The Freundlich adsorption isotherm is mathematically expressed as
In Freundlichs notation (used for his experiments dealing with the adsorption of organic acids on coal in aqueous solutions),  signifies the ratio between the adsorbed mass or adsorbate  and the mass of the adsorbent , which in Freundlichs studies was coal. In the figure above, the x-axis represents , which denotes the equilibrium concentration of the adsorbate within the solvent.
Freundlich's numerical analysis of the three organic acids for the parameters  and  according to equation 
 were: 
Freundlich's experimental data can also be used in a contemporary computer based fit. These values are added to appreciate the numerical work done in 1907.  
△ K  and △ n values are the error bars of the computer based fit. The K and n values itself are used to calculate the dotted lines in the figure. 
Equation  can also be written as
Sometimes also this notation for experiments in the gas phase can be found:
: = mass of adsorbate 
: = mass of adsorbent
: = equilibrium pressure of the gaseous adsorbate in case of experiments made in the gas phase (gas/solid interaction with gaseous species/adsorbed species)
 and  are constants for a given adsorbate and adsorbent at a given temperature (from there, the term isotherm needed to avoid significant gas pressure fluctuations due to uncontrolled temperature variations in the case of adsorption experiments of a gas onto a solid phase).
: = distribution coefficient 
: = correction factor
At high pressure , hence extent of adsorption becomes independent of pressure.
The Freundlich equation is unique; consequently, if the data fit the equation, it is only likely, but not proved, that the surface is heterogeneous. The heterogeneity of the surface can be confirmed with calorimetry. Homogeneous surfaces (or heterogeneous surfaces that exhibit homogeneous adsorption (single site)) have a constant  of adsorption. On the other hand, heterogeneous adsorption (multi-site) have a variable  of adsorption depending on the percent of sites occupied. When the adsorbate pressure in the gas phase (or the concentration in solution) is low, high-energy sites will be occupied first. As the pressure in the gas phase (or the concentration in solution) increases, the low-energy sites will then be occupied resulting in a weaker  of adsorption.