Friction head loss (ftH2O per 100 ft pipe) in water pipes can be estimated by using the empirical Hazen-Williams equation
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:
f = 0.2083 (100 / c)1.852 q1.852 / dh4.8655 (1)
f = friction head loss in feet of water per 100 feet of pipe (fth20/100 ft pipe)
q = volume flow (gal/min)
dh = inside hydraulic diameter (inches)
Note that the Hazen-Williams formula is empirical and lacks a theoretical basis. Be aware that the roughness constants are based on "normal" condition with approximately 1 m/s (3 ft/sec).
- free apps for offline use on mobile devices.
Online Hazens-Williams Calculator
The calculators below can used to calculate the specific head loss (head loss per 100 ft (m) pipe) and the actual head loss for the actual length of pipe:
The flow velocity can be calculated as
v = 0.408709 q / dh2 (2)
v = flow velocity (ft/s)
- 1 ft (foot) = 0.3048 m
- 1 in (inch) = 25.4 mm
- 1 gal (US)/min =6.30888x10-5 m3/s = 0.227 m3/h = 0.0631 dm3(liter)/s = 2.228x10-3 ft3/s = 0.1337 ft3/min = 0.8327 Imperial gal (UK)/min
The results for the formula is acceptable for cold water at 60 oF (15.6 oC) with kinematic viscosity 1.13 cSt. For hot water with a lower kinematic viscosity (0.55 cSt at 130 oF (54.4 oC)) the error will be significant.