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.