Guide for the Use of the International System of Units (SI)

4.2.1.1 Degree Celsius
In addition to the quantity thermodynamic temperature (symbol T), expressed in the unit kelvin, use 
is also made of the quantity Celsius temperature (symbol t) defined by the equation
t = T − T
0,

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Guide for the Use of the International System of Units (SI) 
 
6 
where T
0
= 273.15 K by definition. To express Celsius temperature, the unit degree Celsius, symbol ºC, 
which is equal in magnitude to the unit kelvin, is used; in this case, “degree Celsius” is a special name used 
in place of “kelvin.” An interval or difference of Celsius temperature, however, can be expressed in the unit 
kelvin as well as in the unit degree Celsius (see Sec. 8.5). (Note that the thermodynamic temperature T
0
is 
exactly 0.01 K below the thermodynamic temperature of the triple point of water (see Sec. A.6).) 
4.2.2 Use of SI derived units with special names and symbols 
Examples of SI derived units that can be expressed with the aid of SI derived units having special 
names and symbols are given in Table 4.
Table 4. Examples of SI coherent derived units expressed with the aid of SI derived units having special 
names and symbols. 
 
SI coherent derived unit 
Derived quantity 
Name 
Symbol 
Expression in terms of 
 SI base units 
dynamic viscosity 
pascal second 
Pa · s 
m
−1
· kg · s
−1
moment of force 
newton meter 
N · m 
m
2
· kg · s
−2
surface tension 
newton per meter 
N/m 
kg · s
–2
 
angular velocity 
radian
per second 
rad/s 
m · m
−1
· s
−1
= s
−1
 
angular acceleration 
radian
per second squared
rad/s
2
m 
· m
−1
· s
−2
= s
−2
heat flux density,
irradiance 
watt per square meter 
W/m
2
kg 
· s
−3
 
heat capacity, entropy 
joule per kelvin 
J/K 
m
2
· kg · s
−2
· K
−1
specific heat capacity,
specific entropy 
joule per kilogram kelvin 
J/(kg · K) 
m
2
· s
−2
· K
−1