The output is in either imperial or metric units, depending on your selection. Units that are commonly used to measure volumetric flow rate are: m 3 /s (cubic meter per second), L/min (liters per minute), ft 3 /sec (cubic feet per second), ft 3 /min (cubic feet per minute, or CFM), and gal/min (gallons per minute, or GPM). By filling in the flow rate, it calculates the velocity of flow inside pipes in different diameter from DN15 to DN400 automatically. depth of flow. Cross-Sectional Area. This tool will calculate the flow velocity in any units from the specified volumetric flow rate of a substance flowing through a round duct or pipe of a chosen diameter. Enter the cross-sectional area perpendicular to the direction of flow of the substance through a pipe, duct or channel at the point of measurement. Hydraulic and Pneumatic Knowledge document.write('