SI is the symbol for the Systěme International d’Unites, the modernized version of the metric system that the USA and other nations have agreed to use.  (Do not abbreviate it as S.I.)

 

This list is provided to point out the correct way to use the metric system and to show many of the incorrect examples of its usage that may be given on package labels and in other printed matter.  These correct ways to use SI are set by the international standards that define SI.

 

General Guidelines:

1.      The short forms for SI units (such as mm for millimeters) are called symbols, not abbreviations.

2.      SI symbols never end with a period unless they are the last word in a sentence.

·         RIGHT: 20 mm, 10 kg

·         WRONG: 20 mm., 10kg.

3.      SI symbols should be preceded by digits and a space  must separate the digits from the symbol.

·         RIGHT: It was 300 mm wide.  The millimeter width was given.

·         WRONG: It was 300mm wide.  The mm width was given.

4.      Symbols always are written in the singular form (even when more than one is meant).

·         RIGHT: 1 mm, 500 mm, 1 kg, 36 kg

·         WRONG: 500 mms, 36 kgs

·         BUT: It is correct to pluralize written-out metric unit names: 25 kilograms, 250 millimeters

5.      The symbol for a compound unit that is a quotient of two units is indicated by a solidus or by a negative exponent.

·         RIGHT: km/hg\ or km·h1

·         WRONG: kmph or kph (do not use p as a symbol for “per”)

·         BUT: It is correct to say or write “kilometers per hour.”

6.      The meaning of an SI symbol can be changed when substituting a capital letter for a lower case letter.

·         RIGHT: mm (for millimeter, which means 1/1000 of a meter)

·         WRONG: MM or Mm (M is the prefix for mega, which means one million; a megameter is a million meters)

 

Note: A 5K race would be a five Kelvin race, while a 5k race would be a five kilo race, neither of which would be accurate.  Kilometer should be pronounced KILL-oh-meet-ur, not kill-AHM-it-ur.

 

The information above was adapted from the U.S. Metric Association Website, http://www.metric.org/

Students are encouraged to visit the website for more information.