About Control Valve Cv & Kv Calculator — Liquid Flow Sizing
The valve Cv calculator sizes control valves for liquid service using the flow-coefficient equation every manufacturer's catalog is built on: Cv = Q·√(SG/ΔP), with flow in US gpm, pressure drop in psi, and specific gravity relative to water. Cv is defined as the US gallons per minute of 60 °F water that pass the valve at a 1 psi drop; its metric twin Kv uses m³/h of water at 1 bar, and the two convert by Kv = 0.865·Cv (Emerson Control Valve Handbook; the same factor appears as the ratio of numerical constants in IEC 60534-2-1 / ISA-75.01.01). Enter a flow duty to get the required coefficient, or flip the direction to find what flow an installed valve of known Cv passes at your available ΔP.
This is the ISA/IEC liquid equation reduced to its classic textbook form, which means its scope is specific: turbulent, non-flashing, non-cavitating liquid flow with no piping-geometry correction. It is the right tool for selecting a valve size class and sanity-checking a datasheet; it is not a substitute for the full ISA-75.01.01 procedure when the service flashes, cavitates, is viscous (low Reynolds), or the valve is between reducers. Practical sizing convention: pick a valve whose rated Cv puts your duty at 60–80% of valve opening, leaving margin at both ends of the control range.
How It Works
- Pick the unit system — US customary (gpm, psi, Cv) or metric (m³/h, bar, Kv). Toggling converts your entries in place, rounded to the input step and clamped to the valid range, so a valid entry never turns into an error.
- Pick the direction: "required coefficient" computes the Cv/Kv a flow duty needs; "flow from coefficient" computes what an installed valve passes.
- Enter the flow (or coefficient), the specific gravity of the liquid (water = 1.0; gasoline ≈ 0.74; 30% glycol ≈ 1.05), and the pressure drop across the valve at the sizing flow — not the whole system drop.
- Read both coefficients: the tool always reports Cv and Kv together via Kv = 0.865·Cv, so a US datasheet and a European one can be compared directly.
- Check the service assumptions before trusting the number: liquid, turbulent, no flashing (outlet pressure well above vapor pressure) and no cavitation. If ΔP approaches the choked-flow limit or the fluid is near boiling, use the full ISA-75.01 procedure with the valve's FL factor.
Worked Example
A cooling loop must pass 100 gpm of water (SG = 1.0) with 4 psi allocated across the control valve at design flow. Required coefficient: Cv = Q·√(SG/ΔP) = 100 × √(1/4) = 100 × 0.5 = 50.0. The metric equivalent is Kv = 0.865 × 50.0 = 43.25 — the number a European catalog would quote. Selecting a valve rated around Cv 70–85 puts this duty at roughly 60–70% opening, comfortable control territory. Flipping the tool's direction confirms the selection: the same Cv 50 valve at 4 psi passes exactly 100 gpm, and at a 9 psi drop it would pass 150 gpm — flow grows with the square root of ΔP.
Valve Cv chart for water
Required Cv and Kv for common water flows (SG = 1.0) at a 1 psi pressure drop — at ΔP = 1 psi and SG = 1 the Cv equals the gpm figure by definition, which makes this the reference column of every catalog chart. Enter any row in the tool, or change ΔP to rescale: coefficient shrinks with √ΔP.
| Water flow at ΔP = 1 psi | Required Cv | Kv equivalent |
|---|---|---|
| 10 gpm | 10.0 | 8.65 |
| 25 gpm | 25.0 | 21.63 |
| 50 gpm | 50.0 | 43.25 |
| 100 gpm | 100.0 | 86.50 |
| 200 gpm | 200.0 | 173.00 |
Formulas
- Required coefficient (US customary)
Cv = Q · √(SG / ΔP)- Required coefficient (metric)
Kv = Q · √(SG / ΔP)- Coefficient conversion
Kv = 0.865 · Cv- Flow from an installed valve
Q = C · √(ΔP / SG)
Standards & References
- ISA-75.01.01 / IEC 60534-2-1 — Flow equations for sizing control valves; this tool implements the liquid equation's turbulent, non-flashing simplification without piping-geometry corrections
- Emerson (Fisher) Control Valve Handbook, 5th edition — Cv/Kv definitions and the Kv = 0.865·Cv conversion
- Scope: liquid, turbulent, non-flashing, non-cavitating service only — flashing, cavitation, viscous flow, and choked-ΔP checks require the full standard with the valve's FL and FR factors
Frequently Asked Questions
What exactly does Cv mean?
Cv is the number of US gallons per minute of 60 °F water that flow through the valve at a pressure drop of exactly 1 psi, at a stated opening (usually fully open for the rated value). It rolls the valve's geometry into one measurable number, so sizing reduces to Cv = Q·√(SG/ΔP). The scenario table's first column shows the definition directly: at ΔP = 1 psi with water, required Cv equals the gpm figure.
What is the difference between Cv and Kv, and where does 0.865 come from?
Same concept, different reference units: Cv uses US gpm at 1 psi, Kv uses m³/h at 1 bar. Converting both the flow unit and the pressure unit gives Kv = 0.865·Cv — the factor published in the Emerson Control Valve Handbook and implicit in the IEC 60534-2-1 numerical constants (Cv = 1.156·Kv, and 1/1.156 = 0.865). The tool always reports both, so a US and a European datasheet can be compared without hand conversion.
Which pressure drop do I enter?
The drop across the control valve alone, at the flow you are sizing for — not the pump head and not the whole loop's losses. In a typical system the valve is allocated a portion of the total dynamic head (a common rule of thumb is 25–50% of the friction losses, or at least a few psi) so it retains authority over the flow. Entering the whole system drop undersizes the valve badly; entering too small a ΔP oversizes it into poor, hunting control.
Why should the valve operate at 60–80% opening rather than exactly at its rated Cv?
Because a control valve must modulate. If the design duty needs the full rated Cv, the valve has no headroom for upset flows and will run pinned open; if the duty sits below ~20% opening, the valve throttles near its seat where control is nonlinear and erosion concentrates. Selecting a rated Cv about 1.25–1.6× the required value lands the design point in the sweet band. The calculator gives the required Cv; the margin decision is yours.
When is this simplified liquid equation NOT valid?
Four cases, all in the full ISA-75.01.01 standard: flashing (outlet pressure below the liquid's vapor pressure — the flow chokes and downstream is two-phase), cavitation (local pressure dips below vapor pressure and recovers, hammering the trim), viscous service (low valve Reynolds number needs the FR correction), and significant fittings — reducers around the valve change the effective coefficient (Fp). Gas and steam sizing use entirely different, compressible equations.
Does specific gravity really matter? Water is SG 1 anyway.
It enters under a square root, so it is gentle but real: seawater (SG ≈ 1.03) needs 1.5% more Cv than fresh water, 30% propylene glycol (≈ 1.03 at supply temperature) similarly, while gasoline (SG ≈ 0.74) needs 14% less. The equation's √SG comes from the fluid's density driving the pressure-to-velocity conversion. For anything far from water — viscous oils especially — check the viscosity correction before trusting the simple form.