The first thing the designer will do is calculate the required wall thickness. He/she will then compare this with the minimum wall thickness for a standard pipe schedule. If he/she can’t find a perfect match, which is often the case, he/she will select the next pipe schedule up; so the minimum wall thickness for the schedule is greater than the minimum wall thickness required by the design. Sometimes when the required wall thickness is very big or there isn’t a suitable schedule covering it, the designer will specify pipe as “minimum-wall”. Where this is the case, it is up to the manufacturer to decide on the nominal wall thickness so that the pipe wall thickness is guaranteed to be equal to or greater than the minimum wall thickness required by the designer.