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. Divide the amount of shrinkage by the original size to find the shrinkage rate. Multiply the shrinkage rate by 100 to find the shrinkage as a percentage. Labthink Shrinkage Testers can help customer measure shrinkage force, contracting force and shrinkage ratio of plastic materials.