Shrinkage occurs when a material, such as felt or cotton, gets smaller when it is washed for the first time. Before starting on a big project with a material that shrinks, make a test object to determine the projected amount of shrinkage, so that you do not end up with the material that does not fit correctly after washing. Calculate the shrinkage percentage after determining the original size and the end size.

Subtract the final size from the original size to find the amount of the shrinkage. For example, if a felt square shrinks from 8 square inches to 6 square inches, subtract 6 from 8, resulting in 2 square inches of shrinkage.

Divide the amount of shrinkage by the original size to find the shrinkage rate. In the example, divide 2 by 8 to get 0.25.

Multiply the shrinkage rate by 100 to find the shrinkage as a percentage. In the example, multiply 0.25 by 100 to get 25 percent.