The Darcy-Weisbach equation with the Moody diagram are considered to be the most accurate model for estimating frictional head loss in steady pipe flow. Since the approach requires a not so efficient trial and error iteration an alternative empirical head loss calculation like the Hazen-Williams equation may be preferred. source: http://www.engineeringtoolbox.com/hazen-williams-water-d_797.html
The stated source gives an online calculator using Hazen -Williams equation. Manning's formula can also be used for gravity driven flows in open channels.