Describing the ratio of gravitational to capillary forces, the Eötvös or Bond number is given by the equation:
* : difference in density of the two phases, (SI units: kg/m)
* g: gravitational acceleration, (SI units : m/s)
* L: characteristic length, (SI units : m) (for example the radii of curvature for a drop)
* : surface tension, (SI units : N/m)
The Bond number can also be written as
where  is the capillary length.
A high value of the Eötvös or Bond number indicates that the system is relatively unaffected by surface tension effects; a low value (typically less than one) indicates that surface tension dominates. Intermediate numbers indicate a non-trivial balance between the two effects. It may be derived in a number of ways, such as scaling the pressure of a drop of liquid on a solid surface. It is usually important, however, to find the right length scale specific to a problem by doing a ground-up scale analysis. Other similar dimensionless numbers are:
where Go and De are the Goucher and Deryagin numbers, which are identical: the Goucher number arises in wire coating problems and hence uses a radius as a typical length scale while the Deryagin number arises in plate film thickness problems and hence uses a Cartesian length.
In order to consider all three of the forces that act on a moving fluid front in the presence of a gas (or other fluid) phase, namely viscous, capillary and gravitational forces, the generalized Bond number, which is denoted commonly as Bo, can be used. This is defined as: