text: In fluid dynamics, the Cunningham correction factor, or Cunningham slip correction factor (denoted ), is used to account for non-continuum effects when calculating the drag on small particles. The derivation of Stokes' law, which is used to calculate the drag force on small particles, assumes a no-slip condition which is no longer correct at high Knudsen numbers. The Cunningham slip correction factor allows predicting the drag force on a particle moving a fluid with Knudsen number between the continuum regime and free molecular flow.
  The drag coefficient calculated with standard correlations is divided by the Cunningham correction factor, , given below.
  Ebenezer Cunningham derived the correction factor in 1910 and with Robert Andrews Millikan, verified the correction in the same year.
  where
  * is the correction factor
  * is the mean free path
  * is the particle diameter
  * are experimentally determined coefficients.
  :For air (Davies, 1945):
  ::A = 1.257
  ::A = 0.400
  ::A = 0.55
  The Cunningham correction factor becomes significant when particles become smaller than 15 micrometers, for air at ambient conditions.
  For sub-micrometer particles, Brownian motion must be taken into account.