Buyer reference tool

Steel Pipe Size and Schedule Selection Reference

Nominal pipe size and schedule are naming systems, while outside diameter and wall thickness are physical dimensions. A buyer should move between those terms without treating the table as a pressure calculation. Use the reference to identify a dimensional starting point, then confirm the exact standard table, tolerance, grade, service, and engineering basis.

SSR reference

Start with the published comparison

Illustrative NPS dimensional reference
NPS	Outside diameter	Schedule examples	RFQ note
2	60.3 mm	40, 80 and other standard-listed walls	State actual wall and standard instead of schedule alone
4	114.3 mm	40, 80 and other standard-listed walls	Confirm OD tolerance, wall tolerance, ends, and process
6	168.3 mm	40, 80 and other standard-listed walls	Confirm product standard and dimensional table edition
12	323.9 mm	Multiple listed schedules and project-specific walls	Large diameter orders often use explicit OD and wall values

RFQ selector

Record the selection inputs

Nominal pipe size

Use the nominal designation from the applicable dimensional standard.

Outside diameter (mm)

Confirm the physical OD and tolerance in the order standard.

Wall thickness (mm)

Use actual nominal wall for the RFQ even when schedule is also stated.

Pipe length (m)

State random, fixed, or double-random rules and tolerance.

Selection logic

1.Choose the dimensional standard and nominal size, then read the matching outside diameter from that standard.
2.Select a schedule or explicit nominal wall that exists for the size and product route, and transfer the actual wall value into the RFQ.
3.Estimate inside diameter only as OD minus twice the nominal wall. Final bore and mass vary with tolerances, manufacturing, and product requirements.
4.Add grade, process, ends, length, tolerance, testing, coating, service, and design review before treating the line item as order-ready.

Limits

The reference does not provide pressure rating, allowable stress, corrosion allowance, thread engagement, collapse, flow, or code compliance.
Listed examples are not a complete dimensional standard. The current governing table and product specification control the order.

Error recovery

If NPS and OD do not match the same standard row, stop and confirm whether the request uses pipe, tube, or a custom OD designation.
If schedule exists but actual wall is absent, add the wall from the governing table and identify the standard before quoting.
If calculated inside diameter is zero or negative, correct the units or dimensions and request engineering review.

Continue with application and product owners

Oil and gas line pipe selection

Place dimensions inside the service, process, inspection, and coating specification.

Open owner

API 5L carbon steel pipe

Review the registered line pipe product owner.

Open owner

ERW, LSAW and SSAW welded pipe

Review welded pipe manufacturing routes and order fields.

Open owner

RFQ requirements

Use the selector with these remaining commercial and technical fields.

01
Dimensional standard, NPS and outside diameter
02
Schedule and explicit nominal wall thickness
03
Grade, product standard, manufacturing process, and service
04
Length, ends, dimensional tolerance, test, coating, and marking
05
Quantity, destination, handling, certificate, and schedule

Frequently asked questions

Is nominal pipe size the same as outside diameter?

Usually no. NPS is a nominal designation. Each listed NPS maps to an outside diameter in the governing dimensional standard, and several wall schedules can share that OD.

Can schedule determine pressure capacity?

Schedule supplies a dimensional starting point. Pressure capacity also depends on material, temperature, tolerance, corrosion allowance, joint, design factor, code, and the engineering calculation.

Why should an RFQ state both schedule and wall thickness?

The explicit wall reduces ambiguity, helps check the intended table, and supports mass, tolerance, process, and availability review. The governing standard still controls.