How to use the clay shrinkage calculator
- Choose whether to find the wet size for a finished size, or the finished size from a wet one.
- Enter the shrinkage as one total percent, or switch to drying and firing and enter the two stages.
- Enter the size, or add several sizes such as height and width, in any unit.
- Read the scale factor and the size for each dimension.
Formula
wet size = finished size / (1 - total shrinkage / 100)
finished size = wet size x (1 - total shrinkage / 100)
total shrinkage = 1 - (1 - drying) x (1 - firing)
Shrinkage is stated as a percent of the wet size. Dividing by (1 - shrinkage) is what makes the piece start large enough. Subtracting the percent from the finished size instead would leave the piece too small.
A finished 8 inch piece at 12 percent
With total shrinkage of 12 percent, a piece keeps 88 percent of its wet size. To finish at 8 inches it must start at 8 / 0.88 = 9.09 inches. Going the other way, a 8 inch wet piece fires to 8 x 0.88 = 7.04 inches.
When drying and firing are stated separately, the two stages multiply. Drying shrinkage of 5 percent and firing shrinkage of 7 percent leave 0.95 x 0.93 = 0.8835 of the wet size, a total of 11.65 percent, not 12. An 8 inch finished piece then starts at 9.05 inches.
Where the percent comes from
Clay shrinks as water leaves it and again as it is fired and fuses. The percentage depends on the clay body, how wet it is, how it was made, and the temperature of the firing. The 12 percent shown is only an example. Some clays shrink more and some less.
The best figure is your own. Roll a tile of your clay, mark a line 100 mm long, and measure the line after drying and after firing. The total shrinkage is one minus the finished length divided by the wet length. Test again when you change clay.
Units and lists of sizes
The math is a ratio, so you can use inches, centimeters or millimeters. Add a row for each dimension you need, such as height, width and lid diameter. For other craft planning see the Candle Wax Calculator and the Percent Change Calculator.
Assumptions and limits
- The same shrinkage applies in every direction.
- The percent is a share of the wet size, not of the finished size.
- Warping, glaze thickness and uneven shrinkage are not modeled.
- Total shrinkage is limited to 50 percent.
Frequently asked questions
How do I calculate clay shrinkage?
Measure a wet tile, then measure it after firing. Shrinkage is the difference divided by the wet length. A 100 mm line that finishes at 88 mm means 12 percent.
How big should I make a piece so it fires to 8 inches?
Divide by one minus the shrinkage. At 12 percent that is 8 divided by 0.88, which is 9.09 inches.
Do drying and firing shrinkage add up?
Not exactly. They multiply, because the second percent applies to a smaller piece. 5 percent and 7 percent give 11.65 percent, not 12.
Why not just add the percent to the finished size?
Because shrinkage is a share of the wet size. Adding 12 percent to 8 inches gives 8.96, which fires to 7.88 inches, not 8.