A: Area
C: Discharge coefficient
d: density ( water density = 1000kg/m³ )
D: Diameter
g: Gravity 9.8 m/s²
K: Flow coefficient (Kv)(Cv)
Q: Flow rate
SG: Specific gravity (1 for water)
v: Flow velocity
Δh: Head Drop (high of fluid)
ΔP: Pressure Drop
ζ: Pressure Drop coefficient zeta

ζ	=	π² · [	D	]4 / ( [	K		]² · 8000 )
			cm ft in km m mi mm nmi yd		cfs cfh cfm cm³/s gpm (US) in³/s l/h l/min l/s m³/h m³/min m³/s yd³/h yd³/min yd³/s	at atm bar inH2O inHg kp/cm² kPa lbf/ft² lbf/in² mbar mca mce mH2O mmca mmce mmH2O mmHg mmWC mmWs MPa mWC mWs N/mm² Pa PSI torr

K = Cv, when K is referenced in [gpm] [psi] units.
K = Kv, when K is referenced in [m³/h] [bar] units.

ζ	=	ΔP	· 2 / (	d	·	v ²	)
		at atm bar inH2O inHg kp/cm² kPa lbf/ft² lbf/in² mbar mca mce mH2O mmca mmce mmH2O mmHg mmWC mmWs MPa mWC mWs N/mm² Pa PSI torr		g/cm³ g/l g/m³ g/ml g/mm³ kg/cm³ kg/l kg/m³ lb/ft³ lb/in³ lb/gal (UK) lb/gal (US) mg/cm³ mg/l mg/m³ oz (Av)/gal (UK) oz (Av)/gal (US) oz (Av)/ft³ oz (Av)/in³ t/m³		cm/min cm/s ft/h ft/min ft/s fph fps in/s ips km/h km/min km/s m/h m/min m/s mi/s mi/min mph mps nmi/h yd/h yd/min yd/s