International System Of Units

• Symbols do not have an appended period/full stop (.) unless at the end of a sentence.

• Symbols are written in upright Roman type (m for metres, L for litres), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).

• Symbols for units are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa" whereas the Units of measurement itself is written "pascal (unit)".
• The one exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1" or the uppercase letter "i" (depending on the typeface used), at least in many List of countries where English is an official language. The American National Institute of Standards and Technology recommends that "L" be used instead, a usage which is common in the US, Canada, Australia, and New Zealand (but not elsewhere). This has been accepted as an alternative by the General Conference on Weights and Measures since 1979. The cursive "ℓ" is occasionally seen, especially in Japan and Greece, but this is not currently recommended by any Standards organization. For more information, see Litre#Symbol.

• The SI rule for pluralising units is that symbols of units are not pluralised, for example "25 kg" (not "25 kgs").
• The American National Institute of Standards and Technology has defined guidelines for using the SI units in its own publications and for other users of the SI. These guidelines give guidance on pluralizing unit names: the plural is formed by using normal English grammar rules, for example, "henries" is the plural of "Henry (inductance)". The units lux, hertz, and Siemens (unit) are exceptions from this rule: they remain the same in singular and plural. Note that this rule only applies to the full names of units, not to their symbols.

• A space separates the number and the symbol, e.g. "2.21 kg", "7.3 m²", "22 K" . Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.

• Spaces may be used as a thousands separator (1 000 000) in contrast to commas or periods (1,000,000 or 1.000.000) in order to reduce confusion resulting from the variation between these forms in different countries. Space (punctuation), the space used for this purpose is typically narrower than that between words.

• The 10th resolution of General Conference on Weights and Measures in 2003 declared that "the symbol for the Decimal separator shall be either the full stop on the line or the comma on the line". In practice, the decimal point is used in English, and the comma in most other Languages of Europe.

• Symbols for derived units formed from multiple units by multiplication are joined with a space or Interpunct (·), e.g. "N m" or "N·m".

• Symbols formed by division of two units are joined with a solidus (punctuation) (⁄), or given as a negative exponent. For example, the "metre per second" can be written "m/s", "m s−1", "m·s−1" or \textstyle\frac{\mathrm{m-->{\mathrm{s-->. A solidus should not be used if the result is ambiguous, i.e. "kg·m−1·s−2" is preferable to "kg/m·s²". Many computer users will type the / character provided on American computer keyboards, which in turn produces the Unicode character U+002F, which is named solidus but is distinct from the Unicode solidus character, U+2044.

• In Chinese language, Japanese language, and Korean language computing (CJK), some of the commonly used units, prefix-unit combinations, or unit-exponent combinations have been allocated predefined single characters taking up a full square. Unicode includes these in its CJK Compatibility and Letterlike Symbols subranges for back compatibility, without necessarily recommending future usage.

• When writing dimensionless quantities, the terms 'ppb' (parts per billion) and 'ppt' (parts per trillion) are recognised as language-dependent terms since the value of billion and trillion can Long and short scales. SI therefore recommends avoiding these terms . However, no alternative is suggested by BIPM.

• Several nations typically use the spellings meter and liter instead of metre and litre in keeping with standard American English spelling, which also corresponds to the official spelling used in several other languages, such as German, Dutch, Swedish, etc. In addition, the official US spelling for the SI prefix "deca" is deka.

• In some English-speaking countries, the unit "ampere" is often shortened to amp (singular) or amps (plural).

• The many units of time — minute (min), hour (h), day (d) — in use besides the SI second, and are specifically accepted for use according to table 6.
• The year is specifically not included but has a recommended conversion factor.
• The Celsius temperature scale; kelvins are rarely employed in everyday use.
• Electrical energy is often billed in watt-hour instead of megajoules.
• The nautical mile and knot (unit) (nautical mile per hour) used to measure travel distance and speed of ships and aircraft (1 International nautical mile = 1852 m or approximately 1 minute of latitude at the equator). In addition to these, Annex 5 of the Convention on International Civil Aviation permits the "temporary use" of the foot (unit of length) for altitude.
• Astronomical distances measured in astronomical units, parsecs and light-years instead of, say, petametres (a light-year is about 9.461 Pm or about 9 461 000 000 000 000 m).
• Atomic scale units used in physics and chemistry, such as the ångström, electronvolt, atomic mass unit, and barn (unit).
• Some physicists still use the centimetre gram second system of units (CGS) units, with their associated non-SI electric units.
• In some countries, the informal cup (unit) measurement has become 250 ml. Likewise, a 500 g "Pound (mass)#Metric pounds" is used in many countries. Liquids, especially alcoholic ones, are often sold in units whose origins are historical, e.g. beer in pints in the UK, champagne in Wine bottle#Sizess in France.
• In the US, Blood sugar measurements are recorded in milligrams per decilitre (mg/dL); in Canada and Europe, the standard is millimole per litre (mmol/L or mM (millimolar).
• Blood pressure is measured in mmHg instead of Pa.

• Metrology
• Units of measurement
• History of measurement

• Orders of magnitude
• Metrication
• Metrication in the United States

• SI base unit
• SI derived unit
• SI prefix
• Convention du Mètre

• ISO 31
• ISO 1000
• ISO/IEC 80000
• Coordinated Universal Time (UTC)

• BIPM Bureau International des Poids et Mesures (SI maintenance agency) (home page)
• BIPM brochure (SI reference)
• ISO 1000:1992 SI units and recommendations for the use of their multiples and of certain other units
• ISO 31/1000/80000
• NIST Official Publications
• NIST Special Pub 814: Interpretation of the SI for the United States and Federal Government Metric Conversion Policy
• Weights and Measures Act, Canada
• IEEE/ASTM SI 10-2002 Standard for Use of the International System of Units (SI): The Modern Metric System (ANSI approved, joint IEEE/ASTM standard)
• Rules for SAE Use of SI (Metric) Units

• EngNet Metric Conversion Chart Online Categorized Metric Conversion Calculator

• LaTeX SIunits package manual gives a historical background to the SI system.

• The UK Metric Association
• The US Metric Association
• One Metre: Metric in Canada
• Canadian Metric Association
• Metrication US

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