National Physical Laboratory

SI Conventions

The following is a list of the key recommendations when using SI units:

Writing unit names and symbols

  • Only units of the SI and those units recognised for use with the SI should be used to express the values of quantities.
  • All unit names are written in small letters (newton or kilogram) except Celsius.
  • The unit symbol is in lower case unless the name of the unit is derived from a proper name, in which case the first letter of the symbol is in upper case.
  • Unit symbols are unaltered in the plural.
  • Unit symbols and unit names should not be mixed.
  • Abbreviations such as sec (for either s or second) or mps (for either m/s or meter per second are not allowed.
  • For unit values more than 1 or less than -1 the plural of the unit is used and a singular unit is used for values between 1 and -1.
  • A space is left between the numerical value and unit symbol (25 kg, but not 25-kg or 25kg). If the spelled-out name of a unit is used, the normal rules of English are applied.
  • Unit symbols are in roman type, and quantity symbols are in italic type with superscripts and subscripts in roman or italic type as appropriate.

Numerical Notation

  • A space should be left between groups of 3 digits on either the right or left hand side of the decimal place (15 739.012 53). In four digit numbers the space may be omitted. Commas should not be used.
  • The decimal marker shall be either the point on the line or the comma on the line. The decimal marker chosen should be that which is customary in the context concerned.
  • Mathematical operations should only be applied to unit symbols (kg/m2) and not unit names (kilogram/cubic metre).
  • Values of quantities should be expressed as 2.0 µs or 2.0 x 10-6 and not in terms such as parts per million.
  • It should be clear to which unit symbol a numerical value belongs and which mathematical operation applies to the value of a quantity (35 cm x 48 cm, not 35 x 48 cm; or 100 g ± 2 g, not 100 ± 2g).
  • The value must apply to the whole symbol and not any particular unit within the symbol.

NIST have produced a complete 80+ page document covering all aspects of this which can be downloaded at:


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