OD AND NOMINAL PIPE SIZE

Tubing is measured by the OUTSIDE DIAMETER (O.D.), specified in inches (e.g., 1.250) or fraction of an inch (eg. 1-1/4″).

Pipe is usually measured by NOMINAL PIPE SIZE (NPS). Although it is related to the outside diameter, it is significantly different. For example, a 1-1/2″ pipe size actually has an outside diameter of 1.9″ NOT 1.5″. The table below cross-references the nominal dimension of a PIPE with its actual outside diameter.

For sizes from 1/8-inch through 12-inch, the NPS corresponds to the nominal inside diameter, but differs somewhat from its actual inside diameter. Early pipe manufacturers made the walls in the smaller sizes much too thick, and in correcting this error in design, they took the excess from the inside to avoid changing the sizes of the companion fittings.

For sizes larger than 12-inch, NPS corresponds to the actual outside diameter. For each nominal pipe size, the outside diameter (O.D.) remains relatively constant; the variations in wall thickness affect only the inside diameter (I.D.).

The chart on the right, shows the relationship between NPS and the diameter.

WALL THICKNESS AND PIPE SCHEDULE

The wall thickness of tubing is measured in inches (.0035″), or by standard gauge from 7 (heaviest) to 22 (lightest) which correspond to a range of wall thicknesses. You can see by the chart at the right, not all 12 Gauge allows a thickness between 0.101 to 0.111. The typical value, which is not half way between, is used for calculation of section properties.

Once again, pipe wall thickness is a bit more complicated. Three long standing traditional designations are used to indicate wall thickness:
Standard wall (STD)
Extra Strong wall (XS), sometimes referred to as Extra Heavy wall (XH)
Double Extra Strong wall (XXS), sometimes referred to as Double Extra Heavy wall (XXH)

For the purpose of standardizing pipe dimensions, the American National Standards Institute (ANSI) published ANSI B36.10. That standard broadened the range of wall thicknesses, schedule numbers from schedule 10 (SCH10) through schedule 160 (SCH160). These schedule numbers indicate approximate values for 1000 times pressure-stress ratios.

Stainless steel schedule numbers from Schedule 5S through schedule 80S were added by ANSI B36.19 for sizes through 12-inch. The addition of the letter “S” after the schedule number identified it as pertaining to stainless steel.

Certain relationships exist among the traditional designations of STD, XS, and XXS, the ANSI schedule numbers, and the actual wall thicknesses. STD and SCH 40 are the same in sizes through 10-inch; in sizes above 10-inch, STD has a wall of 3/8-inch. XS has a wall thickness of 1/2-inch. XXS has no corresponding schedule number, but in sizes through 6-inch, XXS has a wall thickness twice as thick as XS. Stainless steel schedules 40S and 80S are identical with carbon steel designations Std and XS, respectively, through 12-inch.

References to pipe wall thickness typically mean the nominal, or average, wall thickness. Most ASTM tolerances on regular pipe products specify that wall thickness of pipe at any point is to be no more than 12 1/2% under the nominal wall thickness specified. This means that a minimum wall of 0.875 times the nominal wall could apply for regular mill rolled steel.

The following chart shows the relationship of the various NPS sizes and Schedules and the actual Outside Diameter and Wall Thickness.

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