Cv and Kv measure the same thing in different units: the flow a valve passes at a defined pressure drop. Kv is roughly 0.865 times Cv. Use them to size from flow and allowable pressure drop, never to compare valves by pipe size, and always check the valve's own data sheet before you commit to a model.
What the two coefficients actually measure
Both numbers answer one question: how much water will this valve pass with one unit of pressure across it. The definitions differ only in the units used.
Cv is the flow of water in US gallons per minute at 60 °F that passes through the valve with a pressure drop of 1 psi. It belongs to the US customary system and is defined in ISA practice.
Kv is the flow of water in cubic metres per hour at a temperature between 5 °C and 30 °C that passes through the valve with a pressure drop of 1 bar. It belongs to the metric system and is defined in IEC 60534.
Neither number says anything about the valve's design, its materials, its leakage class or its suitability for a dirty fluid. They are capacity figures, and that is all they are. The practical value is that a capacity figure lets you calculate whether a valve is large enough before you buy it.
The conversion, and the trap in Kvs
Because both definitions use water and a fixed pressure drop, converting between them is a units exercise rather than an engineering one. The relationship is:
- Kv = 0.865 × Cv
- Cv = 1.156 × Kv
The trap is Kvs. Many European datasheets quote Kvs rather than Kv, and the extra letter matters. Kvs is the coefficient at full open travel, which is the figure you want for sizing. Kv quoted at a partial opening describes the valve at that particular position. Reading a Kvs as though it were a Kv at the duty point overstates the capacity you have available, and sizing a valve from the wrong one of the two is a common reason a new installation cannot reach its design flow.
One further caution applies to both. The water temperature in the two definitions is not identical, 60 °F against the 5 to 30 °C band, so the conversion is a units conversion and not a performance claim. For water at ordinary temperatures the difference is negligible; for a hot or viscous liquid it is not, and the viscosity correction has to be applied separately.
Sizing in four steps
The calculation is short. The judgement is in the second step, where the allowable pressure drop has to be decided.
Step one, fix the flow. Take the maximum flow the process needs, not the average.
Step two, decide the allowable pressure drop. This is the design decision, not a calculation. The drop across a control valve has to be large enough to give the valve authority over the system, and small enough to leave pressure for everything downstream. In pump circuits a common starting point is a third of the dynamic pressure drop, and it is worth confirming that convention against the project specification.
Step three, calculate the required coefficient. For liquid service:
Kv = Q × square root of (specific gravity divided by pressure drop in bar), with Q in cubic metres per hour.
The same relationship with Cv uses gallons per minute and psi.
Step four, choose a valve whose rated coefficient is above the required value. Take a worked example. Water at a specific gravity of 1.0, flow of 20 m³/h, allowable drop of 0.5 bar. The required Kv is 20 divided by the square root of 0.5, which is 20 divided by 0.707, so about 28.3. The equivalent Cv is 28.3 divided by 0.865, so about 32.7. A valve rated at Kvs 40 would carry this duty at roughly 70 percent of its capacity, which is a sensible place to sit.
Why you should not simply oversize
The instinct on a difficult sizing job is to take the next size up. With valves that instinct creates new problems, because a control valve that is far too large spends its life near the closed position.
Running at a few percent of travel makes the valve sensitive to small changes in stem position, so control becomes oscillating or hunting rather than stable. It puts the trim in the region where cavitation and noise begin, because the whole pressure drop is being taken across a narrow gap. It also wastes the rangeability you paid for: a valve with a wide stated rangeability still has to be operated inside a sensible band to deliver it.
The practical target is a valve that passes the normal flow somewhere in the middle of its travel and still has capacity left for the maximum demand. That is the opposite of oversizing, and it is where the sizing margin belongs: at the top end of the flow range, not in a blanket factor applied to the coefficient.
Why the same pipe size can have very different Cv
This is the reason the two numbers exist. Flow capacity is set by the internal geometry, and the internal geometry varies enormously between valve types of the same nominal size.
A globe valve and a full bore ball valve of identical nominal size can differ by a factor of several in capacity. That single fact explains why sizing by line size fails: the pipe tells you what will fit, and the coefficient tells you what will pass.
Limits worth checking before you commit
Three conditions sit outside the basic sizing calculation and are worth checking on any duty that is not plain water at ambient temperature.
Cavitation and flashing. When the pressure drop is large relative to the inlet pressure, the liquid can drop below its vapour pressure at the vena contracta. Flashing means vapour persists downstream; cavitation means the bubbles collapse inside the valve and erode the trim. Valve datasheets publish a pressure recovery factor that sets the allowable drop, and exceeding it damages the valve rather than merely making it noisy.
Gas and steam. The liquid formula does not apply. Gas sizing uses a different relationship with an expansion factor, and the metric convention for gases uses normal cubic metres per hour. A coefficient taken from a liquid calculation and applied to a gas duty will be wrong by a wide margin.
Viscosity. Both definitions are based on water. A viscous liquid passes less than the coefficient predicts, and a correction has to be applied. This is the same limit that decides whether a centrifugal pump is viable at all, since viscosity affects every hydraulic device in the line.
What to put in the enquiry
State the fluid and its specific gravity, the maximum and normal flow in the units you actually work in, the inlet pressure, the allowable pressure drop, the temperature and the line size. State also whether the valve is for isolation or for control, because that changes the valve type and with it the capacity you should be asking for. Given those, the coefficient follows from arithmetic, and the model follows from the coefficient.
Frequently asked questions
How do I convert Cv to Kv?
Multiply Cv by 0.865 to get Kv, or multiply Kv by 1.156 to get Cv. The factor comes from the difference between the two definitions: gallons per minute against cubic metres per hour, and psi against bar. Check whether the figure you were given is Kv or Kvs before converting, because Kvs is the full open value and a partial Kv is not.
Why should a control valve not be oversized?
An oversized valve controls near the closed position, where a small change in stem position produces a large change in flow. The result is hunting or oscillating control rather than a stable setpoint, plus cavitation and noise as the full pressure drop is taken across a narrow gap. Size so the normal duty sits mid travel with capacity in reserve for maximum demand.
Why do two valves of the same size have such different Cv?
Because capacity is set by the internal flow path, not by the connection size. A full bore ball valve is close to a straight pipe, while a globe valve forces the liquid through two turns around a plug and seat, and a characterised control trim is shaped for modulation rather than maximum throughput. Nominal size tells you what will bolt into the line; the coefficient tells you what will pass through it.
What is the difference between Kv and Kvs?
Kv is the flow coefficient at a stated opening, while Kvs is the coefficient at full open travel. Datasheets usually quote Kvs as the reference figure for sizing, so a valve you intend to operate at part travel has less usable capacity than the Kvs suggests. Confirm which one a supplier is quoting before you compare two valves against each other.
Can I use Cv for gases and steam?
The coefficient is still used, but not with the liquid formula. Gas sizing introduces an expansion factor that accounts for the change in density across the valve, and the metric convention for gases is based on normal cubic metres per hour rather than cubic metres per hour of liquid. Using a liquid calculation on a gas duty produces a valve that is far too small.
Sizing a valve from the duty point
Send the fluid, the specific gravity, the maximum and normal flow, the inlet pressure, the allowable pressure drop and the line size, and we will return the required coefficient and the valve types that carry it. We supply industrial valves and can confirm the rated coefficient from the manufacturer data sheet rather than from the line size. Start from the brand index, browse the product catalogue or read the glossary entries for the underlying definitions. Two related guides are worth reading next: ball valve versus butterfly valve covers the capacity consequence of valve type, and sizing a pressure relief valve applies the same discipline to a different duty. Contact our team with the duty point, and find more guides on the insights index.
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