Using Claybook · 2026-09-20

How to Calculate Clay Shrinkage in Claybook

Clay shrinks as it dries and again as it fires. If you need a mug that fits a hand, a lid that fits a jar, or a commission that matches a sketch, you plan the wet size from the fired size.

Claybook’s shrinkage calculator does that math. You pick a clay (or type a percent), enter a diameter and height, and get the matching size in the other direction.

The same tool is free on the web: clay shrinkage calculator. The app version uses the clay bodies you already saved, so the rate stays consistent with your journal.

For the studio science behind the numbers, read Pottery Clay Shrinkage.

What the calculator needs

You need a shrinkage rate and at least one dimension.

Shrinkage rate comes from:

  • The clay body you added in Claybook (bag rate, or your own measured rate)
  • A custom percent if you have not saved that clay yet

Most stonewares sit around 10 to 14 percent linear shrinkage, bone dry plus fire. Porcelain can run higher. Trust the bag, then verify with a test tile when fit matters.

Dimensions can be:

  • Diameter or width
  • Height
  • A third “other” measurement (gallery, foot, lid flange)

Leave unused fields blank. The calculator only converts what you type.

Two directions: pick the one that matches the question

Open Tools, then Clay shrinkage calculator.

Target fired size to throw/build size. Use this when you know the finished measurement. Example: “I need a 12 cm bowl after the fire.” The output is the wet size to throw.

Wet plastic size to expected fired size. Use this when the pot is already made. Example: “This cylinder is 16 cm tall wet. What will it be after cone 6?”

Switching direction does not change your clay. It only flips which number is the input.

Step-by-step in the app

  1. Open Tools
  2. Tap Clay shrinkage calculator
  3. Choose your clay, or Custom and type the percent from the bag
  4. Choose direction: fired-to-wet (plan) or wet-to-fired (predict)
  5. Enter diameter, height, or both
  6. Read the converted sizes underneath

If you previously used that clay in the weight estimator, Claybook remembers it so you are not re-picking every session.

Units follow your app setting (cm or inches). Change units from the calculator if your sketch is in one system and your ribs are in the other.

Save the rate on the clay body

A calculator with a guessed percent is a toy. A calculator tied to your clay library is a tool.

Open Crafts, then Clays. Edit the body and set Shrinkage to the bag number, like 12.0%. When you later measure a test tile, update that field. Every future shrinkage and weight estimate uses the corrected rate.

How to store the rest of the clay record: How to Add a Clay Body in Claybook.

Worked example: a lid that actually fits

You want a jar lid that finishes at 10 cm inside the gallery.

  1. Select the clay you will throw with
  2. Set direction to target fired size
  3. Enter 10 in diameter
  4. Throw the gallery to the wet size the calculator shows, not to 10 cm
  5. After the fire, measure. If it missed, update the clay’s shrinkage percent and use that next time

Lids fail when you fit them wet and hope. Shrinkage is not optional on anything that has to mate.

Worked example: “will this mug still be mug-sized?”

You threw a mug 11 cm tall and 9 cm wide, wet.

  1. Set direction to wet plastic size
  2. Enter height 11 and diameter 9
  3. Read the expected fired height and width

If the fired height is shorter than you wanted, you now know to throw taller next time, or to start with a different weight. Pair this with How to Estimate How Much Clay You Need.

Measure your own shrinkage when the bag is vague

Bag rates are a start. Your kiln, your packing, and your water content can disagree.

A simple studio test:

  1. Roll a bar or throw a small cylinder
  2. Measure a marked span while plastic, then bone dry, then fired
  3. Linear shrinkage percent is (wet - fired) / wet * 100
  4. Save that percent on the clay body in Claybook

Do this once per clay you care about. Then stop guessing.

Shrinkage vs weight

Shrinkage tells you size. Weight tells you how much clay to start with. You usually need both.

Use the shrinkage calculator for lids, rims, and commissions. Use the clay weight estimator for “how many grams for a 12 cm bowl?” The estimator already folds your clay’s shrinkage into the suggested grams.

Common mistakes

  • Using 12% for every clay because it is a round number
  • Fitting lids wet and skipping the fired-to-wet conversion
  • Mixing bone-dry shrinkage with total fired shrinkage
  • Updating the calculator custom percent but never saving it on the clay
  • Planning inches, throwing centimeters, and blaming the clay

Takeaway

Pick the clay, pick the direction, enter the size that you know. Let Claybook give you the size you should throw or the size you should expect. Then log the real fired measurement on the piece so the next pot is aimed, not hoped.

The web calculator is here if you want to try it now: clay shrinkage calculator.