Adsorption isotherm data is the basis of several materials characterization techniques. For a homogeneous material (or surface), the amount of gas adsorbed at given conditions should be proprtional to the surface area. If the same experiment has been performed on a reference material of known surface area, the surface area (per unit volume, say) of the sample material can be determined.
In practice, determination of surface area from adsorption at a single pressure and temperature is not very reliable. Instead, one analyzes adsorption over a particular range of pressure in order to extract the monolayer capacity (or related quantity) of the material. The monolayer capacity is the total adsorptive capacity of the material at a coverage (in molecules/area) corresponding to a dense adsorbed layer one molecule thick. In practice, the monolayer capacity is a parameter in various adsorption models, which can be determined by fitting the models to experimental data. Then,
surface area = monolayer capacity / monolayer density
where the inverse monolayer density is the area per molecule in the monolayer. This last quantity is mostly a property of the adsorbing gas, and mostly independent of the nature of the adsorbent. For instance, the area occupied by an adsorbed nitrogen molecule is frequently taken to be 14 (14 square Angstroms). For a typical adsorbent with a surface area of 100 , the monolayer capacity would be approximately mol/g.
In experimental work, amounts of gas are often given in units of cc(STP) - cubic centimeters of gas at Standard Temperature and Pressure (1.0 atm, 273.15 K); the monolayer capacity in these units would be 1.9 cc(STP)/g.
One can also extract a pore size distribution from gas adsorption data by fitting to the Kelvin equation or other thermodynamic model, but this is a more advanced topic.