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Valve Cv Flow Calculator

Estimate the required flow coefficient (Cv) for your valve — liquid, gas, or steam service. Based on ISA S75.01 / IEC 60534 simplified correlations.

What is Cv?

Cv is the flow coefficient — the volume of water (USGPM) that passes through a valve at a pressure drop of 1 psi. Higher Cv = higher flow capacity.

Kv vs Cv

Kv is the metric equivalent: flow in m³/h at 1 bar ΔP. Conversion: Kv = 0.865 × Cv. Used in European and IEC datasheets.

Why it matters

An undersized valve causes excessive pressure drop. An oversized valve operates near closed, causing instability, erosion, and poor control.

INDICATIVE ONLY — Engineering DisclaimerResults are approximate, based on simplified ISA S75.01 correlations. For preliminary sizing and budgetary purposes only. Not for safety-critical, design-basis, or regulatory compliance use. Always verify with a qualified process engineer, using full fluid property data and manufacturer Cv curves. Vajra Industrial Solutions accepts no liability for decisions made based on these calculations.
(USGPM)Volumetric flow in US gallons per minute
(psia)Absolute inlet pressure (gauge + 14.7)
(psia)Absolute outlet pressure
((water = 1.0))Fluid specific gravity relative to water at 60°F
(cSt)Kinematic viscosity in centistokes. 1 cSt for water.

Formula Reference (ISA S75.01)

Liquid: Cv = Q / √(ΔP / SG)

Gas (subsonic): Cv = Q / [N₇ · P1 · √(ΔP / (T · Gg · P1))]

Gas (choked): Cv = Q / [N₉ · P1 · √(1 / (T · Gg))]

Steam: Cv = W / [N₆ · √(ΔP · γ₁)]

All correlations are simplified per ISA S75.01 / IEC 60534. Compressibility (Z), pipe geometry (Fp), and liquid pressure recovery (FL) factors not applied — use full sizing software for final design.

Cv Calculation Methods — Liquid, Gas & Steam

Liquid (ISA S75.01)

  • › Uses: Q, P1, P2, SG, viscosity
  • › Choked flow check via ΔP/P1 ratio
  • › Viscosity correction for FR factor
  • › Cavitation warning at ΔP > 70%

Gas / Vapour

  • › Subsonic vs. choked flow split at P2/P1 = 0.528
  • › Uses: SCFH, psia, °R, specific gravity Gg
  • › Choked: uses N₉ constant
  • › Subsonic: uses N₇ constant

Steam

  • › Mass flow in lb/h
  • › Choked split at P2/P1 = 0.55
  • › Approximate steam density correlation
  • › Full sizing needs steam tables

Common Questions About Valve Cv

What Cv do I need for a 2-inch ball valve?
A standard 2-inch full-bore ball valve has a rated Cv of approximately 120–170 depending on the manufacturer and end connections. Use this calculator to check if that is sufficient for your flow conditions.
How do I convert Cv to Kv?
Multiply Cv by 0.865 to get Kv. For example, Cv 100 = Kv 86.5. Kv is flow in m³/h at 1 bar differential pressure.
Should I oversize or undersize a valve?
Neither. A valve should operate at 60–80% open at design flow. Oversizing causes control instability near the seat; undersizing causes excessive pressure drop and potential cavitation.
What is choked flow in a valve?
Choked (critical) flow occurs when the gas or steam velocity reaches sonic speed at the vena contracta. Further reducing P2 does not increase flow. For gases, choking occurs when P2/P1 < 0.528 (for air; varies with Cp/Cv ratio).