Measurement consists of packing the powder into a cylindrical "bed" having a known porosity (i.e. volume of air-space between particles divided by total bed volume).  A pressure drop is set up along the length of the bed cylinder.  The resulting flow-rate of air through the bed yields the specific surface by the Kozeny–Carman equation:
where:
:S is specific surface, m·kg
:d is the cylinder diameter, m
:ρ is the sample particle density, kg·m
:ε is the volume porosity of the bed (dimensionless)
:δP is the pressure drop across the bed, Pa
:l is the cylinder length, m
:η is the air dynamic viscosity, Pa·s
:Q is the flowrate, m·s
It can be seen that the specific surface is proportional to the square root of the ratio of pressure to flow.  Various standard methods have been proposed:
* Maintain a constant flowrate, and measure the pressure drop
* Maintain a constant pressure drop, and measure the flowrate
* Allow both to vary, deriving the ratio from the characteristics of the apparatus.