Metric Units





Symbols for units begin with a capital letter when they are named after an individual (for example, ampere, A; kelvin, K; hertz, Hz; coulomb, C). Otherwise they always begin with a lower case letter (for example, metre, m; second, s; mole, mol).

SI Metric Base Quantities and Units

There are seven base quantities in the SI system, with the following units:

Quantity Unit Symbol
Length Meter m
Mass Kilogram kg
Time Second s
Electric Current Ampere A
Temperature Kelvin K
Luminous intensity Candela cd
Amount of Substance Mole mol


Derived Units with Special Names in the SI
Derived units are formed by combining the base units according to the algebraic relations linking the corresponding quantities. There is no set number of derived units, as the possibilities are endless. The below 22 derived units have special names for convenience of reference, as they are frequently used.

 Derived
Quantity
Name of
Derived Unit
Symbol
for Unit
 Expression in Terms
of Other Units
plane angle
radian
rad m/m = 1
solid angle steradian
sr
m2/m2 = 1
frequency
hertz
Hz
s-1
force
newton
N
m kg s-2
pressure, stress pascal
 Pa N/m2 = m-1 kg s-2
energy, work, amount of heat
joule
J N m = m2 kg s-2
power, radiant flux watt
W
J/s = m2 kg s-3
electric charge, amount
of electricity
coulomb
C
sA
electric potential difference
volt
V
W/A = m2 kg s-3A-1
capacitance
farad
F C/V = m-2 kg-1 s4 A2
 electric resistance ohm
 O V/A = m2 kg s-3 A-2
electric conductance siemens S
A/V = m-2 kg-1 s3 A2
magnetic flux weber
Wb
V s = m2 kg s-2 A-1
magnetic flux density
tesla
T
Wb/m2 = kg s-2 A-1
inductance
henry
H
Wb/A = m2 kg s-2 A-2
Celsius temperature
degree Celsius
0C
K
luminous flux
lumen
lm cd sr = cd
 illuminance lux
lx
lm/m2 = m-2 cd
activity referred to a radionuclide becquerel
Bq s-1
absorbed dose, specific energy
(imparted), kerma
gray
Gy
J/kg = m2 s-2
dose equivalent, ambient dose
equivalent
sievert
Sv
J/kg = m2 s-2
catalytic activity katal kat
s-1 mol


Non-SI Units Accepted for Use with the International System of Units
There are many non-SI units that are accepted for use with the International System, because they are widely used in everyday life, are of historical importance or provide an advantage or convenience in a particular scientific, technological or commercial specialty. Below are some non-SI units that are accepted for use with the International System because they are widely used with the SI in matters of everyday life. Their use is expected to continue indefinitely, and each has an exact definition in terms of an SI unit.

Quantity
Name of Unit
Symbol for Unit
Value in SI Units
 Time Minute
min
1 min = 60 s
  Hour
h
1 h = 60 min = 3600 s
  Day
d
1 d = 24 h = 86 400 s
Plane angle
Degree
 o 1o = (p/180) rad
  Minute
 ' 1' = (1/60)o = (p/ 10 800) rad
  Second
"
1? = (1/60)' = (p/ 648 000) rad
Area Hectare
ha
1 ha = 1 hm2 = 104 m2
Volume
Liter
L, l
1 L = 1 l = 1 dm3 = 103 cm3 = 10-3 m3
 Mass Tonne
t
1 t = 103 kg

Notes:
  • Numerous other Non-SI Units are accepted for use only in special circumstances. See the BIPM Brochure, International System of Units (SI).
  • The symbol for the liter is an exception to the rule of using capital letters for symbols of units named after an individual. Either a lower case letter or a capital L may be used, the capital being allowed in this case to avoid confusion between the lower case letter l (el) and the number one, 1 (one).
  • In English speaking countries, the tonne is usually called “metric ton”.