Here is a quick and handy calculator that will determine the required pipe internal diameter for a given flow rate and velocity requirement. All you have to do is enter the flow rate, select the flow rate units and then select your low and high velocities. Note that the metric flow rate velocities are in m/s and the USCS is in ft/s.
You have a choice of the following units for the flow rate: ft^3/s, ft^3/min, ft^3/h, gal/min, gal/h, gal/dy, MM gal/dy (millions of gallons), L/h, L/min, L/s, m^3/h.
This app has only been tested on the iPhone 3GS. There are no guarantees with it working on other devices.
Please leave feedback regarding this application at our support site. We will take this information into account for the future updates.