The holding time or mean residence time of materials in a continuous flow reactor required to process one reactor volume of feed under specified conditions to achieve a desired product composition. For a flow reactor with volume, V, and a total volumetric flow rate, Q, at the inlet:

The reciprocal is the space velocity, which is the number of reactor volumes under specified conditions that can be processed per unit time. The space time yield is the net yield of a product from a reactor per unit time per unit of effective reactor volume.