What is a flow coefficient?
The flow coefficient tells you how much fluid can pass through a valve at a given pressure drop. In valve sizing, this value is one of the first numbers engineers check because it quickly links valve capacity, pressure loss, and fluid properties.
In US practice, the coefficient is usually expressed as Cv. In metric practice, it is typically Kv. They represent the same idea, but with different units.
Cv = Q × √(SG / ΔP) (US: Q in gpm, ΔP in psi)
Kv = Q × √(SG / ΔP) (Metric: Q in m³/h, ΔP in bar)
Cv ≈ 1.156 × Kv and Kv ≈ 0.865 × Cv
How to use this flow coefficient calculator
1) Pick what you want to solve
- Calculate required flow coefficient if you know flow rate and pressure drop.
- Calculate flow rate if you already have a valve coefficient and pressure drop.
2) Select your unit system
Choose US customary for gpm / psi / Cv, or metric for m³/h / bar / Kv. The calculator automatically updates labels and formula interpretation.
3) Enter specific gravity
Specific gravity (SG) adjusts for liquid density relative to water. For water near room temperature, SG is approximately 1.0.
4) Calculate and review
The result panel reports the primary answer and also gives the equivalent Cv or Kv conversion to help with cross-standard valve datasheets.
Example: quick valve sizing
Suppose you need to pass 80 gpm of a fluid with SG = 0.95, and the available pressure drop is 12 psi:
- Cv = 80 × √(0.95 / 12)
- Cv ≈ 22.5
You would typically select a valve trim with a rated Cv somewhat above this minimum to maintain control authority and account for uncertainty.
Important engineering notes
Applicability
This tool is intended for incompressible liquid flow and preliminary sizing. Gas and steam sizing require compressible-flow equations with additional factors (expansion factor, critical pressure ratio, temperature effects, and sonic limits).
Real-world factors that affect final valve selection
- Valve characteristic (linear, equal percentage, quick opening)
- Control range and expected operating points (not only design point)
- Cavitation, flashing, and noise risk at high differential pressure
- Viscosity corrections for non-water-like fluids
- Installed vs. inherent valve behavior within the piping system
Cv vs Kv: when each is used
Manufacturers in North America often quote Cv, while many international catalogs list Kv. If your process data and valve catalog use different systems, use the conversion shown in the calculator output.
Frequently asked questions
Is a higher Cv always better?
Not necessarily. Oversized valves can lead to poor controllability and unstable operation at low openings.
What SG should I use for mixtures?
Use the expected operating specific gravity at process temperature. If SG varies significantly, run a sensitivity check at minimum and maximum values.
Can I use this for gases?
No. Use gas/steam valve sizing equations or ISA/IEC methods for compressible flow.