Friction head loss (ftH2O per 100 ft pipe) in water pipes can be estimated with the empirical Hazen-Williams equation
The Darcy-Weisbach equation with the Moody diagram is considered to be the most accurate model for estimating frictional head loss for a steady pipe flow. Since the Darcy-Weisbach equation requires iterative calculation an alternative empirical head loss calculation like the Hazen-Williams equation may be preferred:
h = 0.2083 (100 / c)1.852 q1.852 / dh4.8655 (1)
h = 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" conditions 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)
The Hazen-Williams equation is assumed to be relatively accurate for water flow in piping systems when
- the Reynolds Numbers is above 105 (turbulent flow)
- the water temperature is in the range 40 - 75 oF (5 - 25oC) and the kinematic viscosity is approximately 1.1 cSt
For hotter water with lower kinematic viscosity (example 0.55 cSt at 130 oF (54.4 oC)) the error will be significant.
- 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
- en: hazen-williams equation