How to Convert Pipe Diameters From Imperial to Metric

Updated February 21, 2017

Until the nearly universal adoption of the metric system, all pipe measurements used British imperial units. With the widespread use of metric piping in new builds, and for repairs to existing pipe work, it is often necessary to join pipes of metric and imperial dimensions. To do this efficiently requires matching their diameters. Converting the diameter of an imperial pipe into metric units requires only basic math skills and a calculator.

Establish the diameter of the pipe in imperial units. Convert any imperial fractions into decimal fractions by dividing the numerator, or top number, by the denominator, or bottom number. For example, 1/2 is the same as 1 divided by 2, or 0.5. An imperial pipe measuring 1 1/8 inches has a diameter of 1.125 inches because 1 divided by 8 equals 0.125.

Multiply the imperial diameter by 25.4, the number of millimetres (mm) in 1 inch. For example, 0.5 multiplied by 25.4 equals 12.7, so a half-inch pipe has a diameter of 12.7mm.

Check for errors by dividing the metric diameter by 25.4. If the result does not equal the imperial diameter of the pipe there was an error in your calculations.


Imperial-to-metric connectors are readily available in most plumbing stores and cost no more than metric-to-metric connectors cost. Not all imperial pipes have a metric equivalent size.


Imperial pipe diameters measure the internal dimensions; metric pipe diameters are for the external dimensions.

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About the Author

David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.