S.I. System of Units | Physics | CSEC CXC lessons online – Study Guide

S.I. System of Units | Physics | CSEC CXC lessons online – Study Guide

S.I. Units

The S.I. System of Units consists of the seven (7) fundamental quantities and ALL derived quantities.

In the system, each physical quantity (fundamental or derived) has only ONE unit.


Standard Prefixes for S.I. Units

Each physical quantity has only one S.I. unit, but these units may lead to very awkward numbers when dealing with very small or very large magnitudes.

Certain prefixes (multiples and sub-multiples of ten) are therefore used.

Scientific notation is also used to represent quantities of very large or very small magnitudes. This involves writing the magnitude: as a number between 1 and 10, times 10 raised to a power. The power to which ten is raised is:

  1. Positive if the magnitude is large (>1), negative if the magnitude is small (<1).
  2. Equal to the number of places the decimal point is moved.


  1. 0.00000145 s = 1.45 x 10-6 s (decimal point moved 6 places to the right) 
  2. 341,000,000 m = 341 x 108 m (decimal point moved 8 places to the left) 


Table 1.1 shows the standard prefixes used.

Table 1.1: Standard Prefixes (multiples and sub-multiples).

Multiples Sub-Multiples
Multiple Prefix Symbol Sub-Multiple Prefix Symbol
103 kilo K 10-3 milli m
106 mega M 10-6 micro μ
109 giga G 10-9 nano n
1012 tera T 10-12 pico p
1015 peta P 10-15 femto f
1018 eta E 10-18 ato a