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In fluid dynamics, the **Cunningham correction factor** or **Cunningham slip correction factor** is used to account for noncontinuum 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 number. 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, C given below.

Ebenezer Cunningham^{[1]} derived the correction factor in 1910 and with Robert Andrews Millikan, verified the correction in the same year.

where

*C*is the correction factor- λ is the mean free path
*d*is the particle diameter*A*are experimentally determined coefficients._{n}- For air (Davies, 1945):
^{[2]}*A*_{1}= 1.257*A*_{2}= 0.400*A*_{3}= 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.

**^**Cunningham, E., "On the velocity of steady fall of spherical particles through fluid medium,"*Proc. Roy. Soc. A*83(1910)357. doi:10.1098/rspa.1910.0024**^**Davies, C. (1945). "Definitive equations for the fluid resistance of spheres".*Proceedings of the Physical Society*.**57**(4): 259. Bibcode:1945PPS....57..259D. doi:10.1088/0959-5309/57/4/301.