The international system of units consists of a set of units together with a set of prefixes. The units of SI can be divided into two subsets. There are the seven base units. Each of these base units are dimensionally independent. From these seven base units several other units are derived.
SI base units
SI writing style
• Symbols are written in lower case, except for symbols derived from the name of a person. The one exception is the liter, whose original symbol "l" is dangerously similar to the numeral "1". The NIST recommends that "L" be used instead, a usage, which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM.
• Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs".
• Symbols do not have an appended period (.) unless at the end of a sentence.
• It is preferable to write symbols in upright Roman type (m for meters, L for liters), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).
• A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×102 m2."
• Symbols formed by division of two units are joined with a virgule (/).
• For example, the "metre per second" is written as "m/s."
Spelling variations
• Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling.
• In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
• The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.
SI base units
Name-Symbol-QuantityA prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and millidenotes a multiple of a thousandth hence there are one thousand millimeters to the metre and one thousand meters to the kilometer.
kilogram-kg-Mass
second-s-Time
metre-m- Length
ampere-A-Electrical current
kelvin-K-Temperature
mole-mol-Amount of substance
candela-cd-Luminous intensity
SI writing style
• Symbols are written in lower case, except for symbols derived from the name of a person. The one exception is the liter, whose original symbol "l" is dangerously similar to the numeral "1". The NIST recommends that "L" be used instead, a usage, which is common in the U.S., Canada and Australia, and has been accepted as an alternative by the CGPM.
• Symbols are written without grammatical markers when used with singular numerals: i.e. "25 kg", not "25 kgs".
• Symbols do not have an appended period (.) unless at the end of a sentence.
• It is preferable to write symbols in upright Roman type (m for meters, L for liters), so as to differentiate from the italic type used for mathematical variables (m for mass, l for length).
• A space should separate the number and the symbol, e.g. "2.21 kg", "7.3×102 m2."
• Symbols formed by division of two units are joined with a virgule (/).
• For example, the "metre per second" is written as "m/s."
Spelling variations
• Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre' in keeping with standard American English spelling.
• In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
• The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and its use is declining.