International System of Units

The International System of Units (SI), adopted by the General Conference on Weights and Measures in 1960. There are six (5) principal units from which the units of

all other physical quantities can be derived. Table 1 below shows the six units, their symbols, and the physical quantities they represent. The SI units are used are used by professionals irrespective of their geographical location.

One great advantage of the SI unit is that it uses prefixes based on the power of 10 to relate larger and smaller units to the basic unit. Table 2 shows the SI prefixes and their symbols.

1,000,000 mm = 1,000 m = 1 km

Table 1 – The six basic SI units
Quantity Basic unit Symbol
Length meter m
Mass kilogram kg
Time second s
Electric current ampere A
Thermodynamic temperature kelvin K
Luminous intensity candela cd
Table 2 – The SI prefixes
Multiplier Prefix Symbol
1018 exa E
1015 peta P
1012 Tera T
109 giga G
106 mega M
103 kilo k
102 hecto h
10 deka da
10-2 centi c
10-3 milli m
10-6 micro µ
10-9 nano n
10-12 pico p
10-15 femto f
10-18 atto a

1. Fundamentals of Electric Circuits, 3rd Ed.