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12 February 2014

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SI, Metric, Imperial and American

by Jill Studholme

Some time ago, the Mars Climate Orbiter crashed into the surface of Mars. One reason cited was a discrepancy over the units used. The navigation software expected data in newton second; the company who built the orbiter provided data in pound-force seconds.

Another, less expensive, disappointment occurs when people used to British pints order a pint in the USA, only to be served what they consider a short measure. Again the reason is confusion over units; this time due to the fact that American units, although bearing the same names as British Imperial units, often refer to very different measures.

Pint of beer, illustrating different engineering units

Why are there different systems of units?

In 1824 a British Act of Parliament gave precise definitions to Imperial units. These included the yard, pound, bushel and gallon. Six years later an American survey found weights and measures in various ports differed significantly. This led to the United States, in 1834, standardising measurements. However, although these were named after British units, they were not always the same measures as those used in Britain. The gallon chosen, for example, was the "Queen Anne wine gallon", which was already obsolete in Britain.

The most widely used system of units and measures around the world is the Systeme International d'Unites (SI), the modern form of the metric system. This originated in France, where in 1790 the French Academy of Science was commissioned to design a new system of units. They decided that

These principles allowed the metric system to evolve and SI units have become the fundamental basis of scientific measurement world-wide.

SI Base Units

There are just 7 base units

Quantity Unit Symbol
length metre m
mass kilogram      kg
time second s
temperature kelvin K
amount of substance    mole mol
electric current ampere A
luminous intensity candela cd

Prefixes are used to indicate powers of ten. They mean that with only one unit of measurement you can express any measurement from the smallest to the largest. They each have a short name and symbol. Large factors have upper case symbols, and small factors lower case symbols.

For example:

giga 1 000 000 000 G
mega 1 000 000 M
kilo 1 000 k
milli 0.001 m
micro 0.000 001 µ

Style Conventions and Rules for using SI Units

General principles for the writing of unit symbols and numbers were first proposed in 1948. These were subsequently adopted and elaborated by ISO, the international standards organisation.

  1. Unit symbols are printed in roman (upright) type, irrespective of how the rest of the text is printed.
  2. Unit symbols are unaltered in the plural.
  3. Unit symbols are written without a final full stop (period) except for normal punctuation such as at the end of a sentence.
  4. Unit symbols are placed after the numerical value, leaving a space between the value and the symbol. For example 5 V not 5V.
  5. Unit symbols are generally written in lower case letters, except when the name of the unit is derived from a proper name. (Note that when the name of a unit which is derived from a proper name is written out in full, such as ampere or herz, the name is not capitalised. The only exception to this is Celsius.)
  6. The the given SI unit symbol should be used. The symbol for second, for example, is s. To use sec or secs is incorrect.
  7. Unit symbols and unit names shouldn't be mixed. Metre per second, not metre/second or metre/s.

Summary of Advantages of SI Units: Why they are the Global Choice


Further Reading

You can find more information on units on these web sites.

National Physical Laboratory (UK)

NIST: National Institute of Standards and Technology (USA)

Bureau International des Poids et Mesures

The Metric System [SI]: A concise reference guide

Metric Units and Conversion Charts, T Wildi, 1996, IEEE Press.

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