The numerical value always precedes the unit, and a space is always used to separate the unit from the number. Thus the value of the quantity is the product of the number and the unit, the space being regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle, °, ', and ", respectively, for which no space is left between the numerical value and the unit symbol.
This rule means that the symbol °C for the degree Celsius is preceded by a space when one expresses values of Celsius temperature t.
Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. Only when the name of the unit is spelled out would the ordinary rules of grammar apply, so that in English a hyphen would be used to separate the number from the unit.
In any one expression, only one unit is used. An exception to this rule is in expressing the values of time and of plane angles using non-SI units. However, for plane angles it is generally preferable to divide the degree decimally. Thus one would write 22.20° rather than 22° 12', except in fields such as navigation, cartography, astronomy, and in the measurement of very small angles.
m = 12.3 g where m is used as a symbol for the quantity mass, but = 30° 22' 8", where is used as a symbol for the quantity plane angle.|
t = 30.2 °C,
but not t = 30.2°C,
nor t = 30.2° C
a 10 k resistor
a 35-millimetre film
L = 10.234 m,
L = 10 m 23.4 cm