where is known as "friction factor" and is defined by,
                                        (20)
Now, recalling the energy equation for a steady incompressible flow,
                                        (21)
where is the head loss between two sections. With assumption of horizontal constant diameter pipe with fully developed flow,
                                        (22)
From Eqs. (19) and (22), we can determine head loss as,
                                        (23)
This is known as Darcy-Weisbach equation and is valid for fully developed, steady, incompressible horizontal pipe flow. If the flow is laminar, the friction factor will be independent on and simply,
                                        (24)