Si-International System of Units
A Metric Guide for Students
The International System of Units (SI) is a comprehensive
and practical system of units of measurement of all physical
quantities for technical, scientific and general use. The unit
of measurement of every physical quantity is derived from,
and described in terms of, one or more base units. The seven
base units are the metre (m), kilogram (kg), second (s),
ampere (A), kelvin (K), candela (cd) and mole (mol). Larger or
smaller multiples of these units of more convenient size are
obtained by combining the unit with an appropriate prefix
selected from a specified series.
THE INTERNATIONAL SYSTEM
OF UNITS (SI)
THE COMMON PREFIXES AND UNITS
The various units of measurement may be used alone or combined
with a prefix to form a unit multiple of more convenient magnitude,
e.g. kilo is combined with metre to form kilometre.
Prefix
Factor
18
Physical Quantity
peta (P)
10
tera (T)
1012
giga (G)
109
length
area
volume
capacity
mass
mass
mega (M)
106
density
kilo (k)
103
hecto (h)
102
deca (da)
101
exa (E)
10
15
1
deci (d)
io-
centi (c)
10-2
milli (m)
10-3
micro ( ju )
10'6
nano (n)
10-9
pico (p)
femto (f)
atto (a)
io-12
io-15
IQ-18
time
velocity
acceleration
temperature
luminous intensity
force
pressure
energy
power
potential difference
resistance
electric charge
electrical capacitance
inductance
electric current
frequency
chemical substance
Unit
metre (m)
square metre (m2)
cubic metre (m3)
cubic decimetre (dm3)
gram (g)
kilogram (kg)
megagram (Mg)
kilogram per cubic
metre (kg/m3)
second (s)
metre per second (m/s)
metre per second
squared (m/s2)
kelvin (K)
candela (cd)
newton (N)
pascal (Pa)
joule (J)
watt (W)
volt(V)
ohm ( Q )
coulomb (C)
farad (F)
henry (H)
ampere (A)
hertz (Hz)
mole (mol)
NOTES
•
When stating the numerical value of a measurement, a prefix should
preferably be selected which will give a value for the measurement
between 0.1 and 1000, e.g. 1234 litres should preferably be f.234
kilolitres.
• Multiples of the unit of mass are formed with the unit "gram" instead of
the base unit "kilogram".
• Pronunciation of the prefix and unit is not altered by their combination.
Kilogram, for example, is pronounced as if it were two words with
approximately equal emphasis on each. The prefix "kilo" is
pronounced "killo" regardless of the unit to which it is attached, e.g.
kilogram, kilometre, kilovolt etc.
•
For scientific and technical calculations all measurements should be
expressed in terms of the SI unit. Non-Si units should not be used in
such calculations, e.g. whereas it is perfectly in order in everyday
speech to talk of a 15 ml dose of medicine, a scientist performing
a calculation in SI would use 15 x 10'6 m3.
Prefix
& Symbol
giga (G)
mega (M)
kilo (k)
hecto (h)
deci (d)
centi (c)
milli (m)
micro (p.)
Multiplied by
Value
Factor
one thousand million
one million
one thousand
one hundred
one tenth
one hundredth
one thousandth
one millionth
1000000000
1000000
1000
100
0.1
0.01
0.001
0.000001
109
106
103
102
io-1
1Q-2
10~3
10'6
* Centimetre is a recognised unit of length but centigram is not a
recognised unit of mass.
* It is preferable to use prefixes to powers of ± 3.
TABLES OF MEASURES FOR
MASS, LENGTH, AREA AND VOLUME
ARE SET OUT BELOW
MASS SI Base unit: kilogram (kg)
1000micrograms(>"g)
1000 milligrams (mg)
1000 grams (g)
1000 kilograms (kg)
LENGTH SI Base unit: metre (m)
1000 micrometres (v m)
10 millimetres (mm)
10 centimetres (cm)
1000 millimetres (mm)
1000 metres (m)
AREA SI unit: square metre (m2)
100 square millimetres (mm2)
10000 square centimetres (cm2)
10000 square metres (m2)
100 hectares (ha)
VOLUME SI unit: cubic metre (m3)
1000 cubic centimetres (cm3)
=
=
=
=
=
1 milligram (mg)
1 gram (g)
1 kilogram (kg)
1 megagram (Mg)
1 tonne (t)
=
=
=
=
=
1 millimetre (mm)
1 centimetre (cm)
1 decimetre (dm)
1 metre (m)
1 kilometre (km)
=
=
=
=
1 square centimetre (cm2)
1 square metre (m2)
1 hectare (ha)
1 square kilometre (km2)
= 1 cubic decimetre (dm3)
= 1 litre (L)
1000 cubic decimetres (dm3)
= 1 cubic metre (m3)
= 1 kilolitre (kL)
Or alternatively, for use with liquids and gases:
1 cubic centimetre (cm3)
= 1 millilitre (ml)
lOOOmillilitres(mL)
= 1 litre (L)
= 1 kilolitre (kL)
1000 litres (L)
= 1 cubic metre (m3)