Pottery Shrinkage Calculator
Created by: Liam Turner
Last updated:
Calculate pottery clay shrinkage from wet to fired dimensions and find the wet size needed to achieve your desired fired measurements.
Pottery Shrinkage Calculator
PotteryCalculate clay shrinkage from wet to fired dimensions or find the wet size needed to achieve your desired fired measurements.
What is a Pottery Shrinkage Calculator?
A pottery shrinkage calculator determines how much a clay piece will change in size from its wet state through drying and firing. All clay bodies shrink as water evaporates during drying and as particles fuse during kiln firing. The total shrinkage depends on the clay body composition, water content, and peak firing temperature, typically ranging from 8 to 18 percent for common pottery clays.
Understanding shrinkage is essential for any potter who needs pieces to come out at a specific finished size. A mug that needs to be 3.5 inches in diameter when fired must be thrown larger to compensate for shrinkage. Without accounting for this dimensional change, pieces will consistently come out smaller than intended, lids will not fit their jars, and sets of dinnerware will not match.
Shrinkage occurs in distinct stages. The first and largest phase happens during drying, when mechanical water between clay particles evaporates and the particles draw closer together. A smaller amount of shrinkage occurs during bisque firing as chemically bound water is driven off. The final phase occurs during the glaze firing as the clay vitrifies and the silica and alumina particles begin to fuse into a glassy matrix.
This calculator handles both directions of the shrinkage problem. You can enter a wet measurement to predict the fired size, or enter a desired fired size to determine how large to make the piece when wet. It also breaks down the shrinkage by stage and provides volumetric shrinkage, which is important for calculating how much liquid capacity a fired vessel will have compared to its wet-formed volume.
How the Pottery Shrinkage Calculator Works
The calculator uses the total linear shrinkage percentage for the selected clay type to convert between wet and fired dimensions. For the wet-to-fired direction, it multiplies the wet measurement by (1 minus the shrinkage fraction). For the fired-to-wet direction, it divides the desired fired measurement by (1 minus the shrinkage fraction). The shrinkage values are derived from standard ceramic engineering data for each clay body type at the selected firing cone.
The stage breakdown splits the total shrinkage into its component phases: wet-to-dry accounts for approximately 45 percent of total shrinkage, dry-to-bisque about 6 percent, and bisque-to-fired about 49 percent. Volumetric shrinkage is calculated by applying the linear shrinkage in all three dimensions using the formula: volumetric shrinkage equals 1 minus (1 minus linear shrinkage)^3. This is always significantly higher than the linear value because volume contracts cubically.
Clay Shrinkage Formulas
Fired Size = Wet Size x (1 - Shrinkage% / 100)
Wet Size Needed = Fired Size / (1 - Shrinkage% / 100)
Volumetric Shrinkage = 1 - (1 - Linear Shrinkage / 100)^3
Total Shrinkage = 1 - (1 - Wet-to-Dry/100)(1 - Dry-to-Bisque/100)(1 - Bisque-to-Fired/100)
Shrinkage Per Inch = Shrinkage% / 100
Example Calculations
Example 1: Stoneware mug at cone 6
A mug thrown at 4 inches in diameter using stoneware with 13.5% total shrinkage will fire to 4.0 x (1 - 0.135) = 3.46 inches. The potter needs to throw the mug about half an inch wider than the target finished size to compensate for shrinkage.
Example 2: Porcelain plate sized for fired target
A dinner plate needs to be 10.5 inches when fired. Using porcelain with 15.5% shrinkage, the wet size needed is 10.5 / (1 - 0.155) = 12.43 inches. The potter must throw or roll the plate nearly 2 inches wider than the desired finished diameter.
Example 3: Raku bowl with low shrinkage
A raku bowl thrown at 6 inches using a raku body with 10% shrinkage will fire to 6.0 x (1 - 0.10) = 5.40 inches. Raku clays contain more grog and coarse particles, resulting in lower shrinkage than refined stoneware or porcelain bodies.
Common Pottery Applications
- Size thrown or hand-built pottery to achieve specific finished dimensions after firing.
- Design lids, flanges, and fitted components that must match after shrinkage.
- Calculate the wet dimensions needed for production dinnerware sets with consistent sizing.
- Estimate the volume change in functional vessels like mugs, bowls, and pitchers.
- Build shrinkage rulers calibrated to your specific clay body and firing cone.
- Compare clay bodies when choosing materials for a project with tight dimensional requirements.
- Troubleshoot sizing mismatches between pieces fired in different kilns or to different cones.
Tips for Better Pottery Results
Always make test tiles with your exact clay body and firing schedule before committing to a production run. Manufacturer shrinkage specifications provide a range, but your actual results depend on how wet you work, your drying environment, and your kiln firing curve. A set of test tiles fired alongside your pieces gives you the most accurate shrinkage factor for your specific conditions.
When making fitted pieces like casseroles with lids, throw both the body and the lid from the same batch of clay and fire them together in the same kiln load. Even small differences in clay moisture or kiln temperature can cause enough variation in shrinkage to make a lid too tight or too loose. Consistency in materials and process is more important than precise calculation for fitted work.
For production pottery, create a shrinkage ruler by marking inch increments on wet clay and firing it. The fired ruler lets you measure directly on wet pieces to predict their finished size without any calculation. Make separate rulers for each clay body and cone you use, and replace them periodically as your clay supplier may adjust formulations over time.
Frequently Asked Questions
How much does clay shrink when fired?
Total shrinkage from wet to fired depends on the clay body. Earthenware typically shrinks 10 to 13 percent, stoneware 12 to 15 percent, and porcelain 13 to 18 percent. The exact amount varies with clay formulation, water content, and peak firing temperature. Most shrinkage occurs in two phases: drying (wet to bone-dry) and the final glaze firing.
What is the difference between linear and volumetric shrinkage?
Linear shrinkage measures the percentage change in a single dimension such as length, width, or diameter. Volumetric shrinkage measures the total change in three-dimensional volume. Because shrinkage acts in all three dimensions, volumetric shrinkage is considerably larger than linear shrinkage. A clay with 13 percent linear shrinkage will have roughly 34 percent volumetric shrinkage.
Why does porcelain shrink more than stoneware?
Porcelain contains a higher proportion of fine-particle kaolin clay and feldspar, which vitrify more completely at high temperatures. This greater degree of vitrification causes the clay particles to fuse and densify more, resulting in higher shrinkage. Porcelain also tends to be thrown with more water, increasing the initial drying shrinkage.
How do I make a shrinkage ruler?
Roll a slab of your clay body and mark exactly 10 inches on the wet clay. Let it dry completely and fire it to your target cone. Measure the fired length and calculate the shrinkage percentage. You can then mark ruler increments on wet clay that will shrink to standard measurements after firing, allowing you to measure target fired sizes directly on wet pieces.
Does firing temperature affect shrinkage?
Yes. Higher firing temperatures cause more vitrification and greater shrinkage. A stoneware body fired to cone 6 will shrink less than the same body fired to cone 10. The difference can be 1 to 3 percentage points. This is why it is important to test shrinkage at your actual firing temperature rather than relying solely on manufacturer specifications.
Can I reduce clay shrinkage?
You can reduce shrinkage by adding non-plastic materials like grog, sand, or molochite to the clay body. These materials act as fillers that do not shrink during drying or firing. Adding 10 to 20 percent grog by weight can reduce total shrinkage by 2 to 4 percentage points, though it will also change the texture and throwing characteristics of the clay.
How accurate are shrinkage calculations for production pottery?
Calculated shrinkage values are useful starting points, but real-world results vary by plus or minus 1 to 2 percentage points depending on how wet the clay is when measured, drying conditions, kiln loading, and firing schedule. For production work, make test tiles with your specific clay and kiln to establish a precise shrinkage factor for repeatable sizing.
Sources and References
- Rhodes, Daniel. Clay and Glazes for the Potter, Revised Edition. Martino Fine Books, 2015.
- Hamer, Frank and Hamer, Janet. The Potter's Dictionary of Materials and Techniques, 5th Edition. A&C Black, 2004.
- Peterson, Susan and Peterson, Jan. The Craft and Art of Clay, 5th Edition. Laurence King Publishing, 2012.
- Norton, F. H. Elements of Ceramics, 2nd Edition. Addison-Wesley, 1974.
- Britt, John. The Complete Guide to Mid-Range Glazes. Lark Ceramics, 2014.