Clay Shrinkage Calculator
Enter your clay dimensions and shrinkage rate to find the fired size, or work backwards from a target finished size to get the wet clay size you need to throw. The calculator splits shrinkage into the drying stage and the firing stage so you can see exactly where the clay changes. Switch between metric and imperial, choose forward or reverse mode, and use the reference table to find typical rates for porcelain, stoneware, earthenware, ball clay, and fire clay.
What is clay shrinkage and why does it matter?
Clay shrinks as it dries and fires because water leaves the clay body in two distinct stages. In the first stage, as the piece air-dries from soft and workable to bone-dry greenware, the water between the clay particles evaporates and those particles pack closer together. In the second stage, when the piece is fired in the kiln, remaining chemically bonded water and organic material burn off, and the clay body sinters and densifies further. The combined result can be a finished piece that is 8 to 22 percent smaller in every linear dimension than the wet original, depending on the clay body and firing temperature. Understanding this contraction is essential for throwing functional pieces to a target size, matching lids to pots, tiling a specific area, or reproducing a broken piece. Even a 1 percent error in the assumed shrinkage rate can make a lid that does not fit.
How to use this calculator: forward and reverse modes
In forward mode you enter the wet clay dimensions you are starting with and the calculator tells you the expected fired size. This is useful when you are throwing freely and want to know what you will end up with. In reverse mode you enter the target finished dimensions you want and the calculator tells you the wet size you need to throw. This is the more common workflow for production potters who need consistent, predictable results. Either way, enter your total shrinkage rate and the drying shrinkage portion (from your clay data sheet), and optionally enter separate length, width, height, and diameter fields for round pieces. The bone-dry length column shows the intermediate size after drying but before the kiln.
Finding your clay shrinkage rate
The most reliable source is the technical data sheet from your clay supplier, which lists linear shrinkage at each cone or temperature. Many manufacturers publish these on their websites. If you blend your own body or want to confirm the figure, fire a test tile: mark a 10 cm or 4 inch line on a freshly thrown slab using calipers, note the wet measurement, let it dry completely, measure again for drying shrinkage, then fire to your target temperature and measure the fired length. Shrinkage rate equals (wet minus fired) divided by wet, expressed as a percentage. A simple spreadsheet with a column for each cone level lets you build a personal shrinkage database over time. For the most accurate results, make several tiles from different batches and average the readings.
Shrinkage across different clay bodies
Earthenware is the most forgiving body, typically shrinking only 8 to 12 percent in total because it fires at lower temperatures and retains more porosity. Stoneware sits in the middle at 11 to 15 percent and is the most popular choice for functional ware because its shrinkage is predictable and consistent. Porcelain shrinks the most, often 14 to 20 percent, because it contains a high proportion of fine-particle kaolin and fires at the highest temperatures. Ball clay, which is added to bodies to improve plasticity, has extremely fine particles and can shrink 15 to 22 percent on its own. Fire clay, used for refractory pieces, has coarser particles and shrinks less. The reference table above lists typical ranges; your actual clay body may fall anywhere within those ranges, and the only way to know precisely is to fire a test tile.
Typical shrinkage rates by clay type
| Clay type | Drying shrinkage | Firing shrinkage | Total shrinkage | Typical firing temp | Water absorption |
|---|---|---|---|---|---|
| Earthenware | 4-6% | 4-6% | 8-12% | 950-1100 C | 10-20% |
| Fire clay | 3-5% | 5-7% | 8-12% | 1100-1300 C | 5-15% |
| Stoneware | 5-7% | 6-8% | 11-15% | 1200-1280 C | 1-6% |
| Porcelain | 6-8% | 8-12% | 14-20% | 1260-1300 C | 0-3% |
| Ball clay | 7-10% | 8-12% | 15-22% | 1200-1300 C | 0-2% |
Linear shrinkage percentages measured from wet to bone-dry, dry to fired, and total. Values are typical ranges; always verify with your clay supplier or a test tile.
Frequently asked questions
What is a typical clay shrinkage rate?
Most commercial clay bodies shrink between 10 and 15 percent in total linear dimensions from wet to fully fired. Earthenware sits at the lower end (8-12%), stoneware in the middle (11-15%), and porcelain at the higher end (14-20%). Ball clay can reach 22% on its own. Always check the technical data sheet for your specific clay body and confirm with a fired test tile.
How do I calculate the wet size I need to throw?
Use the reverse mode. Divide your desired fired dimension by the shrinkage multiplier, which equals 1 minus the shrinkage rate expressed as a decimal. For example, if you want a fired length of 20 cm and your clay shrinks 12%, the multiplier is 0.88, so you need to throw at 20 / 0.88 = 22.73 cm wet. This calculator does that division automatically for all your dimensions at once.
Does shrinkage happen evenly in all directions?
In general, linear shrinkage is approximately equal in all directions for hand-built or slip-cast work. Thrown pieces can shrink slightly less in the vertical direction than horizontally because wheel-throwing aligns clay particles differently. For production work where tight tolerances matter, measure shrinkage in both directions using test tiles.
Why is my actual shrinkage different from the published rate?
Published shrinkage rates are averages measured under controlled conditions. Your kiln temperature, the actual peak temperature reached, the hold time, the rate of cooling, the thickness of the piece, and even the humidity when drying all affect the final size. Using a pyrometer and consistent firing schedules will make your results more repeatable. Always fire confirmation test tiles from each new batch of clay.
What is the difference between linear and volumetric shrinkage?
Linear shrinkage is measured along one dimension, for example the length of a test bar, and is the standard measurement used by clay manufacturers. Volumetric shrinkage is roughly three times the linear figure because it compounds in three directions: a piece with 10% linear shrinkage has a volume about 27% smaller when fired. For most pottery planning, linear shrinkage is the useful number because you are fitting lids, tiles, or measuring wall thickness.
How do I make a shrink rule?
Mark off 10 equal divisions (say, each 1 cm) on a fresh clay slab. Fire the slab. Measure the total fired length of those 10 divisions. The ratio of the original length to the fired length is your shrinkage multiplier. You can then mark a wooden ruler at intervals equal to the original clay spacing so that measuring with it on wet clay gives you a reading in fired-size units, without any arithmetic.