How to Measure Pipe Size: Outside Diameter and NPS Chart

To measure pipe size, measure the outside diameter (OD) of a clean, unthreaded section and compare it with the pipe-size chart below. This guide uses NPS steel pipe dimensions. If calipers cannot span the pipe, measure its circumference and divide by π. Identify any threaded connection separately.

For the sizes covered here, the nominal size differs from the measured OD. NPS 1/2 steel pipe, for example, has a reference OD of 0.840 inch. Find the matching nominal size in the chart and use that designation when ordering.

Pipe Size Terms in Plain English

• NPS (Nominal Pipe Size): a size designation used for pipe, such as NPS 1/2 or NPS 2. The applicable dimensional standard defines its OD.

• DN (nominal size): a size designation defined in ISO 6708. The number after DN is not a measured diameter in millimeters. For the steel pipe series in this guide, NPS 1/2 corresponds to DN 15.

• OD (outside diameter): the distance across the outside of the pipe.

• ID (inside diameter): the bore diameter, which depends on OD and wall thickness.

• Wall thickness (t): the thickness of one pipe wall.

• Schedule (Sch): a wall-thickness designation such as Sch 40 or Sch 80. Read it together with NPS and the pipe standard to determine the nominal wall.

• TPI (threads per inch): the number of thread-pitch intervals per inch of axial length.

• NPT (National Pipe Taper): a tapered pipe thread specified in ASME B1.20.1.

First Identify What You Are Measuring

A pipe body, a thread, an opening and a fitting body all give different numbers. Decide which feature you are measuring before you pick up a tool.

This guide covers round steel pipe sized by NPS. Copper tube, PEX and metric tubing follow different dimensional rules, covered in the section on mismatched readings below.

Method 1: Measure the Outside Diameter with Calipers

1. Pick an accessible section. Use a straight, round, unthreaded stretch of pipe. Follow the site’s isolation and access procedures, and never loosen a live joint to get a measurement.

2. Prepare the surface. Clear loose rust and scale. Keep the jaws away from weld beads, fittings and insulation. Leave required protective coatings in place and note them.

3. Measure across the pipe. Zero the calipers, keep the jaws square to the pipe axis, and contact opposite sides without excessive force.

4. Repeat at 90°. A second reading helps reveal obvious ovality or inconsistent contact. If readings differ, repeat at nearby positions and investigate the cause.

5. Compare with the correct chart. Use the material and dimensional standard to select a size candidate, then confirm it against markings or records. Investigate a poor match rather than choosing a row solely because it is nearest.

On larger pipe, standard calipers may not open wide enough. Use a large outside caliper, a diameter tape, or the circumference method below.

Method 2: Calculate the OD from the Circumference

Wrap a flexible tape around a round section, square to the pipe axis. Keep one flat turn against the surface and read where it meets the starting mark. Check whether your tool reads circumference or diameter, because a diameter tape may already display the OD.

OD = circumference ÷ π

Example: an outside circumference of 83.8 mm gives 83.8 ÷ π ≈ 26.67 mm. For the NPS steel pipe series below, that is consistent with NPS 3/4. An approximate reading of 3.30 inches gives the same size candidate.

Without a flexible tape, wrap a strip of paper around the pipe, mark the overlap, then flatten it and measure it with a ruler. A non-stretch string gives a rough reading. Coating thickness and ovality can shift the reading away from the reference circumference.

NPS Pipe Size Chart: OD and Circumference

Reference inch ODs follow the steel pipe dimensions in ASME B36.10. ASME B36.19 stainless steel pipe uses the same ODs for these listed sizes. Millimeter ODs are inch conversions rounded to two decimal places; they are not separate inspection limits. Circumferences are calculated from the unrounded inch OD using π and rounded for reference. Use the applicable product standard for manufacturing tolerances.

What If Your Reading Does Not Match the Chart?

If the reading does not fit a row, recheck tool zero, jaw alignment and the surface condition. Compare several positions if the pipe looks oval or damaged. Then look for a material or standard marking: a different pipe or tube series may be the explanation.

ASTM B88 copper water tube has an OD 1/8 inch larger than its nominal size: nominal 1/2-inch tube has a 0.625-inch OD, and nominal 3/4-inch tube has a 0.875-inch OD.

PEX sold in copper tube size (CTS) uses corresponding copper-tube ODs. PEX is also made in other size series, and many metric tubing products are specified by actual OD in millimeters. Confirm the markings and product standard before applying these conventions.

Outside Diameter, Inside Diameter and Wall Thickness

For a round section with uniform wall thickness, ID = OD − 2t, where t is one wall thickness. Use the same units throughout.

Take NPS 1/2 steel pipe with a 0.840 in. OD. At a nominal Schedule 40 wall of 0.109 in., the ID is 0.840 − 2 × 0.109 = 0.622 in. At a nominal Schedule 80 wall of 0.147 in., the ID is 0.546 in. The OD is the same, but the bore is smaller.

The table uses nominal steel pipe ODs and wall thicknesses. Each ID is calculated as OD − 2t; manufacturing tolerances, corrosion and deposits affect the actual bore. ASME B36.19 uses stainless steel wall designations such as 40S and 80S. Check the specified standard rather than assuming all schedule designations are equivalent.

To find the actual remaining wall of installed pipe, an OD measurement is not enough. Use a wall-thickness method such as an ultrasonic thickness gauge, following the inspection procedure for the service.

How to Measure a Threaded Pipe Connection

Measure the pipe size and identify the thread as two separate jobs. Record whether the end is male or female, then work through these steps:

1. Measure across the crests of a male thread. Use calipers on undamaged, fully formed crests and note where along the thread you measured. Use a thread-identification reference to interpret the reading; the plain-pipe OD column alone cannot identify the connection.

2. Check for taper. Measure across fully formed crests at two axial positions. A consistent diameter change indicates taper. Combine this observation with pitch, thread form and markings to identify the standard.

3. Measure the pitch. Use a thread pitch gauge where possible. Without one, measure several complete crest-to-crest intervals with a ruler and divide the length by the number of intervals. TPI = 1 ÷ pitch when pitch is in inches.

4. Identify the standard. Compare the measurements and markings with the equipment drawing or a suitable thread-identification reference. NPT has a 60° included thread angle; BSP pipe threads have a 55° form. Close or unclear matches may require suitable gauges or specialist inspection.

At nominal sizes 1/2 and 3/4, NPT and BSP both use 14 TPI, so pitch alone cannot distinguish them. ISO 7-1 covers pressure-tight thread joints with tapered external threads and parallel or tapered internal threads. ISO 228-1 covers parallel G threads whose pressure seal is made outside the threads. Identify the complete connection before selecting its sealing method.

A female thread is harder to read. Check the pitch with a gauge, look for markings, and consult the equipment drawing. A known mating part can provide supporting information, but hand engagement alone does not prove compatibility. Recessed or unclear threads may need suitable gauges or specialist inspection.

For the full size and pitch reference, see our NPT thread chart. Treat field measurements as identification evidence, not acceptance gauging.

Check More Than Size Before Ordering

A matching nominal size is only one requirement. The replacement must also have compatible connection ends, material, wall or class, and pressure-temperature capability.

For a replacement order, record:

• Measured pipe OD, measurement location and tool used.

• Nominal size and the dimensional standard, if known.

• Connection type and the specification of each end.

• Wall designation or measured wall, where required.

• Material, service fluid, operating pressure and temperature.

• Required overall length, center-to-end dimensions and available clearance.

Use the pipe fitting types guide to name the shape you need. Then compare specifications in the pipe fittings catalog. The outside measurement of a fitting body cannot replace the dimensions of its connection.

FAQ

Can a phone photograph determine pipe size?

A photograph helps identify markings and fitting shape. For dimensional identification, include the measured OD and show the ruler or caliper in the same plane as the measured feature. A perspective view alone gives an unreliable scale.

What if only a short section of pipe is exposed?

Use a tool whose contact surfaces can reach the pipe body squarely. If nearby fittings prevent correct contact, consult the drawing or installation record, or arrange suitable access. Measuring the fitting shoulder or hex will not recover the pipe-body OD.

Can I identify the original size from a badly corroded end?

Use an intact section, surviving markings or original records to support identification. A damaged end may have lost metal or accumulated deposits. Identifying the original size and assessing remaining fitness for service are separate tasks.

Technical Reference

ASME B36.10 — Welded and Seamless Wrought Steel Pipe provides the dimensional reference for the steel pipe examples. Other references used to distinguish dimensional and connection systems are listed below.

• ASME B36.19: dimensions of welded and seamless wrought stainless steel pipe.

• ASME B1.20.1: general-purpose inch pipe threads, including NPT.

• ISO 6708: definition and selection of DN.

• ISO 7-1: pipe threads for joints made pressure-tight on the threads.

• ISO 228-1: pipe threads for joints whose pressure seal is made outside the threads.

• ASTM B88: seamless copper water tube.

If your measurements do not point to a clear match, contact us with the readings, photographs, markings and service details.