Despite the use of the international system of units in British education for the last four decades there are widespread myths about the metric system.

*Metric is a French plot*

Metric is the international system of units (SI) officially adopted by all countries apart from the United States, Myanmar and Liberia. Read more

*The metre is wrong*

The metre is defined in terms of the distance travelled by light in a very precisely defined fraction of a second. Read more

*Centimetres are not valid in SI*

In the international system (SI) *centi*- is a standard prefix and *metre* is a base unit. Centimetres are therefore valid in SI. Read more

*Metric requires decimal time*

The second is an SI base unit. Minutes, hours and days are metric units. Read more

*Metric means compulsory use of decimal numbers*

metric usually uses decimal numbers but there are specific cases like angular measure or time when decimal numbers are not used Read more

This myth is based on the fact that the metric system originated in 18th Century France. By the mid-19th century the metric units were widely adopted by other countries in Western Europe and Latin America. This adoption was based on the usefulness of metric units and not due to political, military or commercial pressure from France. In the UK the Select Committee on Weights and Measures of 1862 recognised the advantages of metric units.

In the mid-19th century many scientists had started to use metric units and among others British scientists contributed ideas on how to make the system of units more effective. In 1875 a diplomatic treaty called the Metre Convention was signed to be responsible for the development of the metric system internationally. Britain signed the convention in 1884.

In the late 19th century and 20th century most countries adopted the international system of units. Britain started its conversion in 1965 but unlike other Commonwealth countries has failed to complete it. Metric is the international system of units (SI) adopted by all countries apart from the United States, Myanmar and Liberia; and, even in these countries, metric is used for some purposes.

This myth is based on the old definition of the metre. Initially the metre was defined to be one ten millionth of the distance between the pole and equator. Two surveyors – Mechain and Delambre – spent 6 years surveying the arc of the meridian from Dunkirk in France to Barcelona in Spain. The results of the survey were used to define the metre. It is oft-stated that because their measurement was not accurate (in modern terms) that the metre is "wrong".

Of course, the underlying assumption behind the measurement was that the earth is a perfect sphere. For their time, Mechain & Delambre’s measurements were very good; they had defined the meridian circumference of the earth to be 40 000 km and today it is generally agreed to be 40 007.86 km. An error of 0.02% is really not bad for eighteenth century scientists!

Since then we have learned that the earth is not perfectly spherical but is an irregular oblate spheroid and that the length of the arc can vary. This means that the approach originally taken to define the metre is not ideal. The same, of course, applies to the definition of the nautical mile which is one minute of an arc of a great circle of the earth. So the 'imperialists' who argue that the "metre is wrong" should acknowledge that the internationally agreed nautical mile is also "wrong".

By the late 19th century it was recognised that more accurate methods were needed to define the metre. Michelson and Benoît worked on defining the metre in terms of the wavelength of the red line of cadmium. Further improvements were made in the course of the 20th century culminating in the current definition which is the length of the path travelled by light in vacuum during a specific fraction of a second. The history of the metre is documented on the BIPM website.

*Centimetres are not valid in SI*

This misunderstanding has its roots in the development of the international system (SI) as a rational and coherent system of units and in the encouragement of best practice in engineering and science. For these purposes it is recommended that the use of multiples and sub-multiples of units be restricted to factors of 10^{3}, e.g. micrometre (μm), millimetre (mm) and metre (m). So engineering drawings and building plans are labelled in millimetres only.

However, the prefixes centi, deci, deca and hecto are all part of SI and so the *centi*metre is an SI unit.

Decimal time (and there are a number of different ideas in this area) basically advocates dividing the day into submultiples of 10. Some of these ideas have even been called 'metric time' despite having nothing to do with the international system of units.

The SI base unit of time is the second. Minutes, hours and days are permitted for use with SI. A number of ancient civilisations divided the day into 12 hours and the Egyptians were the first to divide the night into 12 hours. The Babylonians came up with the idea of dividing intervals by 60. So both metric and imperial owe our concept of time to ancient civilisations.

*Metric means compulsory use of decimal numbers*

Of course, it is true that the standard metric prefixes use decimal multiples and submultiples. However, this only applies to the ratios between units of different sizes. Measured quantities can assume any value.

Though it is rarely acknowledged by the critics that when estimating or approximating we have a natural tendency to speak in multiples of ten, hundred, thousand etc. For example, we wouldn't typically say that something is roughly 147 metres away, we'd more likely say about 150 metres, i.e. round it to the nearest 10 (note that the same would apply if we were talking in yards). Even when aviators talk of altitude in feet they would estimate in hundreds or thousands.

We think decimal because that's the way we count. That is why the metric system is structured that way.