Enter the flow rate, pressure drop and fluid specific gravity to find the required flow coefficient for a liquid valve. Results in both Cv (US) and Kv (metric).
Liquid, turbulent, non-choked flow. For gas/steam, flashing or choked service, use the full ISA/IEC 60534 sizing equations.
For liquid (turbulent, non-choked) service, Cv = Q x sqrt(SG / dP), where Q is flow in US GPM, dP is pressure drop in psi, and SG is specific gravity. The result is the flow coefficient: the GPM of water at 60 deg F that passes through the valve at 1 psi drop.
Cv is the US/imperial flow coefficient (GPM at 1 psi); Kv is the metric flow coefficient (m3/h at 1 bar). They convert directly: Kv = 0.865 x Cv, or Cv = 1.156 x Kv.
Cv quantifies a valve's flow capacity: a higher Cv passes more flow for the same pressure drop. It is used to size valves so they deliver the required flow without excessive pressure loss.
The flow coefficient Cv is the single most fundamental number in liquid valve sizing: it tells you how much flow a valve passes for a given pressure drop. Get it wrong and the valve still 'works' on paper but performs poorly in service. An oversized valve runs near its seat at low travel, where its characteristic is steep and unstable — small stem movements cause large flow swings, control becomes hunting and jittery, and trim wears prematurely. An undersized valve starves the process: it cannot pass the design flow even fully open, so the loop saturates and the plant never reaches setpoint.
This calculator gives you the required Cv (and the metric Kv) directly from three inputs you already know at the design stage — flow rate, available pressure drop across the valve, and the fluid's specific gravity. It uses the standard liquid sizing relation Cv = Q × √(SG ÷ ΔP), so you can quickly check a vendor's published Cv against your duty, compare candidate valve types, or confirm that the valve will land in the controllable 20–80% travel band rather than against either end stop. Once you have the number, the harder questions are valve type and trim — see the Valve Selector and valve types reference.
A cooling-water line needs 100 US GPM and the available pressure drop across the valve is 5 psi. Water has a specific gravity of 1.0. Cv = Q × √(SG ÷ ΔP) = 100 × √(1.0 ÷ 5) = 100 × √0.2 = 100 × 0.4472 = Cv ≈ 44.7. Pick a valve whose full-open Cv comfortably exceeds 44.7 so it operates partly closed at normal load, leaving controllable rangeability in hand.
A light hydrocarbon (specific gravity 0.85) must flow at 60 US GPM with a 10 psi drop. Because the fluid is lighter than water, the required Cv is lower than for the same flow of water. Cv = 60 × √(0.85 ÷ 10) = 60 × √0.085 = 60 × 0.2915 = Cv ≈ 17.5. The lower specific gravity reduces the Cv demand: a denser fluid at the same flow and drop would need a larger valve.
Suppose a sizing exercise gives a required Cv of 18. To work in metric units (m³/h at 1 bar), convert with Kv = 0.865 × Cv = 0.865 × 18 = Kv ≈ 15.6. The relationship is fixed and linear, so it works in either direction: Cv = 1.156 × Kv. Vendor datasheets may quote either coefficient, so always convert to a common basis before comparing valves.
The table below gives indicative, typical full-open (100% travel) Cv ranges by valve type and nominal size for general liquid service. These are order-of-magnitude figures only — actual Cv varies widely with manufacturer, body style, trim, port type (full vs. reduced bore) and pressure class. Always use the specific valve's published Cv from its datasheet for real sizing. Use this table to sanity-check a calculated Cv against a plausible valve size, not as a substitute for vendor data.
| Nominal size | Gate (full-bore) | Globe | Ball (full-port) | Butterfly |
|---|---|---|---|---|
| 1" (DN25) | ~25–35 | ~10–15 | ~30–45 | ~20–35 |
| 2" (DN50) | ~110–130 | ~45–55 | ~120–160 | ~110–160 |
| 3" (DN80) | ~250–300 | ~110–130 | ~280–360 | ~280–380 |
| 4" (DN100) | ~450–550 | ~200–240 | ~500–650 | ~550–750 |
| 6" (DN150) | ~1000–1300 | ~480–560 | ~1100–1500 | ~1400–1900 |
Indicative/typical values for general liquid service; not manufacturer specifications. Globe valves have the lowest Cv (most restriction) and are favoured for fine throttling; ball and butterfly valves have high Cv but characteristics better suited to on/off or coarse control. Confirm every value against the actual valve datasheet.
Liquid valve Cv sizing is a daily task across the process industries. It is used in oil & gas (production, refining, pipeline metering skids), chemical and petrochemical plants, water and wastewater treatment, power generation (cooling, condensate and feedwater liquid loops), HVAC and building services (chilled and hot water balancing), and food, beverage and pharmaceutical processing where hygienic liquid lines must be sized correctly. Typical users include process and control engineers selecting control valves, valve specifiers and instrument engineers preparing datasheets, EPC and detailed-design teams, maintenance and reliability engineers troubleshooting poor control, and procurement staff cross-checking vendor-quoted Cv against the stated duty.
ValveAnswer is an independent valve knowledge and reference site — we do not sell valves, so the method here is vendor-neutral. The calculator uses the standard liquid flow-coefficient relation Cv = Q × √(SG ÷ ΔP) consistent with ISA and IEC 60534 valve sizing practice, and the formula is shown openly on the page so you can verify every result by hand. We state our assumptions and limits plainly rather than presenting a single number as universally correct.
Size for the maximum required flow, but check that at normal flow the valve still operates within a controllable travel band — roughly 20–80% open. A valve sized only for peak duty may sit nearly closed and unstable at normal load. Many engineers add a modest margin to the maximum-flow Cv, then verify the minimum-flow case as well.
Choose the valve whose full-open Cv is comfortably above your required Cv, so it runs partly throttled at design flow with rangeability in reserve. Avoid picking a valve whose full-open Cv barely meets the duty, since it would have to run wide open with no control margin and no allowance for fouling or future flow increases.
No. The basic Cv equation assumes non-choked liquid flow. When downstream pressure approaches the fluid's vapour pressure, the liquid can cavitate or flash, which limits the achievable flow regardless of Cv and can damage the valve. Those cases require choked-flow checks and pressure-recovery factors from the full ISA/IEC 60534 sizing method.