The handy Shrink Ruler from DiamondCore® Tools is a one-of-a-kind measuring device for potters to measure clay shrinkage. This flexible, two-sided plastic ruler has four clay shrinkage percentages to help with projects when an exact match is crucial, such as lids for a teapots, cookie jars, etc.

The photos show both sides of the same ruler. One side shows 6% and 10%, the other side shows 12% and 14%.

Using your Shrink Ruler is easy! Here’s a step-by-step guide example:

1. Measure the dimension of the finished piece you want using a standard inch ruler.
2. Find that measurement on the Shrink Ruler on the scale corresponding to the shrinkage of your clay.
3. Mark or form the wet clay piece to that dimension.
4. When the piece is done firing, it will have shrunk to the correct size.

To use the Shrink Ruler, knowing the shrinkage rate of your clay is important. Generally, the clay supplier can provide this, or you can calculate it using the formula below.

1. Make a rectangular clay slab that’s 4” (101.6 mm) X 12” (304.8 mm) and as thick as you plan to make your piece.
2. With a regular ruler, measure and carve a 10” (254 mm) line in the wet clay.
3. Let the clay dry, then bisque fire the clay slab.
4. Fire the slab to final firing conditions.
5. Measure the length of the same line again with the regular ruler.
6. Use the below formula to solve for S.