The concept of the theory is an extension of the Langmuir theory, which is a theory for monolayer molecular adsorption, to multilayer adsorption with the following hypotheses:
# gas molecules physically adsorb on a solid in layers infinitely;
# gas molecules only interact with adjacent layers; and
# the Langmuir theory can be applied to each layer.
# the enthalpy of adsorption for the first layer is constant and greater than the second (and higher).
# the enthalpy of adsorption for the second (and higher) layers is the same as the enthalpy of liquefaction. 
The resulting BET equation is
where c is referred to as the BET C-constant, is the vapor pressure of the adsorptive bulk liquid phase which would be at the temperature of the adsorbate and θ is the surface coverage, defined as:
Here is the amount of adsorbate and  is called the monolayer equivalent.  The  is the entire amount that would be present as a monolayer (which is theoretically impossible for physical adsorption) would cover the surface with exactly one layer of adsorbate.  The above equation is usually rearranged to yield the following equation for the ease of analysis:
where  and  are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, respectively;  is the adsorbed gas quantity (for example, in volume units) while  is the monolayer adsorbed gas quantity.  is the BET constant,
where  is the heat of adsorption for the first layer, and  is that for the second and higher layers and is equal to the heat of liquefaction or heat of vaporization.
Equation (1) is an adsorption isotherm and can be plotted as a straight line with  on the y-axis and  on the x-axis according to experimental results.  This plot is called a BET plot.  The linear relationship of this equation is maintained only in the range of .  The value of the slope  and the y-intercept  of the line are used to calculate the monolayer adsorbed gas quantity  and the BET constant . The following equations can be used:
The BET method is widely used in materials science for the calculation of surface areas of solids by physical adsorption of gas molecules.  The total surface area  and the specific surface area  are given by
where  is in units of volume which are also the units of the monolayer volume of the adsorbate gas,
 is the Avogadro number,  the adsorption cross section of the adsorbate,  the molar volume of the adsorbate gas, and  the mass of the solid sample or adsorbent.