Pipe Size Chart (A ⇄ B, OD, Sch 40/80)

Understanding the pipe size chart — concept and principle

This chart converts a nominal pipe size between the metric "A" designation (millimetres) and the inch "B" designation, and shows the actual outside diameter and the wall thickness for Schedule 40 and Schedule 80 steel pipe. Use it when reading drawings, ordering pipe and fittings, or working out the inside diameter for a flow calculation.

How it is calculated

There is no formula to convert A to B; the sizes are names, so the chart looks them up in a table based on ASME B36.10M.

• A (mm) and B (inch) are two names for the same nominal size: 50A = 2 B.
• OD is the real outside diameter in mm. For a given nominal size it stays the same for every schedule, so fittings fit.
• Sch 40 / Sch 80 wall is the wall thickness in mm. A higher schedule means a thicker wall.

Inside diameter = OD − 2 × wall thickness.

The principle

Nominal sizes began as rough inside diameters of early iron pipe. When stronger pipe with thicker walls appeared, makers kept the outside diameter fixed so threads and fittings still matched, and made the wall thicker inward. As a result, the nominal size no longer equals either the inside or the outside diameter for sizes up to 12 inches. The schedule number is a wall thickness series, roughly related to how much pressure the pipe can hold.

Worked example

Default input: 50A.

1. 50A corresponds to 2 inches nominal (B).
2. The outside diameter is 60.3 mm, not 50 mm.
3. Sch 40 wall = 3.91 mm, so the inside diameter is 60.3 − 2 × 3.91 = 52.48 ≈ 52.5 mm.
4. Sch 80 wall = 5.54 mm, so the inside diameter is 60.3 − 2 × 5.54 = 49.22 mm.

How to read the result

Use OD to choose clamps, sleeves and insulation, and the inside diameter for flow velocity and pressure drop. Sch 80 pipe has the same OD but a smaller bore and more weight per metre. A useful steel-pipe weight estimate is (OD − t) × t × 0.02466 kg/m with dimensions in mm; for 50A Sch 40 that gives about 5.44 kg/m.

Common mistakes

• Treating 50A as a 50 mm outside or inside diameter.
• Assuming all pipe materials use these ODs. Copper tube, PVC and stainless tubing may follow other standards.
• Mixing schedules and assuming the bore is the same.
• Confusing schedule with pressure rating. The allowable pressure also depends on material, temperature and joints.

Frequently asked questions

What is 25A in inches?

1 inch nominal, with an outside diameter of 33.4 mm.

What is the inside diameter of 2-inch Sch 40 pipe?

About 52.5 mm (60.3 − 2 × 3.91).

Are Korean KS pipe sizes the same as ASME sizes?

The outside diameters of KS carbon steel pipes such as KS D 3562 are nearly the same, but wall thickness tables can differ slightly. Check the standard you order to.

When does the nominal size equal the outside diameter?

For NPS 14 (350A) and larger, the nominal inch size equals the outside diameter.

Sources: ASME B36.10M, Welded and Seamless Wrought Steel Pipe · KS D 3562, Carbon steel pipes for pressure service