Thumb Rule to Identify the schedule number of pipe by direct measurement
Add three inches to the actual inside diameter; divide this
by the wall thickness; the schedule number can then be identified
by the following table:
Schedule 30 40 to 50
Schedule 40 29 to 39
Schedule 60 25 to 29
Schedule 80 20 to 23
Schedule 100 16 to 18
Schedule 120 13 to 15
Schedule 140 11 to 13
Schedule 160 9 to 11
Example. Pipe measures 12.5 inches inside diameter, wall
thickness .75 inch; 15.5/.75 = 20.2; pipe is Schedule 80, 14
inches outside diameter.
Note:
The rule fails on small diameter pipe (under 6 inches), and
also on schedules 10 and 20, because of the use of “standard”
thicknesses in this range.