Things You'll Need
Measure the pipe's inside diameter. If you have access to a cut end of the pipe, place a ruler or tape measure across the end and measure the distance between the pipe's inner walls.
Round down the actual inside diameter to the next smaller nominal size. This is necessary for pipe with an inside diameter of two inches or less. Typical nominal sizes in inches are 1/4, 3/8, 1/2, 3/4, 1, 1 1/4, 1 1/2, 2 and 2 1/2. For the smaller pipe, there may be a significant difference between your measurement and the nominal size. For example, a 3/8 nominal pipe measures almost half an inch.
For larger pipe, you may have to round up. For example, a 2 1/2-inch nominal pipe has an inside diameter of slightly less than 2
Measure the outside diameter of the pipe to estimate the inside diameter. This is necessary if you don't have access to a cut end and cannot measure the inside diameter directly. Find the circumference by wrapping a piece of paper around the pipe, marking where it overlaps, then removing the paper and measuring it's length. Divide the circumference measurement by 3.14 (or pi) to find the outside diameter.
Estimate the inside diameter by subtracting 1/4-inch from the outside diameter, if your pipe is standard Schedule 40 galvanized pipe. If you suspect your pipe is Schedule 80 extra strong pipe, subtract 3/8 inch. Schedule 120 double extra strong pipe is significantly heavier and if you suspect you have it, subtract 11/32 inch.
Use the estimated inside diameter to estimate the nominal pipe size, as in Step 2.