In fluid dynamics, the Morton number (Mo) is a dimensionless number used together with the Eötvös number or Bond number to characterize the shape of bubbles or drops moving in a surrounding fluid or continuous phase, c. It is named after Rose Morton, who described it with W. L. Haberman in 1953.

The Morton number is defined as

where g is the acceleration of gravity, ${\displaystyle \mu _{c}}$ is the viscosity of the surrounding fluid, ${\displaystyle \rho _{c}}$ the density of the surrounding fluid, ${\displaystyle \Delta \rho }$ the difference in density of the phases, and ${\displaystyle \sigma }$ is the surface tension coefficient. For the case of a bubble with a negligible inner density the Morton number can be simplified to

The Morton number can also be expressed by using a combination of the Weber number, Froude number and Reynolds number,

The Froude number in the above expression is defined as

where V is a reference velocity and d is the equivalent diameter of the drop or bubble.