What Is Cv Value? Valve Flow Coefficient

Cv (flow coefficient) is a dimensionless number that expresses a valve’s flow capacity. It is defined as the number of US gallons per minute of water at 60 degF that flows through a fully open valve with a pressure drop of 1 psi. A valve with Cv = 100 passes 100 US GPM of water at 1 psi differential pressure.

Cv is the single most important parameter for sizing control valves. It determines whether the valve is large enough to pass the required flow at the available pressure drop.

Cv Definition and Formula

Parameter	Symbol	Unit
Flow coefficient	Cv	US GPM at 1 psi delta-P (water, 60 degF)
Flow rate	Q	US gallons per minute (GPM)
Pressure drop	delta-P	psi (P1 - P2)
Specific gravity	G	Relative to water (water = 1.0)

Liquid flow formula:

Cv = Q x sqrt(G / delta-P)

Example: A valve must pass 200 GPM of water (G=1.0) with 4 psi pressure drop.

Cv = 200 x sqrt(1.0 / 4) = 200 x 0.5 = 100

A valve with rated Cv of 100 or higher is required.

Cv Values by Valve Type and Size

Valve Type	2”	4”	6”	8”	12”
Full bore ball valve	135	640	1,470	2,640	5,400
Reduced bore ball valve	90	400	900	1,600	3,600
Gate valve (full open)	120	590	1,350	2,500	5,100
Globe valve	30	110	210	340	700
Butterfly valve (90 deg open)	80	480	1,200	2,200	4,800

Globe valves have the lowest Cv per size because of their tortuous S-shaped flow path. Full bore ball and gate valves have the highest Cv because of their unobstructed through-bore.

Cv and Valve Sizing

The ISA/IEC 60534 standard provides comprehensive sizing equations for control valves:

Service	Sizing Considerations
Liquid (non-flashing)	Cv = Q x sqrt(G / delta-P)
Liquid (flashing/cavitating)	Correction for FL (pressure recovery factor)
Gas/vapor	Different formula using specific heat ratio, upstream pressure
Steam	Empirical formula using steam tables
Two-phase	Sum of liquid and gas Cv requirements

Cv at Partial Opening (Flow Characteristic)

Control valves do not operate at full-open Cv. The relationship between valve opening and Cv depends on the flow characteristic:

Characteristic	Behavior	Application
Linear	Cv proportional to % open (50% open = 50% Cv)	Liquid level control
Equal percentage	Small Cv change at low opening; large change at high opening	Most process control loops
Quick opening	Large Cv change at low opening; small change at high opening	On/off, relief bypass

Cv vs Kv

Parameter	Cv	Kv
Units	US gallons per minute	Cubic meters per hour
Test medium	Water at 60 degF (15.6 degC)	Water at 5-30 degC
Pressure drop	1 psi	1 bar
Primary usage	North America, oil and gas	Europe, international
Conversion	Cv = 1.156 x Kv	Kv = 0.865 x Cv

Oversizing and Undersizing

Problem	Cause	Consequence
Undersized	Cv too low for required flow	Valve fully open, no control range; cannot deliver design flow
Oversized	Cv much higher than required	Valve operates at very low opening; poor control, high wear, noise
Correct sizing	Cv provides 60-80% opening at normal flow	Full control range available; good rangeability

Pro Tip

Size control valves so the normal operating point falls at 60-80% of valve travel. This leaves room for upward control (increased flow) while avoiding the very-low-opening regime where control accuracy degrades. A valve operating at 10% open with 90% of its Cv unused is oversized and will hunt and oscillate.

Read the full guide to valve types