Multiply four by the volumetric flow rate to obtain a product. Then multiply the pipe diameter times itself and times pi to obtain a second product. Divide the first product by the second product and set this value equal to the velocity of the fluid to obtain the answer.
There are two different types of flow in a pipe: steady and turbulent. Steady flow means that the fluid is flowing consistently throughout the pipe so all properties are uniform throughout the pipe and can be analyzed at any point of the flow to obtain a value for it. Turbulent flow is not consistent and has to be analyzed locally in order to obtain values for properties in that immediate vicinity. If the flow is turbulent, the flow is much more complex, and many more factors must be considered, such as the density of the fluid, the kinematic viscosity of the fluid and the Reynolds number, all of which vary depending on the type of fluid.
For most everyday applications, it is sufficient to assume the flow is steady. When using equations regarding fluid flow the diameter mentioned is the inner diameter of the pipe, the volumetric flow rate is the amount of water that passes through per unit time, and pi is a constant with an approximate value of 3.14.