The Weber number (We) is a dimensionless number used in fluid mechanics to quantify the relative importance of inertial forces to surface tension forces. It is commonly used in the study of fluid flows, especially those involving interfaces or surface phenomena. The Weber number is defined as the ratio of inertial forces to surface tension forces and is denoted by the symbol We.

In SI units, the Weber number (We) is defined as:

Where:

• $\rho$ is the density of the fluid,
• $U$ is the characteristic velocity of the flow,
• $L$ is a characteristic length scale,
• $\sigma$ is the surface tension of the fluid.