Interstitial velocity is a measurement of how fast water flows through a medium in a particular direction. Knowing how to calculate interstitial velocity can be useful in a variety of situations, such as in evaluating the effectiveness of a water filter. The equation for determining water speed is simple to solve when you know the values for hydraulic conductivity, hydraulic gradient and the porosity of the medium that the water passes through.

Set up the mathematical equation V = (CG)/P. In this equation, V represents interstitial velocity, C stands for hydraulic conductivity, G is the hydraulic gradient and P is the medium's porosity.

Insert the values of the hydraulic conductivity, hydraulic gradient and porosity into the equation. For example, if the conductivity is 100 feet per day with a gradient of 10 feet and the medium's porosity is .01, then the equation is V = (100 X 10)/.01.

Solve the equation by multiplying the conductivity and the gradient. Then, divide that number by the medium's porosity. In the example of V = (100 X 10)/.01, the interstitial velocity is 100,000 feet per day.