Pipes have two types of flow rates. The linear flow rate is the velocity of the fluid inside the pipe. The volumetric flow rate is the quantity of fluid that travels through the pipe over a given period of time. The pipe size can be calculated with both flow rates. Only one specific pipe size can have a certain linear and volumetric flow rate. Pipe size is measured by the internal diameter of the pipe.
Divide the volumetric flow rate by the linear flow rate. This will give the area of a cross section of the pipe.
Divide the area of the pipe by 3.14159.
Take the square root of the result from Step 2. This gives the internal radius of the pipe.
Multiply the internal radius of the pipe by 2. This is the internal diameter of the pipe. This value indicates the pipe size based on the linear and volumetric flow rate.