## Author Archive

## 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 italicrepresents*m*. UPPER CASE*mass***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 theunit*time***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[see footnote 1] is*c***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 V**, write*AC**AC***115 V**or**115 volts**in correct SI.*alternating current* **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. SIis reckoned in the 24-hour system, often with the time zone specified:*time***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.