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Nominal Pipe Size (NPS) Converter

1/2 inch NPS Schedule 40 = OD 0.840" (21.34 mm), Wall 0.109" (2.77 mm)

Inside diameter (ID) = 0.622" (15.80 mm).

Convert Nominal Pipe Sizes (NPS) or DN equivalents to actual outside/inside diameters and wall thickness schedules. Essential for plumbing, HVAC, and industrial piping design.

Interactive Pipe Dimension Estimator

Outside Dia (OD) 0.405" (10.29 mm)
Inside Dia (ID) 0.269" (6.83 mm)
Wall Thickness 0.068" (1.73 mm)

Nominal Pipe Sizes Reference Matrix

NPS Actual OD (in / mm) Sch 40 Wall (in / mm) Sch 80 Wall (in / mm)
1/8" 0.405" / 10.29 mm 0.068" / 1.73 mm 0.095" / 2.41 mm
1/4" 0.54" / 13.72 mm 0.088" / 2.24 mm 0.119" / 3.02 mm
3/8" 0.675" / 17.15 mm 0.091" / 2.31 mm 0.126" / 3.2 mm
1/2" 0.84" / 21.34 mm 0.109" / 2.77 mm 0.147" / 3.73 mm
3/4" 1.05" / 26.67 mm 0.113" / 2.87 mm 0.154" / 3.91 mm
1" 1.315" / 33.4 mm 0.133" / 3.38 mm 0.179" / 4.55 mm
1-1/4" 1.66" / 42.16 mm 0.14" / 3.56 mm 0.191" / 4.85 mm
1-1/2" 1.9" / 48.26 mm 0.145" / 3.68 mm 0.2" / 5.08 mm
2" 2.375" / 60.33 mm 0.154" / 3.91 mm 0.218" / 5.54 mm
2-1/2" 2.875" / 73.02 mm 0.203" / 5.16 mm 0.276" / 7.01 mm
3" 3.5" / 88.9 mm 0.216" / 5.49 mm 0.3" / 7.62 mm
4" 4.5" / 114.3 mm 0.237" / 6.02 mm 0.337" / 8.56 mm

Why NPS Doesn't Match Actual Dimensions

Nominal Pipe Size (NPS) is a historical trade designation that evolved from early iron pipe standards. The original NPS number roughly corresponded to the inside diameter of standard-weight pipe, but as schedules (wall thicknesses) were introduced, the NPS number became purely nominal.

NPS 1/2: OD = 0.840" (not 0.500")
NPS 1: OD = 1.315" (not 1.000")
NPS 2: OD = 2.375" (not 2.000")
Key Rule: Always use the OD from the pipe dimension table, never assume NPS = actual diameter

Pipe Schedule and Pressure Ratings

The pipe schedule system (Sch 5, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160, STD, XS, XXS) defines wall thickness. Schedule 40 is the most common for plumbing and general-purpose applications. Schedule 80 provides extra wall thickness for higher-pressure or corrosive service.

Pressure rating depends on pipe material, diameter, wall thickness, and temperature. For carbon steel Sch 40 pipe, typical working pressure ranges from 150 psi (1/2") to 280 psi (4") at ambient temperature.

Frequently Asked Questions — Pipe Size Conversion

What is Nominal Pipe Size (NPS)?
Nominal Pipe Size (NPS) is a US standard for pipe dimensions that defines pipe diameter by a dimensionless number. It does NOT directly correspond to actual inside or outside diameter. For example, a 1-inch NPS pipe has an actual outside diameter of 1.315 inches (33.4mm), not 1 inch.
What is the difference between Schedule 40 and Schedule 80 pipe?
Schedule numbers indicate wall thickness. Schedule 40 is standard wall and Schedule 80 is extra strong (thicker wall). A 1-inch Sch 40 pipe has 0.133" wall thickness, while Sch 80 is 0.179". Thicker walls mean smaller inside diameter but higher pressure rating.
How do you calculate pipe inside diameter?
Inside Diameter (ID) = Outside Diameter (OD) − 2 × Wall Thickness. For example, a 2-inch NPS Sch 40 pipe: OD = 2.375", wall = 0.154", so ID = 2.375 − 2(0.154) = 2.067".
What does DN mean in pipe sizes?
DN (Diameter Nominal) is the metric equivalent of NPS, used in European and international standards. DN25 corresponds to NPS 1, DN50 to NPS 2, etc. DN numbers are roughly the actual OD in millimeters rounded to the nearest 25mm increment.
How do I select the right pipe schedule for my application?
Select based on operating pressure, temperature, and fluid type. Sch 40 handles most residential and light commercial applications. Sch 80 is used for higher pressure systems, chemical processing, and structural applications. For critical applications, calculate the required wall thickness using ASME B31.3 formulas.

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