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Fluid Mechanics

Liquid Valve Cv Calculator

The valve flow coefficient Cv expresses how much liquid a valve passes for a given pressure drop. In the water-service definition, Cv is the flow in US gallons per minute of water (specific gravity 1.0) that produces a 1 psi drop across the fully relevant opening. This calculator works in three modes: it finds Cv from flow and pressure drop, the pressure drop from flow and Cv, or the flow from Cv and pressure drop, all for a single-phase turbulent liquid. It converts your metric inputs (m³/h, kPa) into the gpm/psi basis the relationship is written in, and also reports the Kv equivalent. Treat the answer as a screening estimate: it is the simple, non-choked liquid relationship and deliberately excludes the corrections that real control-valve sizing needs.

TypeInteractive engineering calculator

Calculator

Result
Flow coefficient (Cv)163.497
Kv equivalent141.434
Liquid flow rate (Q)100 m³/h
Pressure drop (ΔP)50 kPa
Flow (sizing basis)440.287 gpm
ΔP (sizing basis)7.25188 psi
Simple turbulent liquid sizing screen only — not a control-valve selection. Excludes gas, steam, flashing, cavitation, choked flow, two-phase, slurry, viscosity correction, noise, and actuator sizing. Confirm with vendor data and the applicable standard before sizing.

Formulas

Flow from Cv
Q_gpm = Cv × √(ΔP_psi / SG)
Cv from flow and ΔP
Cv = Q_gpm × √(SG / ΔP_psi)
ΔP from flow and Cv
ΔP_psi = SG × (Q_gpm / Cv)²
Unit basis
Q_gpm = Q_m³h × 4.402867 · ΔP_psi = ΔP_kPa / 6.89476 · Kv ≈ Cv / 1.156

Diagram

Liquid Valve Cv: Q_gpm = Cv·√(ΔP_psi / SG)P₁P₂ΔP = P₁ − P₂Q_gpm = Cv·√(ΔP_psi / SG)Kv ≈ Cv / 1.156

Worked example

Water (SG = 1.0) flows at 100 m³/h with a 50 kPa drop across the valve. Find the required Cv and its Kv equivalent.

  1. 01Q_gpm = 100 × 4.402867 = 440.3 gpm
  2. 02ΔP_psi = 50 / 6.89476 = 7.252 psi
  3. 03Cv = 440.3 × √(1.0 / 7.252) = 163.5
  4. 04Kv = 163.5 / 1.156 = 141.4
Result

Required Cv ≈ 163.5 (Kv ≈ 141.4) for 100 m³/h of water at a 50 kPa drop.

FAQ

What is the difference between Cv and Kv?
They are the same idea in different units. Cv is gpm of water per 1 psi drop (US/imperial basis); Kv is m³/h of water per 1 bar drop (metric basis). They are related by Kv ≈ Cv / 1.156. This tool reports both so you can match a vendor datasheet in either convention.
Is this a control-valve sizing tool?
No. It is the simple turbulent-liquid Cv relationship used as a screening check. Real control-valve sizing also accounts for flashing, cavitation, choked flow, viscosity, the installed flow characteristic, rangeability, noise, and actuator forces — use vendor software and the applicable standard for selection.
Which pressure drop do I enter?
The pressure drop across the valve at the flow of interest — not the total system pressure drop. Valve ΔP is only part of the system loss, and it changes with flow, so size against the relevant operating case (often the maximum required flow).
Can I use it for hydrocarbons or other liquids?
For a clean, low-viscosity, single-phase liquid you can enter its specific gravity and get a screening Cv. It is not valid where the liquid flashes or cavitates across the valve, for high-viscosity service, or for slurries — those need the proper corrections and vendor data.

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