Why Does a Pipe's Nominal Size Differ from Its Actual Diameter in Millimeters?

A customer orders a «۱ اینچ» pipe. When they test it with a ۱ اینچ fitting from another brand, it may not fit exactly or may have clearance. This is not something related to poor product quality; its root lies in a less frequently mentioned point: the nominal pipe size is not equal to the exact mathematical conversion of inches to millimeters.

What Does Nominal Size Mean

When you say «۱ اینچ» pipe, you mean an industrial standard designation that developed throughout the history of pipe manufacturing, not that the actual pipe diameter is exactly ۲۵.۴ millimeters (the mathematical equivalent of one inch). For example, a ¾-inch pipe actually has an outside diameter of about ۲۶.۷ millimeters, but in the market it is known by the rounded number close to ۲۵.

Nominal size is an industrial convention; it is not always equal to the precise millimeter measurement of the outside diameter.

Why Does This Difference Matter in Practice

When a customer who works with foreign standards or fittings from another brand places an order, if you conduct the transaction based only on stating the approximate inch or millimeter number, the pipe and fittings may not fit together properly. This problem occurs especially when one party confuses a metal pipe (whose nominal size is usually defined based on the inside diameter) with a plastic pipe (whose nominal size is based on the outside diameter).

How to Use This Knowledge in Sales

When a customer places an order, especially for fittings or pipes from multiple brands, it is better to confirm the actual outside diameter as well instead of relying solely on stating the inch number, or to use the official size chart. These few extra minutes of confirmation prevent returned shipments and subsequent disputes over pipes and fittings not fitting together.

A Brief Reminder

This difference is specific to pipes; for rebar or sheet metal, the stated number is usually the actual size.