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A unit of measure is
a value or magnitude conventionally adopted in physical
measurements. (as of length, surface, volume, time, weight, time,
etc…) , taken as a standard of comparison for others of the same
kind |
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In this web pages we adopted the
International System of Units (sometimes referred to as the
metric system, symbol: SI) that is is the most widely
used system of units in virtually every country of the world except
locally in Liberia, Myanmar and United States, and it is almost
globally used in scientific and engineering work.
There are seven base units and
several derived units, together with a
set of prefixes plus
some Non-SI units accepted for use with SI
Non-SI units can be converted to SI units (or vice versa) according
to the conversion of units. Virtually all non-SI units have been
redefined in terms of SI units. |
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SI Base units of measure
The following are the widely accepted fundamental units.
|
|
Name |
Symbol |
Quantity
|
|
metre |
m
|
Length |
|
kilogram |
kg
|
Mass |
|
second |
s
|
Time |
|
ampere |
A
|
Electrical Current |
|
kelvin |
K
|
Thermodynamic/Temperature
|
|
mole |
mol |
Amount of substance |
|
candela
|
cd
|
Luminous intensity |
SI Derived units with
special names
Base units can be put together to derive units of measurement
for other quantities. |
|
Name
|
Symbol
|
Quantity |
Expressed in base units |
|
hertz |
Hz
|
Frequency |
s-1 metre |
|
newton |
N
|
Force |
kg m s -2 |
|
joule |
J
|
Energy |
N m = kg m2 s-2
|
|
watt |
W |
Power |
J/s = kg m2 s-3
|
|
pascal |
Pa |
Pressure |
N/m2 = kg m -1 s-2
|
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lumen |
lm |
Luminous flux |
cd |
|
lux |
lx |
Illuminance |
cd m-2 |
|
coulomb |
C |
Electrical charge |
A s |
|
volt |
V |
Electrical potential difference |
J/C = kg m2 A-1 s-3
|
|
ohm |
Ω |
Electric resistance |
V/A = kg m2 A-2 s-3
|
|
farad |
F |
Electric capacitance |
Ω-1 s = A2 s4
kg-1 m-2 |
|
weber |
Wb |
Magnetic flux |
kg m2 s-2 A-1
|
|
tesla |
T |
Magnetic flux capacities |
Wb/m2 = kg s-2 A-1
|
|
henry |
H |
Indutance |
Ω s = kg m2 A-2 s-2
|
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siemens |
S |
Electric condutance |
Ω-1 = kg-1 m-2
A2 s3 |
|
becquerel |
Bq |
Radioactivity (decays per unit time)
|
s-1 |
|
gray |
Gy |
Absorbed dose (of ionising radiation)
|
J/kg = m2 s-2
|
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sievert |
Sv |
Equivalent dose (of ionising radiation)
|
J/kg = m2 s-2
|
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katal |
kat |
Catalytic activity |
mol/s = mol s-1 |
|
degree Celtsius |
°C |
thermodynamic temperature |
K - 273.15</sup> |
|
SI prefixes
Used to prefix any of the above units to produce a multiple or
submultiple of the original unit. |
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10n
|
Prefix |
Symbol |
Short scale
|
Long scale Name
|
Decimal Equivalent
|
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1024
|
yotta
|
Y
|
Septillion
|
Quadrillion
|
1 000 000 000
000 000 000 000 000 |
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1021
|
zetta
|
Z
|
Sextillion
|
Trilliard
(thousand trillion) |
1 000 000 000
000 000 000 000 |
|
1018
|
exa
|
E
|
Quintillion
|
Trillion
|
1 000 000 000
000 000 000 |
|
1015
|
peta
|
P
|
Quadrillion
|
Billiard
(thousand billion) |
1 000 000 000
000 000 |
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1012
|
tera
|
T
|
Trillion
|
Billion
|
1 000 000 000
000 |
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109
|
giga
|
G
|
Billion
|
Milliard
(thousand million) |
1 000 000 000
|
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106
|
mega
|
M
|
Million |
1 000 000
|
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103
|
kilo
|
k
|
Thousand |
1 000
|
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102
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hecto
|
h
|
Hundred |
100
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101
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deca
|
da
|
Ten |
10 |
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10−1
|
deci |
d
|
Tenth |
0.1
|
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10−2
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centi |
c
|
Hundredth |
0.01
|
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10−3
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milli |
m
|
Thousandth |
0.001
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10−6
|
micro |
μ
|
Millionth |
0.000 001
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10−9
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nano |
n
|
Billionth
|
Milliardth
|
0.000 000 001
|
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10−12
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pico |
p
|
Trillionth
|
Billionth
|
0.000 000 000
001 |
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10−15
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femto |
f
|
Quadrillionth
|
Billiardth
|
0.000 000 000
000 001 |
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10−18
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atto |
a
|
Quintillionth
|
Trillionth
|
0.000 000 000
000 000 001 |
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10−21
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zepto |
z
|
Sextillionth
|
Trilliardth
|
0.000 000 000
000 000 000 001 |
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10−24
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yocto |
y
|
Septillionth
|
Quadrillionth
|
0.000 000 000
000 000 000 000 001 |
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Non-SI units
accepted for use with SI |
|
Name
|
Symbol
|
Quantity
|
Equivalent SI
unit |
|
minute |
min
|
time |
1 min = 60
s |
|
hour |
h
|
time
|
1 h = 60 min =
3600 s |
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day |
d
|
time
|
1 d = 24 h =
1440 min = 86400 s |
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degree of arc |
°
|
angle |
1° = (π/180)
rad |
|
minute of arc |
′
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angle
|
1′ = (1/60)° =
(π/10800) rad |
|
second of arc |
″
|
angle
|
1″ = (1/60)′ =
(1/3600)° = (π/648000) rad |
|
litre |
l or
L |
volume |
0.001
m3 |
|
tonne |
t
|
mass |
1 t = 103
kg |
|
Empirical non-SI units accepted for use with SI |
|
electronvolt |
eV
|
energy |
1eV =
1.60217733 (49) × 10-19
J |
|
atomic mass unit |
u
|
mass |
1u = 1.6605402
(10) × 10-27 kg |
|
astronomical unit |
au
|
length |
1au =
1.49597870691 (30) × 1011
m |
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Other Non-SI units currently accepted for use with SI |
|
nautical mile |
nautical
mile |
length |
1 nautical mile
= 1852
m |
|
knot |
knot
|
speed |
1 knot = 1 nautical mile per hour =
(1852/3600) m/s
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are |
a
|
area |
1a = 1dam2
= 100
m2 |
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hectare |
ha
|
area
|
1ha = 100a =
10000 m2 |
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bar |
bar
|
pressure |
1 bar = 105Pa
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ångström, angstrom |
Å
|
length
|
1 Å = 0.1 nm =
10-10
m |
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barn |
b
|
area
|
1b = 10-28
m2 |
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Common Non-SI units
(non accepted for use with SI) |
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Name
|
Symbol
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Quantity
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Equivalent SI
unit |
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atmosphere |
atm. |
pressure |
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