There are several standards for pipe sizing worldwide. In the United States, NPS (Nominal Pipe Sizing) is the most common method of sizing pipes. The size carries two values: SCH (Schedule, or wall thickness) and NPS (roughly related to the actual diameter of the pipe). The best method for calculating the pipe size is to measure the circumference and the wall thickness, then compare the values to a standardised chart.
Wrap a piece of string around the outside of the pipe. For the most accurate measurement, press the string up against a fixture on the pipe or the very edge of the pipe.
Mark the string where it crosses itself.
Measure the length of the string between the marks in inches. Write down this value.
Divide the circumference of the pipe by pi, or 3.14. This is the OD (outer diameter).
Measure the thickness of the pipe. With the edge of the pipe exposed, place the ruler against the pipe and measure the width of the wall in inches.
Compare the values with a standardised chart (see Resources). Find the OD from your measurement under OD on the chart. The number labelling the row for the OD is the NPS. Go across the row until you find the width of the pipe wall. The number labelling the column for the pipe wall width is the SCH. The values may not match exactly so use the closest match.