Drawing No. EH–FM–004 // Fluid Mechanics & Piping
Control Valve Cv / Kv Calculator
Reviewed September 2026
Size Cv/Kv, inspect liquid choking risk and compare metric and US flow coefficients with transparent assumptions.
01 // Service
02 // Sizing result
Liquid mode uses the conventional turbulent-flow coefficient definition and checks the IEC/ISA liquid choking limit using FL and the liquid critical-pressure factor.
Gas mode is an ideal-gas effective-area screening model only. It does not convert area into Cv or Kv because that conversion is not universal for compressible valve sizing. Final selection should use IEC 60534-2-1 / ISA sizing factors for the actual valve geometry and manufacturer data.
Worked example — water at 25 m³/h
Water (SG = 1) from 6 bar to 4 bar across the valve, FL = 0.9, vapour pressure 0.03 bar.
Step 1 — the pressure drop across the valve. ΔP = 6 − 4 = 2.0 bar.
Step 2 — the flow coefficient. For non-choked turbulent liquid flow:
Kv = Q √(SG / ΔP) = 25 × √(1/2) = 17.68
Step 3 — the same valve in US units. Cv = 1.156 × Kv = 20.44.
Step 4 — check it is not choked. Choking begins at
ΔPchoked = FL2 (P1 − FF Pv)
With FF ≈ 0.96 − 0.28√(Pv/Pc) = 0.957, that gives 0.81 × (6 − 0.029) = 4.84 bar. The actual 2.0 bar is well below it, so the flow is not choked and the simple equation holds.
Cv, Kv and Av — the same idea, three units
These are not different properties. They are one property — the valve's flow capacity at a reference pressure drop — expressed in three unit systems, and mixing them is the most common error in valve selection.
| Coefficient | Definition | Convert |
|---|---|---|
| Cv (US) | US gallons per minute of 60 °F water at 1 psi drop | Cv = 1.156 Kv |
| Kv (metric) | m³/h of water at 1 bar drop | Kv = 0.865 Cv |
| Av (SI) | equivalent orifice area, m² | Av = 2.4 × 10−5 Kv |
The 1.156 factor is where money gets lost. Take a European datasheet quoting Kv 17.68, read it as Cv, and you specify a valve 15% smaller than required. Read a US Cv as Kv and you oversize by the same margin. Neither error announces itself: the valve works, it simply sits at the wrong part of its travel.
Always check which coefficient a datasheet quotes before comparing two manufacturers. A "Cv 20" valve and a "Kv 20" valve differ by 15% in capacity.
The number the calculation does not give you: valve authority
A valve sized purely on Cv will pass the flow. Whether it can control it is a separate question, and it is decided by valve authority:
N = ΔPvalve, open / ΔPtotal, circuit
If the valve takes only a small share of the circuit's total pressure drop, closing it barely changes the flow until it is nearly shut, and then the flow collapses. The installed characteristic bears no resemblance to the inherent one, and the loop hunts. Aim for N ≥ 0.25, and 0.5 where tight control matters. Achieving it usually means accepting a larger permanent pressure drop across the valve — control costs pumping energy, and that is a real design trade rather than a mistake.
Oversizing is the standard failure. Rounding up to the next valve size “for safety” pushes the operating point down towards the seat, where the characteristic is steep and the seat and trim erode fastest. A control valve should do its work between roughly 20% and 80% of travel across the expected flow range. If the sized Cv places the normal duty below 20% travel, the answer is a smaller valve or different trim, not a bigger one.
Characteristic matters too. Equal-percentage trim suits circuits where much of the drop is elsewhere and varies with flow — heating and cooling coils, most process duties. Linear trim suits high-authority circuits where the valve holds most of the drop. The wrong characteristic can undo careful sizing.
Assumptions and where this stops being valid
Turbulent, non-choked, single-phase flow. The square-root relation assumes all three. Viscous service needs the Reynolds number correction factor FR; heavy oils can require a valve substantially larger than the simple equation suggests.
Cavitation is not sized here. Choking is flagged, but cavitation damage begins well before the choked limit. It is assessed with the cavitation index σ = (P1 − Pv)/(P1 − P2) against the manufacturer's incipient and damage coefficients. Sustained cavitation destroys trim in weeks, and the fix is anti-cavitation trim or staged pressure reduction, not a different size.
Flashing is a different problem. If P2 falls below the vapour pressure the fluid flashes and stays vapour downstream. No trim design prevents it; it needs hardened materials and a body designed for two-phase velocity.
Piping geometry is ignored. Reducers, bends and close-coupled fittings change the effective capacity. IEC 60534 handles this with the piping geometry factor FP, which can cost several percent.
Gas sizing needs the expansion factor. Compressible service uses the expansion factor Y and the pressure drop ratio xT; treating gas with the liquid equation undersizes badly at high pressure ratios.
Standards and what to do next
Sizing: IEC 60534-2-1 is the international standard for control valve sizing equations for incompressible and compressible flow; ISA 75.01.01 is the equivalent and is technically harmonised with it. Both define FL, FF, FP, xT and the choked-flow treatment used above.
Inherent characteristic and rangeability: IEC 60534-2-4. Noise prediction for gas and liquid service: IEC 60534-8-3 and 8-4 — worth running, because aerodynamic noise above about 85 dB(A) usually means the trim selection needs revisiting.
Next steps: size at minimum, normal and maximum flow, not just the design point — a valve that is well placed at full load is often near its seat at turndown. Confirm the resulting travel at each, check valve authority against the real circuit resistance, and run the cavitation index for any liquid service where P2 approaches vapour pressure. Then choose the characteristic to suit the authority you actually have, and check the actuator can close against the maximum differential, which for a tight-shutoff duty is the full pump head, not the operating ΔP.
Background & method
IEC 60534-2-1 contains the harmonized IEC/ISA sizing framework for incompressible and compressible control-valve flow. This page implements a strong liquid screening calculation and clearly separates the less complete gas screening model from final standard sizing.
FAQ
Cv is the US customary flow coefficient and Kv is the metric flow coefficient. For the same valve, Cv is approximately 1.156 × Kv.
At sufficiently large pressure drop the local pressure at the vena contracta can reach the vapour-pressure region, limiting flow and creating cavitation or flashing risk.
No. Gas mode is intentionally labelled as a screening model. Final compressible sizing needs the actual valve xT, Fp, Fγ, Reynolds corrections and manufacturer data required by IEC/ISA procedures.