Clay Shrinkage Calculator (Wet to Fired and Reverse)

1. Pick a mode: Wet to fired (how big a wet piece will end up), Target to wet size (how big to build it so it fires to a target), or Measure my clay rate (work out your own shrink rate from a test tile).
2. Choose your units (mm, cm or inches). The math is the same in any unit, so just keep both measurements in the same one.
3. For modes 1 and 2, enter the dimension and your clay's total linear shrinkage percent. Add a second dimension if you want to convert two measurements at once. For mode 3, enter the wet and fired lengths of a marked line.
4. Read the result, the linear percent and the volume shrinkage below it.

What each result tells you:

Clay shrinkage is the way a pottery piece gets smaller as it dries and is fired. Water leaves the clay during drying, and the clay particles pack together and fuse during firing, so the finished piece is smaller than the wet one you shaped.

Potters describe shrinkage in two ways. Linear shrinkage is the change along one direction, for example a length or a diameter. It is the number clay suppliers print on a data sheet. Volume shrinkage is the change in the whole three-dimensional size, and because it applies in every direction at once it is always a much bigger number than the linear figure.

Shrinkage matters whenever a finished size has to be right: a lid that must fit a jar, a tile that must meet a grid, or a bowl that must hold a set volume. This tool is deterministic geometry built around simple ratios and proportions, so the answer is only as good as the shrink rate you give it.

All three modes come from one relationship between wet size, fired size and the shrink fraction s (the linear shrinkage percent divided by 100).

The volume formula cubes the linear factor because the piece shrinks in all three dimensions together. That is why a clay that shrinks 12 percent in length loses about 31.85 percent of its volume, not 12 percent.

Notice that none of these formulas contains a fixed clay constant. The shrink rate is always your input, because every clay body, and even the same body fired to a different cone, behaves differently.

Suppose your clay body's data sheet lists a total linear shrinkage of 12 percent, so s = 0.12 and the factor (1 - s) = 0.88.

Volume: at 12 percent linear, volume shrinkage = (1 - 0.88³) x 100 = (1 - 0.681472) x 100 = 31.85 percent. So the 200 mm piece loses 24 mm of length but close to a third of its volume. Always confirm the rate against your own clay, because 12 percent is only an example here.

The shrink rate you type into this tool is not fixed by nature. Several things move it, which is why measuring your own body beats trusting any single number.

Clay Body

Porcelain usually shrinks more than coarse stoneware or clay with heavy grog. As a general guide only, many stoneware bodies fall somewhere near 10 to 14 percent linear and porcelains often sit higher, but your supplier's data sheet is the number to use.

Firing Temperature

The same clay fired hotter (a higher cone) packs and fuses more, so it shrinks more. A rate measured at bisque will not match the rate at a vitrified glaze firing.

Water Content and Forming

Wetter throwing slip and heavier compression change how much the piece moves between wet and dry. Measuring at a consistent stage keeps your rate repeatable.

Drying

Shrinkage happens in two stages, drying then firing. A data-sheet "total" figure usually covers both from wet to fired, which is what this calculator expects.

You can get a shrink rate two ways. Both are valid; they answer slightly different questions.

Data sheets quote linear shrinkage, not volume. If a number looks very large (around 30 percent or more) it is probably a volume figure, so convert back or re-check. For comparing your measured rate with a published one, the percent error calculator and the percentage change formula are handy.

Sizing to a Finished Target

Use Target to wet size when the fired size must be exact: a lid for a jar, a tile for a set grid, or a stamp that must match a mould. Build the piece to the wet size the tool gives.

Checking How a Piece Will End Up

Use Wet to fired to preview the finished size of something you have already thrown or rolled, so you can adjust before it dries.

Calibrating a New Clay

Use Measure my clay rate the first time you fire a new body or a new cone. Keep that number for every future project in that clay.

1. Using a Generic Percent as Fact

A number from a blog or another potter's clay can be well off. Use your own body's figure or measure a tile.

2. Confusing Linear and Volume Shrinkage

A 12 percent linear clay shrinks about 31.85 percent by volume. Do not plug a volume figure into a length calculation.

3. Measuring Too Short a Line

A 20 mm mark has large rounding error. Mark 100 mm so a 1 mm reading error is only about one percent.

4. Ignoring Firing Temperature

A rate from a bisque firing will not match a hotter glaze firing. Measure at the cone you actually use.

5. Forgetting Both Stages

Some sheets split drying and firing shrinkage. This tool wants the total wet-to-fired figure.

The arithmetic is exact standard geometry, so any difference from your kiln comes from the inputs and from real-world variation, not the formula.

Fire a test tile with a marked line, then use linear shrinkage = (wet length - fired length) / wet length x 100. For a 100 mm mark that fires to 88 mm, that is (100 - 88) / 100 x 100 = 12 percent.

It varies by body and firing temperature. As general background only, many stoneware bodies sit near 10 to 14 percent linear and porcelains often higher, but you should use your own clay's data-sheet figure or measure a test tile rather than treat any single number as fact.

Divide the target fired size by (1 - s). To finish at 150 mm with 12 percent shrinkage, build it 150 / 0.88 = 170.45 mm wet.

Multiply the wet size by (1 - s). A 200 mm wet piece at 12 percent fires to 200 x 0.88 = 176 mm.

Because the piece shrinks in all three dimensions at once. Volume shrinkage = (1 - (1 - s) cubed) x 100, so 12 percent linear is about 31.85 percent by volume.

No. It depends on the clay body, how hot you fire it, water content and forming. That is why this calculator never assumes a rate and always asks for yours.

Use the linear figure for lengths, diameters and heights, which is almost always what you need. Volume shrinkage is mainly useful for understanding how much the whole piece changes.

Yes. Firing hotter, to a higher cone, generally increases shrinkage because the clay fuses more. Measure your rate at the temperature you actually use.

Mark at least 100 mm. A longer line makes a small measuring error a smaller percentage, so your rate is more reliable.

Yes. Shrinkage is a ratio, so any unit works as long as both measurements use the same one. Switch the units toggle to mm, cm or inches.

• Drying and firing shrinkage of ceramic bodies (overview of clay shrinkage behaviour) (Digitalfire technical glossary).
• Measuring clay shrinkage with a test bar (standard studio method) (Digitalfire test procedure).
• Ratios and proportions explained (MultiCalculators, the scaling math behind the tool).