Pipe Schedule, Outside Diameter, And Inside Diameter

Use pipe schedule data before any velocity or pressure-loss calculation that starts from a nominal pipe designation instead of a measured inside diameter.

What This Means

Nominal pipe size, or NPS, is a standardized designation. It is not generally the physical inside diameter and should not be entered directly into a flow equation.

For a given NPS, the applicable dimensional standard establishes an outside diameter. Schedule identifies a wall-thickness series. Increasing wall thickness while keeping the same outside diameter reduces the inside diameter, increases velocity at a fixed flow rate, and can increase pressure loss substantially.

Material and product standard matter. ASME B36.10 covers welded and seamless wrought steel pipe, while ASME B36.19 covers welded and seamless wrought stainless steel pipe. A schedule label without the governing standard and material is incomplete dimensional information.

Key Relationships

D_i = D_o - 2 t
A = pi D_i^2 / 4
V = Q / A

• D_i is calculated inside diameter.
• D_o is standardized or measured outside diameter.
• t is the wall thickness used for the hydraulic model.
• A is flow area.
• Q is volumetric flow rate and V is average velocity.

The nominal designation selects a standards row; it does not replace D_i in these relationships.

Use This When

• Converting an NPS and schedule callout into a diameter for a pipe-flow calculator.
• Comparing Schedule 40 and Schedule 80 velocity or pressure-loss effects.
• Reviewing a model that appears to use nominal diameter as hydraulic diameter.
• Checking whether a stainless schedule designation comes from the correct dimensional standard.
• Reconciling drawing, purchase specification, and field-measured dimensions.

Assumptions

• The pipe conforms to the identified material and dimensional standard.
• Outside diameter and wall thickness are taken from the same standard row and edition.
• The hydraulic model uses a defensible inside diameter for the installed condition.
• Linings, coatings, deposits, corrosion, and manufacturing tolerances are handled when material to the decision.

Limitations

• The simple subtraction gives a nominal geometric inside diameter; it is not a minimum guaranteed bore.
• Design pressure, allowable stress, corrosion allowance, mill tolerance, threading, erosion, and code rules require the governing piping or pressure code.
• Tube sizing, duct, hose, plastic pipe, cast pipe, and proprietary products may use different dimensional systems.
• A schedule number is not a pressure rating and does not by itself establish suitability for temperature, material, joining method, or service.
• Use the current ASME standard or verified manufacturer data for final pipe dimensions.

Common Mistakes

• Entering 2 in as the actual inside diameter of NPS 2 pipe.
• Assuming the same schedule has the same wall thickness for every NPS.
• Mixing an outside diameter from one product standard with a wall thickness from another.
• Treating Schedule 40S and Schedule 40 as universally interchangeable.
• Ignoring reduced bore from lining, scale, or corrosion products in an existing system.
• Using schedule alone as proof of pressure-code compliance.

Related Calculators

• Pipe velocity from flow rate
• Pipe Reynolds number
• Darcy-Weisbach pressure loss

Sources

ASME B36.10 and B36.19 are the dimensional authorities for the steel and stainless-steel pipe scopes described here. Crane TP-410 corroborates the practical use of actual inside diameter in pipe-flow calculations.

1. American Society of Mechanical Engineers. Welded and Seamless Wrought Steel Pipe (ASME B36.10), 2022, ASME, 2022. ASME B36.10-2022. Source page.
2. American Society of Mechanical Engineers. Welded and Seamless Wrought Stainless Steel Pipe (ASME B36.19), 2022, ASME, 2022. ASME B36.19-2022. Source page.
3. Crane Co.. Flow of Fluids Through Valves, Fittings, and Pipe (Crane TP-410), Technical Paper No. 410, Crane Co., 2009. TP-410.