The length of a wraparound is always based on the o.d., which is pi x the outside diameter of the pipe. The nominal sizes of standard pipe (A53) are i.d.'s, not o.d.'s. That is not the case with round HSS (A500). A round HSS 3 is 3" o.d., and a round HSS 4 is 4" o.d., so, if HSS round are being used, the o.d. must be entered.