The ASTM calculation assumes that the tube is manufactured with a consistent wall thickness. The actual wall is always thicker than the specified minimum. The producing mills and R- S Matco, Inc. use the ASTM A-450 formula, but multiply the thickness by 1.10 for tubes with an OD less than 1-1/2” and 1.11 for 1-1/2” OD and larger and use that number for the wall thickness in the formula. Even though the formula is an accepted industry standard, it has been our experience that the mills will produce a cold drawn tube that is around 6-8% above the specified minimum. This is for informational purposes only and should not be used for the purpose of engineered standards.