Steel Pipe Inside Diameter by Schedule

Values are calculated using ID = OD minus 2 times wall thickness from standardized ASME B36.10 outside diameter and wall thickness data. For a complete size range including NPS 10 through 24, see the Pipe Schedule Chart and Pipe Wall Thickness Chart.

Inside diameter controls or strongly influences internal flow area, fluid velocity, hydraulic capacity, pressure loss, and internal volume. For nominal geometric calculation, the relationship is:

Per ASME B36.10-2022, for NPS 12 (DN 300) and smaller, the actual outside diameter is numerically larger than the nominal pipe size.

ASME B36.10 distinguishes pipe from tube and confirms that NPS 12 and smaller do not have an OD numerically equal to the nominal size.

Increasing wall thickness reduces ID because more material occupies the same outside envelope. For NPS 2:

This relationship between schedule, wall thickness, and bore is a fundamental concept for this entire chart.

Full wall-thickness interpretation by size is covered in the Pipe Schedule Chart and Pipe Wall Thickness Chart.

For full Schedule 40 dimensional coverage including weight and larger sizes, see the Schedule 40 Pipe Chart.

For full Schedule 80 dimensional coverage, see the Schedule 80 Pipe Chart.

This comparison matters directly for plumbers, hydraulic calculations, pump sizing, and process piping, where the actual bore, not the nominal label, determines flow behavior.

For a complete wall-thickness reference by NPS and schedule, see the Pipe Wall Thickness Chart.

ASME B36.10-2022 covers dimensional standardization of welded and seamless wrought steel pipe for high or low temperature and pressure service. The standard provides standardized OD and nominal wall thickness by NPS and schedule; the reference ID shown throughout this chart is derived from those dimensions using ID = OD – 2t.

Common carbon steel pipe products may be manufactured to material specifications such as ASTM A53 or ASTM A106, or API-related piping products. The dimensional standard (B36.10) and the material specification (such as A53 or A106) are separate concepts: a pipe can follow the B36.10 dimensional envelope while its material requirements come from a different standard entirely. This chart addresses dimensions and bore, not material grade selection.

For pipe weight based on these same dimensions, see the Steel Pipe Weight Chart.

ASME B36.19-2022 standardizes dimensions for welded and seamless wrought stainless steel pipe and uses schedule designations with an “S” suffix, such as 5S, 10S, 40S, and 80S, to distinguish its schedule system from the carbon-steel B36.10 schedules.

Conversion: 1 in = 25.4 mm. This U.S.-focused chart keeps inches as the primary unit; metric values are provided for reference only.

For NPS 2, the roughly 6 percent ID reduction from Schedule 40 to Schedule 80 corresponds to about a 12 percent reduction in flow area, illustrating why area, not diameter alone, should guide hydraulic decisions.

Higher velocity generally increases friction losses. Calculations such as the Darcy-Weisbach or Hazen-Williams equations require an appropriate actual or effective internal diameter rather than the nominal pipe size, since using NPS in place of actual ID can meaningfully distort hydraulic results. Detailed hydraulic design calculations are outside the scope of this dimensional reference chart.

The reference ID calculated from ID = OD – 2t uses nominal dimensions. Actual manufactured ID can differ because OD carries permitted manufacturing tolerances, wall thickness carries its own tolerances, and corrosion, lining, or coating can further affect the physical bore.

Factors affecting actual bore versus the calculated reference value include OD tolerance, wall thickness under or over tolerance, ovality, the manufacturing process used, and any internal coating or lining. For critical clearance or flow applications, verify the governing product specification or manufacturer data rather than relying on the reference chart value alone.

Over time, effective ID can decrease due to corrosion products, mineral scale, or process deposits building up on the interior wall. This chart distinguishes nominal new-pipe ID from effective in-service bore; it does not attempt to quantify a universal corrosion allowance, since scaling and buildup rates vary widely by fluid, water chemistry, temperature, and service conditions.

Common internal linings include cement mortar, rubber, epoxy, and specialty process linings. Effective ID for lined pipe is smaller than the bare-pipe ID by the added lining thickness. Use manufacturer or project-specific data for lined pipe rather than the bare-pipe reference values in this chart.

PVC Schedule 40 and Schedule 80 pipe share the nominal size and IPS outside diameter convention with steel pipe at many sizes, but each has a controlled OD, a schedule-specific wall thickness, and consequently its own inside diameter. Manufacturers maintain their own dimensional catalogs for Schedule 40 and Schedule 80 PVC pressure pipe systems. Do not assume the steel B36.10 wall thickness or ID applies directly to a PVC product even where nominal sizes align.

For complete PVC-specific dimensional data, see the PVC Pipe Size Chart.

For the same nominal PVC size, Schedule 80 has a thicker wall and therefore a smaller bore than Schedule 40. Comprehensive PVC-specific numerical tables are maintained on the dedicated PVC Pipe Size Chart to avoid duplicating that page’s scope here.

Copper tube dimensional references publish separate tables for Type K, Type L, Type M, and DWV copper tube. For nominal 1/2-inch Type K copper tube, OD is approximately 0.625 in and wall thickness is approximately 0.049 in, giving an inside diameter of approximately 0.527 in. The same nominal copper size is not dimensionally equivalent to NPS steel pipe of the same nominal label.

For complete copper-specific dimensional data, see the Copper Pipe Size Chart.

For a nominal 1/2-inch copper tube, published dimensional references show inside diameter of approximately 0.528 in for Type K, 0.545 in for Type L, and 0.569 in for Type M, confirming the pattern above. Use current copper tube manufacturer or industry dimensional references for design work; see the Copper Pipe Size Chart for full tables.

Pipe uses NPS-style nominal sizing, where the nominal label does not directly equal either OD or ID. Tubing dimensions generally follow a different convention, where OD is directly specified rather than inferred from a nominal pipe size. Do not use a pipe-ID table for structural or mechanical tubing products unless the dimensional system actually matches; copper water tube and NPS steel pipe, for example, use entirely separate dimensional systems despite both being called “pipe” or “tube” in casual usage.

Verify pressure ratings for the selected schedule using the Pipe Pressure Rating Chart.