I see you asked this same question on the Math Help Forum - I think the response there is appropriate:

Calculating the friction of fluid flow in a pipe is a big subject. Factors to consider include the flow's Reynolds number, turbulent versus non-turbulent flow, and pipe roughness. I suggest you start by reviewing the following site, and if you have further follow-up questions we'll try our best to answer them:

Flow in pipe - Bernoulli equation