What Is Nominal Bore in Engineering? ISO 6708 Standard Guidelines

24 April 2026

Verified by Rachel Mayfield, Supply Chain Analyst - April 2026

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Nominal Bore is nb, a pipe sizing convention that roughly corresponds to the internal diameter but isn't an exact measurement. Nominal bore sizes are used extensively in UK engineering for specifying pipe diameters, fittings, flanges, and valves. This standardization simplifies procurement and ensures compatibility across various components from different manufacturers. Common nominal bore values range from 15 mm (0.6") to over 1400 mm (56"), with some sizes being more prevalent in specific industries. For instance, the 20-30 mm (0.8-1.2") range is frequently used in plumbing and heating systems, while larger bores are common in industrial applications like oil refineries. The ISO 6708 standard provides guidelines for nominal bore pipe dimensions internationally, complemented by British Standards such as BS EN 10255 for smooth carbon steel pipes. These standards outline the allowable tolerances and dimensional requirements that ensure consistent performance across different suppliers and countries. For example, a DN 20 (Nominal Bore of 20 mm) pipe will have an internal diameter slightly larger than 20 mm to accommodate industry-specific fittings and ease of installation. When selecting engineering components like flanges or valves, the nominal bore directly influences compatibility and flow rates. A 50 mm NB valve must fit onto a 50 mm NB pipe. Ensuring that both components are interchangeable within the same system. This standardization is important for maintenance engineers who need to replace parts quickly without having to measure every component individually. Maintenance teams often refer to trade bodies like the British Fluid Power Association (BIAFD) or the British Pump Manufacturers' Association (BPMA) for best practices and standards in specifying nominal bore sizes. These organizations provide detailed guidelines on material grades, such as A2-70 stainless steel or EN8 carbon steel, which are critical factors in selecting components that meet both durability and performance requirements. Understanding nominal bore is important for procurement managers and workshop owners who need to balance cost efficiency with system reliability. Choosing the correct size based on actual flow needs can prevent costly over-specification while ensuring adequate capacity and longevity of installed systems.

Frequently Asked Questions

What is Nominal Bore?

NB, a pipe sizing convention that roughly corresponds to the internal diameter but is not an exact measurement.

What does Nominal Bore refer to in engineering specifications?

Nominal Bore is a sizing convention used for pipes and fittings that roughly corresponds to the internal diameter but isn't an exact measurement. It simplifies procurement by ensuring compatibility across various components from different manufacturers.

Which standard guides the use of Nominal Bore pipe dimensions internationally, including in the UK?

The ISO 6708 standard provides guidelines for nominal bore pipe dimensions globally, complemented by British Standards such as BS EN 10255 for smooth carbon steel pipes. These standards outline tolerances and dimensional requirements to ensure consistent performance across different suppliers.

How do engineers choose the correct Nominal Bore size for a valve or fitting?

Engineers select the appropriate Nominal Bore based on the specific application, considering factors like flow rate, pressure requirements, and compatibility with existing piping systems. For example, DN 20 (Nominal Bore of 20 mm) is commonly used in plumbing and heating systems due to its suitability for low-pressure applications.

What are some typical Nominal Bore sizes used in UK engineering?

Common nominal bore values range from 15 mm (0.6") to over 1400 mm (56"). The 20-30 mm (0.8-1.2") range is frequently used in plumbing and heating systems, while larger bores are common in industrial applications like oil refineries.

Where can UK engineers find reliable suppliers for Nominal Bore components?

UK engineers can source nominal bore components from reputable suppliers listed on industry directories or trade associations. These suppliers typically offer a range of sizes and materials to meet diverse engineering needs, ensuring availability and cost-effectiveness.

What does Nominal Bore mean in the context of pipe dimensions?

Nominal Bore (nb) refers to a sizing convention for pipes that roughly correlates with the internal diameter but isn't an exact measurement. This standard simplifies procurement and ensures compatibility across various components from different manufacturers.

Which standards should UK engineers refer to when working with Nominal Bore sizes?

Engineers in the UK should consult ISO 6708 for international guidelines on nominal bore pipe dimensions, complemented by British Standards such as BS EN 10255 for smooth carbon steel pipes. These standards provide specifications that ensure consistent performance and fit across different suppliers.

How do engineers choose the appropriate Nominal Bore size for industrial applications?

When selecting engineering components like flanges or valves, engineers should consider the specific application requirements such as flow rate, pressure, and material compatibility. For instance, a DN 20 pipe has an internal diameter slightly larger than 20 mm to accommodate various fittings and ensure ease of installation.

Are there common nominal bore sizes that UK suppliers frequently stock?

Common Nominal Bore values range from 15 mm (0.6") to over 1400 mm (56"), with some sizes being more prevalent in specific industries. For example, the 20-30 mm (0.8-1.2") range is frequently used in plumbing and heating systems, while larger bores are common in industrial applications like oil refineries.