Clay Shrinkage Calculator

What is clay shrinkage?

Clay shrinkage is the reduction in size that happens as a piece loses moisture and then undergoes vitrification in the kiln. It occurs in two distinct stages: wet to bone dry as water evaporates from the clay body, and dry to fired as the silica and alumina fuse under heat. Most stoneware shrinks between 10 and 15 percent total from the plastic state to the finished fired piece, though the exact rate depends on the clay body, firing temperature, and how the piece was built.

How do you calculate clay shrinkage percentage?

Measure your piece wet, then again after firing. Subtract the fired measurement from the wet measurement, divide by the wet measurement, and multiply by 100. A test bar that starts at 120mm wet and comes out of the kiln at 105mm has a total shrinkage of (120 - 105) divided by 120, times 100 - which is 12.5 percent. The Calculate Shrinkage tab above handles this automatically; entering a bone dry measurement as well splits the result into wet-to-dry and dry-to-fired stages.

How do I plan a build size from a target fired dimension?

Once you know your clay body's shrinkage rate, divide the target fired size by one minus the shrinkage expressed as a decimal. For a target of 100mm with a 12 percent shrinkage rate: 100 divided by 0.88 equals approximately 113.6mm wet. The Plan Build Size tab does this in one step. The most reliable approach is to fire a dedicated shrinkage bar - a 100mm strip of fresh clay - alongside your work each time you use a new clay batch, so the rate stays accurate across different bodies and temperatures.