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International System of Units (SI) Checklist

Metrology is the science of measurement. The International System of Units, also called SI, consists of standards which result from meticulous negotiations among international metrologists. The purpose of the International System of Units is to communicate quantitative information clearly across languages and cultures.

This check-list summarizes the most important elements of those standards. For more detail, you may wish to download the PDF document NIST Special Publication 811, 2008 Edition, by Ambler Thompson and Barry N. Taylor: Guide for the Use of the International System of Units (SI). A relevant Wikipedia article is also useful. The Bureau International des Poids et Mesures (BIPM) also publishes useful information in French and English.

Here is a simple checklist to help you use SI correctly:

  • Except for degrees Celsius, SI units like microfarads and millivolts are always written in lower case and are almost always pluralized. (See below for exceptions to the usual pluralizing standard.) Because of their phonetics, some SI units like megahertz are written the same in singular and plural.
  • SI symbols like kHz and µV are written in lower case or UPPER CASE Latin or Greek characters or in combinations. SI symbols are the most universal parts of SI, and are never pluralized.
  • Be careful with UPPER and lower case: UPPER CASE M is the SI symbol for the mega- prefix; lower case m is the symbol for meters or metres as well as the milli- prefix; lower case Greek µ is the symbol for the micro- prefix. With the advent of computer word processors, using u as a substitute for µ is an obsolete practice. Lower-case italic m represents mass. UPPER CASE K is the SI symbol for thermodynamic temperature in kelvins and lower case k is the SI symbol for the kilo- prefix. One should not be used in place of the other.
  • With three exceptions, SI values and SI symbols are always separated with spaces and never with anything else. Those exceptions are the symbols for angular degrees, minutes and seconds. The 100 m dash and a 10 A fuse are correct expressions. It is also correct to write: The summit of 6190 m Denali in Alaska is located at 63°04’08.7”N 151°00’25.5”W.
  • When used in an adjectival sense in English, SI values and spelled-out SI units are separated with hyphens and are not pluralized: the 100-meter dash and a 10-ampere fuse are correct expressions. When accompanying values of exactly 1 or -1, SI units are not pluralized.
  • Abbreviations do not exist in SI. Instead of abbreviations like amps and secs, use SI symbols like A and s or fully spelled-out SI units like amperes and seconds. Note that the symbol for the time unit minutes is min, which is not an abbreviation, and therefore it is not pluralized and it is not followed by a period.
  • Never use SI prefixes in isolation. Avoid using expressions like 10 kilos of flour or 5 K run; use 10 kilograms of flour or 10 kg of flour or 5-kilometer run or 5 km run instead.
  • Except at the end of a sentence, an SI symbol is never followed by a dot or period. To avoid confusion, try not to end sentences with SI symbols if possible.
  • Fractional SI values are decimalized and preceded with a zero or other integers: 0.529 µm or 0.529 micrometers.
  • Since either a dot or a comma may be used in SI as a decimal marker, the comma should never be employed as a separator for long integers or long fractions. Segment values with five digits or more utilizing spaces or half spaces. Using a word-processor, create a half space by changing the font size of a regular space to about half the value of the rest of the text. The speed of light, whose symbol is italic c [see footnote 1] is 299 792 458 m/s or 299 792.458 km/s or 299.792 458 Mm/s when written in SI. The speed of light may also be written as 299 792,458 km/s or 299,792 458 Mm/s without any change in meaning.
  • SI symbols should never include suffixes. Instead of 115 VAC, write AC 115 V or 115 volts alternating current in correct SI.
  • Avoid orphaned values. Instead of 9-15 volts or 9-15 V, write 9 volts to 15 volts or 9 V to 15 V in SI.
  • SI dates are rendered with numerals in descending order. The origin of what became the International System of Units began in Paris on 1875-05-20 with an international treaty. SI time is reckoned in the 24-hour system, often with the time zone specified: 1445 UTC or 0657 EST.
  • SI standards have changed over time. Avoid obsolete expressions. The old degrees kelvin should be kelvins (symbol K). The obsolete mhos should be siemens (symbol S), which is followed by an s in both singular and plural unit forms. The old cubic centimeters unit is still commonly used in medicine, but milliliters or millilitres (symbol mL) [see footnote 2] should be used instead. The obsolete microns unit is now micrometers or micrometres (symbol µm). Multiple prefixes like µµ or micromicro- are no longer allowed in SI. Use the pico- unit prefix or the p- symbol prefix instead. An acceptable SI substitute for the obsolete parts per million (ppm), parts per billion (ppb) has not yet been developed. If international metrologists eventually agree on an SI unit and symbol for nominal-scale entities, then fractional prefixes combined with that unit or symbol will do a good job of carrying out that proportional function.

¹ Quantities to be measured and their symbols are written in italics: current and inductance are examples. Think of e=mc² and I=E/R.

² Although lower-case l may be used as a symbol for liters of litres, that character may be mistaken for the numeral 1, so most writers prefer the upper case L for that symbol.

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