Read the problem and write down all the information that it gives you. Calculating volumetric flow through a pipe requires only three pieces of information: the cross-sectional area of the pipe, the forward velocity or speed of the fluid involved, and the angle at which your liquid flows through the pipe. In most cases, the problem gives you this information directly.
Derive any information that you still need from the numbers that the problem gives you. For example, if the problem only tells you the radius of the pipe, you will have to use this information to find the cross-sectional area. You can do this by squaring the radius and then multiplying it by pi.
Multiply the cross-sectional area of the pipe by the forward velocity of your fluid. For example, if the cross-sectional area of your pipe is 2 square meters and water flows through it at 5 meters per second, then you would multiply 5 by 2 to get a flow rate of 10 meters cubed per second.
Multiply this number by the cosine of the angle at which your liquid or gas flows. Most grade-school level physics problems will not require you to do this step, because physicists commonly use it in special cases reserved for times when the pipe is at an angle or the liquid cannot fill the pipe.
Format your answer with the correct units. Scientists usually express volumetric flow rates as units of fluid cubed/time, which makes sense because you get a volume of fluid over a time. The most common units for volumetric flow are meters cubed per second, the most basic standard scientific units involved.