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 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 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.
A higher Cv passes more flow for the same pressure drop. Engineers size a valve so its Cv delivers the required flow without excessive pressure loss or choking.
The flow coefficient Cv is the single number that ties a valve to the hydraulic system it sits in. By definition, Cv is the flow of water at 60 °F, in US gallons per minute, that passes through a fully open valve at a pressure drop of 1 psi. A valve with Cv = 100 will therefore pass 100 GPM of water at 1 psi, roughly 316 GPM at 10 psi, and so on — flow scales with the square root of the available pressure drop. For liquid, turbulent, non-choked service the relationship the calculator uses is Cv = Q × √(SG ÷ ΔP), with Q in US GPM, ΔP in psi and SG the specific gravity referenced to water. The metric equivalent, Kv, follows from Kv = 0.865 × Cv.
From a specification standpoint, Cv is not a property of the pipe size — it is a property of the trim and body geometry, and two valves of the same nominal diameter can have very different coefficients. That is why Cv, not line size, belongs on a valve datasheet. Getting the number wrong has visible consequences. Specify a coefficient that is too large and a control valve spends its working life nearly closed, where the trim is non-linear, gain is high, and stable control is hard to hold; the valve also becomes a candidate for seat erosion and cavitation as it throttles a large pressure drop across a small opening. Specify a coefficient that is too small and the valve becomes the bottleneck of the loop, forcing the pump to work against an unnecessary restriction and wasting energy for the life of the plant. Sizing to the correct Cv — with margin, but not excess — is what keeps a valve controllable, efficient, and durable. For the broader workflow this calculator supports the valve selection process and pairs with our valve selector.
A condenser cooling-water line carries 250 GPM of plain water (SG = 1.0) and the hydraulic study allows an 8 psi drop across the valve at design flow. Apply the formula directly:
Cv = 250 × √(1.0 ÷ 8) = 250 × 0.354 = 88.4
The valve must offer at least Cv 88.4 fully open to pass design flow within the 8 psi budget. In practice you would round up to a standard valve whose rated Cv comfortably exceeds 88 — here a line-size full-bore valve easily clears it, confirming this duty is pressure-drop limited rather than Cv limited.
A glycol/water heating loop circulates 160 GPM (SG = 1.03) and the control valve is allocated a 10 psi drop at design. First find the required coefficient:
Cv = 160 × √(1.03 ÷ 10) = 160 × 0.321 = 51.4
For a modulating valve you do not pick a rated Cv equal to 51.4, because operating at full travel leaves no room to open further and pushes the plug into a non-ideal part of its characteristic. A common rule of thumb is to land the design point near 60–80% of travel. Choosing a valve with a rated Cv of about 75 puts this duty at 51.4 / 75 ≈ 68% of rated Cv — squarely in the controllable band, with headroom for fouling, future flow increases, and turndown.
A supplier-neutral datasheet quotes a valve at Cv = 50, but the project's specification and pump curves are in metric units, so the equivalent Kv (m³/h at 1 bar) is needed:
Kv = 0.865 × Cv = 0.865 × 50 = 43.3
So Cv 50 corresponds to Kv 43.3. To go the other way, Cv = 1.156 × Kv, which returns 1.156 × 43.3 ≈ 50. Keeping the conversion explicit on mixed-unit projects avoids the classic error of comparing a US-coefficient valve against a metric-coefficient requirement as if the numbers were interchangeable — they differ by about 13.5%.
| Valve type | Typical inherent characteristic | Indicative rangeability | Relative Cv per nominal size | Where Cv character helps |
|---|---|---|---|---|
| Globe (control) | Linear or equal-percentage trim | ~30:1 to 50:1 | Low–moderate | Precise throttling; design point set on the trim curve |
| Segmented / V-port ball | Modified equal-percentage | ~100:1 to 300:1 | High | Wide turndown control with high capacity |
| Butterfly (high-performance) | Roughly equal-percentage to ~70° | ~20:1 to 50:1 | High | High capacity in large lines; control mid-stroke |
| Full-bore ball (isolation) | Quick-opening | Not for modulation | Very high | On/off and low-drop isolation duties |
| Gate | Quick-opening (on/off) | Not for modulation | Very high | Full-open isolation; minimal pressure penalty |
| Plug (eccentric) | Linear to equal-percentage | ~50:1 to 100:1 | Moderate | Slurry and dirty-service throttling |
Indicative ranges only; rangeability and Cv depend on trim selection, size, and manufacturer design. Use vendor Cv-vs-travel tables for sizing. Modulating duties should generally land the design point near 60–80% of travel, away from the extremes where gain and erosion behave poorly.
Flow-coefficient sizing is a daily task across the process and energy industries. In chemical and petrochemical plants it sets control-valve trim for reactors, columns, and transfer lines; in oil & gas it governs separators, metering skids, and pressure-let-down service where cavitation and flashing margins matter as much as the raw Cv. Power generation relies on it for cooling-water, condensate, and feedwater loops; water and wastewater utilities use it to size pump-control and throttling valves without starving the network or over-pressuring mains. General process manufacturing — food, pharma, pulp and paper — uses the same method for hygienic and utility service. On the engineering side, EPC and consulting firms embed Cv calculations in hydraulic studies and valve datasheets long before a single valve is procured. Typical roles that run these numbers include control and instrumentation engineers, process engineers, piping and hydraulic engineers, and valve specifiers building tender specifications.
This tool implements the standard liquid flow-coefficient relationship used across the industry, Cv = Q × √(SG ÷ ΔP), with the direct metric conversion Kv = 0.865 × Cv. There is nothing proprietary in the math — it is the textbook turbulent, non-choked liquid equation, shown openly so you can reproduce every result by hand. ValveEngineeringHub is an independent reference site and does not sell valves, so the output is not steered toward any product line. That said, a one-line equation has real boundaries, and honest sizing means knowing them:
Use the result as a first-pass engineering estimate, then confirm critical or borderline duties against the full standard and the manufacturer's Cv-and-travel data.
Use the pressure drop available across the valve at design flow, not the total system pressure. A common practice is to allocate the valve a meaningful share of the dynamic loss — often on the order of a quarter to a third of the friction drop, or a fixed allowance from the hydraulic study — so the valve retains authority over the loop. Too small a ΔP leaves the valve unable to control; too large wastes pump energy.
No. For a modulating valve, landing the maximum-flow design point near 60–80% of travel is usual practice. Sizing for 100% travel leaves no headroom to open further if flow demand rises or trim fouls, and it pushes the plug into the part of the characteristic where control is poorest. Always pick a rated Cv with margin above the calculated required Cv.
Because SG sits under the square root, its effect is moderate. Going from water (SG 1.0) to a fluid at SG 1.3 raises the required Cv by only about 14% (√1.3 ≈ 1.14), while a light hydrocarbon at SG 0.7 lowers it by roughly 16%. It is not negligible on tight sizing, but flow rate and pressure drop dominate the result.