Pipe Size Chart in mm: NB to Inch & Pipe Dimensions

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What Is NB, DN and NPS?

The dimension of pipe size is called Nominal Bore (NB) and is measured in millimetres. Commonly used in all markets across India and most Asian and European markets, 50 NB is the same as DN50.

The normative international term for this metric sizing is its designation in nominal diameter, DN. The terms NB and DN are synonymous in commercial documents.

The system is the same, and in North America it's called Nominal Pipe Size, which is also measured in inches. The same pipe size is represented by both the NPS 2 and DN50.

All three of these numbers are not exact inside diameter measurements. They are size labels that were standardised decades ago, inherited for compatibility with fittings, flanges and valves. The actual number that remains constant for a particular size is the outside diameter (OD); see the table below.

Pipe Size Chart: NB to Inch to mm Conversion Table

Convert directly between NB (Nominal size), DN, NPS (inch) and outside diameter using this table. Values are based on ASME B36.10M (carbon and alloy steel pipe) and ASME B36.19M (stainless steel pipe).

Once again, you see the pattern: the NPS and the OD in inches are equal for pipes 14" or larger (DN350 and up). Below that, they diverge — a "1 inch" pipe has an OD of 1.315", not 1". This is the one mistake most people make when choosing pipe sizes in procurement and drafting; always check the OD from the table instead of assuming that NPS = OD.

Pipe Dimensions Chart: OD and Wall Thickness by Schedule

The outside diameter remains the same for a given size; the wall thickness varies with schedule. For convenience, the two schedules that are most often specified are presented below.

The full SCH 5S, 10S, 20, 40S, 80S, 160 and XXS breakdown by all pipe sizes, and a description of how schedule relates to pressure rating is available on our Pipe Schedule Chart. See the SS Pipe Weight Chart for each wall thickness for weight per metre.

How to Convert Pipe Size (mm to Inch and Inch to mm)

Formula: mm = inch × 25.4, and inch = mm ÷ 25.4.

This is a transformation of a raw measurement, but pipe sizing has one additional step to it because NB and NPS are labels rather than actual measurements. When converting between an NB size and an NPS size, use the standardised OD, not a direct mathematical conversion of nominal number. 

Worked example 1: Convert 2 inches NPS to mm: 2 inches NPS is equal to DN50. From the table, OD = 60.33 mm. Not a straight 2 × 25.4 = 50.8 mm — this is the figure to use for flange, fitting and clearance calculations.

Worked example 2: Convert DN50 to inches. DN50 is equivalent to NPS 2, which has an OD of 2.375 inches (60.33 mm).

Worked example 3: Find the inside diameter of NPS 2, SCH 40S: Outside diameter = 60.33 mm. Wall thickness (SCH 40S) = 3.91 mm. Inside diameter ≈ OD − (2 × wall thickness) = 60.33 − 7.82 = 52.51 mm. Always confirm against certified mill dimensions for final engineering use; this is an estimate for planning purposes only.

Common NB to Inch Conversions — Quick Reference

Pipe Size and Material Grade

Pipe size and OD stay the same regardless of material grade — a 50 NB (2") pipe has an OD of 60.33 mm whether it is SS 304, SS 316/316L, SS 904L or ASTM A106 Gr. B carbon steel. With grade, corrosion resistance, temperature rating and even minimum order quantities can change. SS 321/321H is normally used for high-temperature applications, whilst Duplex S31803/S32205 is used when both strength and corrosion resistance to chloride are needed.

Also, the manufacturing route influences tolerance and availability at a given size, seamless pipes versus welded pipes; the construction method also influences which sizes and schedules are readily available.

Common Industries Using These Pipe Sizes

• Oil, gas and refinery process piping, from instrumentation lines to large-diameter transmission lines, all the way from small to large bore.
• Petrochemical and chemical plants which call for specific OD for flange and fitting compatibility.
• Where DN sizing is a common practice in marine and offshore projects according to industry standards across all vendors' documentation.
• When NPS and schedule are both given for piping of a power generation or heat exchanger, the pipe size is determined by meeting the NPS requirement. For power generation and heat exchanger piping, where NPS and schedule are specified together in the same line list, the pipe size is determined by meeting the NPS requirement.
• Pharmaceutical, food and water treatment systems using smaller NB sizes with tight OD tolerance.

Metinox Overseas provides supplements of matching pipe fittings and flanges to enable pipe size, schedule and end connection to be coordinated throughout the entire pipe system from a single source.

Frequently Asked Questions