# How can I calculate a Reynolds number if I do not know flow or the diameter of a pipe

I am a freshman student working on a capstone project. I am doing a microhydro feasibility study for a homesteader in my area.

I am trying to optimize the size of the pipe that delivers water.

I am having trouble calculating the dynamic head. I am using the darcy weisbach equation to reduce the static head. In order to use this equation I need a coefficient called the "frictional factor". This coefficient is usually obtained using the reynolds number and the moody diagram. The problem with this for me is that I do not know flow through the pipe, the flow will be determined by the size of the pipe, and I want to choose the size of the pipe that minimizes friction (but isn't too expensive).

I have heard that one can do something called an "iteration". to find the frictional factor. something that involves supposing a value for flow and then running the calculations (for friction factor, and then the darcy weisbach, and then plugging them back in to the original equation for velocity?) I'm not exactly sure what that is, how to do it, or why it works.