do I measure my clay's shrinkage rate?
Make a test tile, mark a line exactly 100 mm long on the wet clay, fire it, then measure the line. If it reads 90 mm, the shrinkage is 10 percent. Using a 100 mm reference makes the percentage read directly. Enter that measured percentage here for accurate results.
How do I find the wet size for a target fired size?
Divide the target fired size by (1 minus shrinkage as a fraction). To finish at 90 mm with 10 percent shrinkage, make it 90 divided by 0.9, which is 100 mm wet. The calculator does this so you can build to the right wet dimension and land on your finished size.
Does shrinkage apply equally in all directions?
For most clay bodies linear shrinkage is roughly uniform in all directions, so the same percentage applies to length, width, and height. The calculator treats it as a single linear rate. Strongly directional forming or wedging can introduce small differences, so test pieces matter for precision work.
Why is wet-to-fired not just subtracting the percentage?
Going from wet to fired you do multiply by (1 minus shrinkage). But going the other way, from a target fired size back to wet, you must divide by (1 minus shrinkage), not add the percentage. Adding it back overshoots. The calculator handles both directions correctly.
Official sources
- NIST: Units of length (millimetre).
- NIST: Office of Weights and Measures.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.