Theoretical ‘cut-size' of a cyclone is the particle size above which all particles will be collected. A theoretical expression considering drag and centrifugal forces on a particle, has been obtained to estimate the ‘cut size' of cyclone. The calculation takes into account the experimental observation that the equilibrium rotation-radius of all captured particles in cyclone is do, where do is the diameter of the nozzle at the top of the cyclone though which the gas exits.
The settling velocity of captured particles,
where,
Form , the theoretical cut–diameter, dp is determined from the settling velocity equation:
(Note that it is assumed that particles settle in Stoke's regime)
do, where do is the diameter of the nozzle at the top of the cyclone though which the gas exits. 
, the theoretical cut–diameter, dp is determined from the settling velocity equation:
