System International (SI) Notation

Circuit : Andy Collinson

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Many of the schematics on this site have parts that are labeled using the System International (SI) notation system. The SI system is an easy way to describe values without the need for a decimal point. Circuits that have been previously drafted on paper may have a decimal point that has faded or hard to see. The SI system thereby overcomes this problem and offers a clear advantage. Multipliers are used in the SI system, taking the place of the decimal point in the conventional naming system.

The following table indicates the multipliers for the commonest values used in electronics:-

Table of Multipliers

Number | Prefix | Symbol |

10 ^{12} | tera | T |

10 ^{9} | giga | G |

10 ^{6} | mega | M |

10 ^{3} | kilo | k |

10 ^{-3} | milli | m |

10 ^{-6} | micro | u |

10 ^{-9} | nano | n |

10 ^{-12} | pico | p |

SI notation replaces the decimal point "." with the unit of measurement. So for example in old notation a 4.7k resistor is quoted as 4k7. If the value does not contain a decimal point, than the unit appears at the end. For example, a 10Ω resistor would appear on a parts list as 10R. Resistor values are in ohms (R), capacitance is measured in farads (F) and inductors are measured in Henrys (H). See the table below for more examples.

More Examples

Component | Old System | SI System |

Resistor | 10 ohms | 10R |

Resistor | 0.1 ohms | 0R1 |

Resistor | 6.8 k | 6k8 |

Capacitor | 100 uF | 100u |

Capacitor | 4700 uF | 4m7 |

Inductor | 2.2 mH | 2m2 |

Inductor | 4.7 H | 4H7 |

Inductor | 100 nH | 100n |

Fuse | 0.5 Amp | 0A5 |