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Converting Imperial units to SI units

6 January, 2016 - 09:32

Because the metric system is already widely used globally, the United Kingdom and United States published guidelines to the public for converting Imperial units to SI units respectively:

  • United Kingdom: British Standard: BS350
  • United States: The International System of Units (SI) — Conversion Factors for General Use by NIST.

Read the following document for a further introduction to Imperial units, and to see how they are converted to SI units.


Butler, K, Crown, L and Gentry, E (eds) (2006) The International System of Units (SI): Conversion Factors for General Use, NIST Special Publication 1038.

You should now be aware of the relevant conversion factors to convert SI units to Imperial units.

When a product is to be shipped to a country where Imperial units are used, but which was weighed and labelled with SI units in its country of origin, conversion of units is necessary. In such cases, you will find this online calculator helpful; you can use it for conversion of units without memorizing all the conversion factors:

This article also answers some frequently asked questions about SI units.

In this section we have discussed the metric and Imperial unit systems. When we conduct a test or measurement with reference to a country's requirements/standards, we need to know what unit system must be followed. Some electronic instruments can display results in different unit systems. In this case, we need to know what conversion factor has been used. Otherwise, this conversion can be an error source and cause measurement uncertainty.

In the next section we will discuss measurement traceability.