ISO 80000/SI

ISO 80000/SI

ISO 80000/SI

ISO 80000/SI

a handful of useful hints to write technical prefixes and units


There is nothing wrong in defining your own standard. However, why not use one that is already established? Don't you think it is a pity to make a typo on your new QSL card or in the front plate of your new amplifier?

The purpose of this page is to tell a bit about how you can write, and read, units and prefixes in compliance with the ISO 80000 standard.

Fundamental rules

The basic rules for ISO 80000 are

  • Everything is in lower case except if it follows a full stop from a previous sentence, e.g. meter, m
  • Units and prefix are always in singular, e.g. 10 m and 10 km
  • There is a spare between the numerical value and unit symbol, e.g. 12 V/m, 400 mA
  • There is not a full stop after a unit unless it finishes a sentence, e.g. "... to the top there is 800 m. From the top you can ..."
  • A prefix must always be follow by a unit and two prefixes cannot be combined

Unfortunately, even in ISO 80000 there are some exceptions

  • In case a unit is named after a person the first letter of the unit is written in capital, e.g. V, volt after Alessandro Volta, W, watt after James Watt, Hz, hertz, after Heinrich Hertz, and F, farad after Michael Faraday. This is easy to remember because all names begins with a capital letter e.g. Jones and not jones
  • Prefixes greater than 1000 are in capital, e.g. Mega, M. Because the absolute temperature is measured in kelvin after Lord Kelvin the prefix for 1000 is k and not K. Rule to remember: all prefixed greater than 1000 are in capital
  • Time units can be in plural, e.g. seconds, but there are no plural "s" after the symbol for seconds, s, i.e. 100 seconds are not written 100 ss

Most common electrical units and prefixes

Most common electrical units.

Type Unit Symbol
Length Meter m
Area Square meter m2
Volume Cubic meter m3
Weight Kilogram kg
Voltage Volt V
Current Ampere A
Resistance Ohm Ω or Ohm
Conductivity Siemens S
Power Watt W
Temperature Degree  
Capacity Farad F
Inductivity Henry H

Most common prefixes used in combination with electronics.

Prefix Quantity Symbol Example
Tera 1012 T THz
Giga 109 G GHz
Mega 106 M MW
Kilo 103 k (lower case!) kg
Deci 10-1 d dB
Milli 10-3 m mm
Micro 10-6 µ or u µH
Nano 10-9 n nF
Pico 10-12 p pF

Common mistakes

The most common mistakes are

  • Writing units in plural, e.g. watts, Watts, Ws for watt, W and Ohms for Ohm, CM, cms, meters for cm and m, grams, kgs for g and kg, KM and kms for km
  • The correct prefix for 1000 is k and not K which is the unit for the absolute temperature, K after Lord Kelvin
  • The s in fractions of a second in capital S, e.g. 100 mS. However, the symbol S represents the unit siemens which is the measure for conductivity. The correct way is to write 100 ms
  • Omitting the space between the numerical value and (prefix symbol)unit symbol, e.g. 20m, 20M. The correct is to write 20 m

Decimal separator

How is it with the decimal separator? It is a full stop/dot or a comma? Initially it was comma. However, ISO probably became soft so both are valid. It depends upon the context/origin.

Useful links

International Organization for Standardization (ISO)


National Institute of Standards and Technology - Check List for Reviewing Manuscripts

Bo, OZ2M,

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