The Kv to Cv conversion for butterfly valves is the linear IEC 60534-2-1 factor Cv = 1.156 × Kv, equivalently Kv = 0.865 × Cv, with the constant originating from the unit gap between US gallons per minute at 1 psi (Cv) and cubic metres per hour at 1 bar (Kv) [S1][S2][S3].
The two coefficients describe the same physical flow capacity of a butterfly valve at a defined opening; they differ only in unit system and the stated reference pressure drop, not in the underlying hydraulic test [S4][S7]. For a 6-inch DN150 resilient-seated wafer butterfly, the published full-open Cv of 1,450 equals a Kv of 1,255 using the 0.865 inverse factor [S4].
Where the 1.156 Factor Comes From
The 1.156 multiplier is not a fitted constant; it is a closed-form unit conversion built from the two reference conditions, with 1 bar = 14.5038 psi and 1 m³ = 264.172 US gallons, so Kv/Cv = (3.7854 L × 60 / 1000) × √14.5038 = 0.2271 × 3.8084 ≈ 0.8650 [S2]. Stated the other way, Cv = Kv × 1/0.8650 = 1.156 [S3][S6][S8].
The reference fluids are also slightly different: Cv uses water at 60 °F, while Kv uses water at 15–20 °C (IEC 60534-2-1 allows the 5–40 °C band), and the 1 °C–2 °C offset in water density changes the resulting flow by under 0.1%, an order of magnitude smaller than the unit-system gap [S2][S5]. This is why a single linear factor is adequate for control-valve sizing work; the more subtle temperature correction is folded into the published test data [S1].
Reading Manufacturer Curves: Concentric vs High-Performance Butterfly
A DN100 (4-inch) resilient-seated concentric butterfly valve typically carries a full-open Cv of 550 and Kv of 476, while the same nominal size in a double-offset high-performance butterfly (HPBV) lists Cv 700 and Kv 606, roughly 27% more capacity from the same line size because the disc clears the seat on every stroke [S4]. By DN300 (12 inch) the gap widens to Cv 8,500 (Kv 7,353) for HPBV versus Cv 7,500 (Kv 6,488) for concentric designs, a 13% advantage that compounds when the valve is operated below 90° [S4].
For DN150 (6 inch) throttling service, the Cv-at-60° value typically falls to about 110–140 on a resilient-seated unit, and that partial-open Cv is what the control loop actually has available, not the full-open number most catalogs headline [S4]. The 1.156 conversion applies at every opening, so a published partial-open Kv 95 is exactly Kv 109.8 in Cv terms, but only if the Kv itself is read at the same disc angle the Cv value was tabulated at [S1][S4].
IEC 60534-2-1 Sizing Equation and Where 1.156 Lives in It
The turbulent, non-choked liquid sizing form is Cv = Q × √(SG / ΔP) and Kv = Q × √(SG / ΔP), with Q in US gpm and m³/h respectively and ΔP in psi and bar; the 1.156 factor is what bridges the two forms when the same process duty is written in mixed units [S1][S5][S8]. The IEC shorthand is often shown as Cv = 1.156 × Q × √(G/ΔP) when the engineer plugs metric inputs into a US-unit datasheet, a common error in projects where European and American vendors quote on the same line [S3][S5].
Compressible service breaks the simple linear relationship by 30–50% because the IEC 60534-2-1 gas and steam modules add the expansion factor Y and the pressure-drop ratio xT, and the 1.156 short-cut is only valid while the flow stays in the turbulent, non-choked, low-ΔP regime [S5]. For a 6-inch butterfly on a 4 bar steam let-down, the naive liquid Cv underestimates required capacity because choked flow dominates past the valve pressure recovery limit; the correct path is the Mod. 1 (gas) or Mod. 2 (steam) iteration, not a 1.156 multiplication [S5].
Comparison: Which Conversion to Use Across Valve Types
The 1.156 factor is universal across valve geometries, but the ratio of Cv across types is not: a 4-inch full-port ball valve carries about Cv 1,300, a 4-inch gate carries about Cv 1,100, and a 4-inch butterfly only reaches Cv 500, so a project mixing ball valve and butterfly valve skids needs the same 1.156 conversion on both, but different rated values to hit the same line capacity [S2][S4].
A 1/2-inch globe at Cv 8 versus a 2-inch butterfly at Cv 110 illustrates the size-dependent spread; the 1.156 × Kv number is the same on both, but the Cv/Kv ratio the engineer pays for is dominated by geometry, disc clearances, and seat geometry rather than the conversion constant [S2][S4]. For control valve duty where inherent rangeability matters, the HPBV table shows DN300 HPBV at Kv 7,353 versus a comparable globe of about Kv 380 at the same size, a 19× gap that the 1.156 factor never closes [S4].
Limitations, Failure Modes, and Standards Caveats
The 1.156 conversion is exact only for water at the reference temperature; for fluids with specific gravity SG, the sizing equation still uses √(SG/ΔP), so a slurry at SG 1.4 needs a Cv that is √1.4 = 1.183 times larger than the water figure, and that SG correction compounds independently of the unit-conversion factor [S1][S5]. Cavitation onset at σ_incipient (per IEC 60534-1) is not detected by any Cv/Kv number, and damage onset in the 500–2000 hr range is silent, so the conversion only helps the sizing arithmetic, not the materials or trim selection [S5].
The piping-geometry factor Fp from IEC 60534-2-1 §8 is also outside the 1.156 conversion, and when the valve bore is smaller than the line size with concentric reducers, Fp reduces effective Cv by 5–15%, which the engineer must apply as a manual multiplier on the calculated Cv [S5]. Laminar flow with low Reynolds number adds the FR factor; the Kv/Cv 1.156 figure still holds, but the underlying sizing equation no longer scales with √ΔP, and the converted number is a guide, not a working point [S5].
Selection and Sourcing Cues for the Field
For a process engineer quoting a US-spec Cv against a European vendor's Kv datasheet, the safe workflow is: compute the required Cv in US units from the duty, multiply by 0.865 to read the equivalent Kv, then select a Kv-rated valve with at least 25% margin and size it to operate at 30–80% of its rated value at normal flow [S1][S5]. This keeps the valve out of the low-turndown saturation region and inside the controllability band that IEC 60534-2-1 implicitly assumes [S5].
On the procurement side, butterfly valves above DN600 show wider Cv-to-Kv scatter between manufacturers because disc geometry, shaft offset pattern, and seat profile diverge sharply, so the published Cv of 40,000 for a 24-inch concentric design in one catalog may read Cv 45,000 in another at the same nominal size [S4]. For a wider view on sizing margin across the same family, see butterfly valve Cv at 6 inch 60 degrees open sizing data, and for face-to-face dimensional differences that change which HPBV fits a given flanging standard, see API 609 vs EN 558 butterfly valve face to face dimensional series compared.
Trackable next nodes: IEC 60534-2-1 Mod. 1 gas module inputs (Y, xT) for any butterfly above 4 bar ΔP, and Fp correction per IEC 60534-2-1 §8 for reducers ahead of any valve smaller than line size; both are where the 1.156 conversion stops being enough.