Most pipes have standard sizes, regardless of their material. Those standard sizes are indicated with the nominal pipe diameter (or bore) and the pipe schedule. These two numbers set the outside pipe diameter (OD) and the wall thickness. With this information, the internal diameter (ID) can be calculated.
Nominal pipe diameter
There are two general methods to note the outside diameter of pipes:
- The North American method, NPS (Nominal Pipe Size/National Pipe Thread Straight) based on imperial units, inches.
- The European method, DN (Diametre Nominal/Nominal Diameter) based on metric units, millimeters.
- (A third term often used, is the NB (Nominal Bore), this is a European designation corresponding with NPS.)
Be aware that those ways of noting the nominal pipe diameter, are not always the same as the outside diameter. With this NPS, DN or NB it is possible to exact outside diameter can be acquired from reference tables, like the one here.
As the nominal pipe diameter sets the outside diameter, the pipe schedule sets the wall thickness of the pipe. A higher schedule stands for a thicker wall. A nominal pipe diameter has a fixed outside diameter, but the inside diameter can still vary. This fixed outside diameter makes it possible to connect pipes with different schedules/wall thicknesses. When knowing the schedule, the wall thickness can be obtained from tables.
Measuring the outside diameter
Is the nominal pipe diameter unknown, it is possible to measure it. With a flexible measuring tape, it easy to read out the outside diameter by wrapping the measuring tape around the pipe. Is there no flexible measuring tape within reach? It is also possible to use a string instead. Wrap the string around the pipe, mark the point where the string touches each other, and measure the length from the mark you made till the tip of the string. With the measured outside diameter, it is possible to find in the tables the Nominal pipe diameter when needed.
What to do, if the internal diameter is unknown and it is not given in any table? No problem, it is still possible to calculate it. To get the internal diameter (ID), you need to know the outside diameter (OD) and from this, subtract the wall thickness twice:
ID = OD – (2 x the wall thickness)
Be aware that the wall thickness can deviate. The tolerance on a typical wall thickness is 12,5%, so the actual internal pipe diameter can be slightly different. When you do not know the wall thickness, but you know the schedule of the pipe, you can simply look for the wall thickness in standard tables. The table below gives the internal diameter in inches and millimeters belonging to those schedules.
If only the outside diameter is known and there is no way to get other measurements of the pipe, there is another way to obtain the internal diameter. It is possible to measure the wall thickness with an ultrasonic wall thickness gauge. Ultrasonic thickness measurement (UTM) works with a sound pulse, produced by an ultrasonic transducer. It measures exactly how long it takes for the sound pulse to travel through the pipe and back. From this measurement the device can calculate the wall thickness of the pipe. Once knowing the wall thickness and the outside diameter, the inside diameter can be calculated as described above.