The bottom most curve shown for represents the friction factor for nearly hydraulically smooth tube or pipe, for which the friction factor has reached the smallest value with increasing smoothness.
Now, we have an expression to calculate energy loss for a pipe–flow under both laminar and turbulent conditions:
where,
A common empirical formula to calculate friction factor is proposed in literature:
represents the friction factor for nearly hydraulically smooth tube or pipe, for which the friction factor has reached the smallest value with increasing smoothness. 
for a pipe–flow under both laminar and turbulent conditions:
