To understand this concept you have to learn the way pipes are manufactured.
Copper pipe sizes in inches and mm.
In plumbing pipe size is referred to as nominal pipe size nps or nominal pipe size.
Nominal pipe size nps is the number that defines the size of the pipe.
Wall thickness of pipes available in different standard schedules sch for nps nominal pipe size in inch as well as dn diameter nominal in mm are shown in below chart.
Nominal refers to pipe in non specific terms and identifies the diameter of the hole with a non dimensional number for example 2 inch nominal steel pipe consists of many varieties of steel pipe with the only criterion being a 2 375 inch 60 3 mm outside diameter.
Pipes are identified by nominal or trade names that are loosely related to the actual dimensions.
A nominal 3 4 in copper pipe in australia has measured diameters of 0 750 in outside and 0 638 in inside whereas a nominal 3 4.
Metric rules of thumb 1 mm about 1 25 inch thickness of a dime 25 mm about 1 inch 1 25 4 mm 300 mm about 1 foot 12 304 8 mm 1 m roughly 10 sq.
For instance a 2 inch galvanized steel pipe has an inside diameter of about 2 1 8 inches and an outside diameter of about 2 5 8 inches.
1 m 10 76 sq.
For example when you say 6 pipe the 6 is the nominal size of that pipe.
While pipe sizes in australia are inch based they are classified by outside rather than inside diameter e g.
For example dn20 is the size for copper pipe with an outside diameter of 19 05 mm or 3 4 inch.
Nominal pipe size nps is a north american set of standard sizes for pipes used for high or low pressures and temperatures.
The wall thickness unit shown in the table without bracket is in metric system i e millimetres mm the british equivalents dimensional conversion i e inch is shown in bracket.