Here are a few ways to measure the diameter of steel pipe:

1.  Direct Diameter measurement is the easiest.  Measure at the end of the pipe.

2.  Direct Diameter measurement with a caliper (useful if the end is not accessible)

3.  Measure circumference using a flexible tape and divide the outside circumference by 3.1415.  Here the circumference is about 1/32″ (0.03″) less than 6″ = 5.97″.  So 5.97 ÷ 3.1415 = 1.900″ diameter – 1 1/2″ nominal pipe.

Many people like using pi tapes to measure the pipe size.  Pi tapes, which wrap around the pipe the same way but have units in diameter, can be expensive.  With calculators everywhere today, dividing by 3.14 is a snap, making these pi tapes not even necessary.  In fact, even using an inexpensive sewing tape works fine!

Round hollow sections can be a true round tube where 2″ is really 2.00″ outside diameter.  This is called “2 inch O.D. tubing”.  And if you are really lucky enough, the tubing is already marked and you don’t need to even measure.