The Weber number may be written as:
where
*  is the drag coefficient of the body cross-section.
*  is the density of the fluid (kg/m).
*  is its velocity (m/s).
*  is its characteristic length, typically the droplet diameter (m).
*  is the surface tension (N/m).
The modified Weber number,
equals the ratio of the kinetic energy on impact to the surface energy,
where
and