The activity coefficient , which is also a dimensionless quantity, relates the activity to a measured mole fraction  (or  in the gas phase), molality , mass fraction ,  molar concentration (molarity)  or mass concentration : 
The division by the standard molality  (usually 1 mol/kg) or the standard molar concentration  (usually 1 mol/L) is necessary to ensure that both the activity and the activity coefficient are dimensionless, as is conventional.
The activity depends on the chosen standard state and composition scale; for instance, in the dilute limit it approaches the mole fraction, mass fraction, or numerical value of molarity, all of which are different. However, the activity coefficients are similar.
When the activity coefficient is close to 1, the substance shows almost ideal behaviour according to Henrys law (but not necessarily in the sense of an ideal solution). In these cases, the activity can be substituted with the appropriate dimensionless measure of composition ,  or . It is also possible to define an activity coefficient in terms of Raoults law: the International Union of Pure and Applied Chemistry (IUPAC) recommends the symbol  for this activity coefficient, although this should not be confused with fugacity.