Calculator
How the calculation works
For an incompressible liquid (no flashing or cavitation), the flow coefficient is derived from the basic valve sizing equation:
Kv = Q × √( SG / ΔP ) (Q in m³/h, ΔP in bar)
Cv = 1.156 × Kv (Cv in US gpm, ΔP in psi)
Where Q is the volumetric flow rate, SG is the specific gravity of the fluid relative to water, and ΔP is the pressure drop across the valve at the desired flow. The tool picks the smallest standard DN whose rated Cv (with a 15–25% margin) covers your requirement, so the valve is never run near its choke limit.
Gas, steam and two-phase flows need additional correction factors (Fk, xt, expansion). For those services, contact our engineering team with your operating data.
Typical Cv capacity by size (butterfly, reference)
| DN | Rated Cv (approx.) | Rated Kv (approx.) |
|---|---|---|
| DN50 | 95 | 82 |
| DN65 | 150 | 130 |
| DN80 | 210 | 182 |
| DN100 | 340 | 294 |
| DN125 | 540 | 467 |
| DN150 | 760 | 657 |
| DN200 | 1400 | 1211 |
| DN250 | 2200 | 1903 |
| DN300 | 3100 | 2681 |
| DN400 | 6000 | 5192 |
Values are indicative for centric/wafer butterfly valves and vary with seat and disc design. Use them for first-pass selection only.
Need the actual valve?
Reguvale manufactures wafer & lug butterfly valves, pneumatically actuated butterfly valves and control valves in DN50–DN400, PN10/PN16, with EPDM / PTFE seats. Send us your calculated Cv, medium and temperature and we will confirm the exact model and quote FOB Tianjin.
Full product range: All Products.
Working out the size and the pressure rating first? Use the DN to inches (NPS) and PN pressure rating chart — it also explains why PN16 is not the same as Class 150.
What Cv and Kv actually mean
Both numbers answer the same question: how much fluid can this valve pass at a given pressure drop? They differ only in the units used to define the test condition.
| Coefficient | Definition | Units |
|---|---|---|
| Cv | Flow of water at 60 °F (15.6 °C) that passes through the valve with a pressure drop of 1 psi | US gpm |
| Kv | Flow of water at 5–40 °C that passes through the valve with a pressure drop of 1 bar | m³/h |
The two are linked by a fixed factor, so you never need to look up two sets of data:
Cv = 1.156 × Kv · Kv = 0.865 × Cv
Which one does your project use? Metric projects (DIN / EN / GOST) normally specify Kv. US and oil & gas projects (ANSI / API) normally specify Cv. This calculator returns both.
Worked example
Water at ambient temperature, 60 m³/h, 0.5 bar allowable pressure drop:
| Step | Calculation | Result |
|---|---|---|
| 1. Required Kv | Kv = Q × √(SG / ΔP) = 60 × √(1.0 / 0.5) | 84.9 |
| 2. Convert to Cv | Cv = 1.156 × 84.9 | 98.1 |
| 3. Add margin | 98.1 × 1.20 (20% margin) | 117.7 |
| 4. Pick the size | Smallest DN with rated Cv ≥ 117.7 | DN65 (rated Cv ≈ 150) |
Step 4 uses the capacity table below. Note that DN50 (rated Cv ≈ 95) would be too small — it is below the requirement even before the margin is added.
Specific gravity of common fluids
SG is the density of your fluid divided by the density of water. Values below are typical for the fluids named, at roughly 20 °C. Always confirm against your own process data — SG changes with temperature and concentration, and it sits inside a square root, so an error here feeds straight into the valve size.
| Fluid | Typical SG | Fluid | Typical SG |
|---|---|---|---|
| Water (fresh) | 1.00 | Ethylene glycol 50% | 1.07 |
| Sea water | 1.02–1.03 | Brine, NaCl 10% | 1.07 |
| Gasoline | 0.72–0.77 | Brine, NaCl 25% | 1.19 |
| Diesel | 0.83–0.87 | Caustic soda 50% | 1.52 |
| Light fuel oil | 0.85–0.90 | Sulphuric acid 98% | 1.84 |
| Heavy fuel oil | 0.92–0.98 | Liquid ammonia | 0.68 |
When the simple formula is not enough
The equation above assumes an incompressible liquid with no flashing and no cavitation. That covers most water, HVAC and general industrial duty — but not all of it. Check these three cases before you order:
1. Flashing
If the pressure downstream of the valve falls below the fluid’s vapour pressure, part of the liquid boils and stays vapour. The valve is then passing a two-phase mixture, the simple Kv formula understates the required capacity, and the trim is exposed to severe erosion. Flashing cannot be avoided by valve selection alone — it has to be handled by the system design.
2. Cavitation
If the pressure at the vena contracta drops below vapour pressure but recovers above it downstream, bubbles form and then collapse against the trim. The symptoms are a rattling noise, vibration and pitting of the disc and seat. Where cavitation is likely, choose a valve with a higher FL (anti-cavitation or cage trim) or split the pressure drop over two valves in series.
3. Choked flow
Increasing the pressure drop across a valve does not increase flow without limit. Once ΔP reaches roughly FL² × (P1 − FF × Pv), the flow is choked and further pressure drop buys nothing. A valve sized below the choked point will simply not deliver the flow you calculated.
Gas, steam and two-phase service
These need additional correction factors — the expansion factor Y, the pressure-drop ratio xT, compressibility Z, and for steam the superheat correction. The procedure is set out in IEC 60534-2-1. Do not reuse the liquid formula for gas or steam: the error can be large enough to make the valve unusable. Send our engineers the operating data (medium, P1, P2, temperature, required mass or normal flow) and we will size it.
How valve type changes Cv at the same size
Two valves in DN100 will not have the same Cv. The internal flow path decides how much of the pipe bore is actually usable:
| Valve type | Relative capacity at the same DN | Why |
|---|---|---|
| Ball valve, full bore | Highest | Straight-through bore, essentially no obstruction |
| Gate valve, fully open | Very high | Full-bore when open; not intended for throttling |
| Butterfly valve (centric / wafer) | High | Disc sits in the flow path; capacity falls quickly as it closes |
| Globe / control valve | Lowest of these | Tortuous S-shaped path — that resistance is exactly what makes it good at modulating |
Practical consequence: if you are replacing a globe valve with a butterfly valve at the same DN to save cost, the Cv will change. Always re-check the sizing rather than assuming the bore size is the answer.
Frequently asked questions
What is the difference between Cv and Kv?
Only the units of the reference condition. Cv uses US gpm and 1 psi; Kv uses m³/h and 1 bar. Convert with Cv = 1.156 × Kv, or Kv = 0.865 × Cv.
What units does this calculator use?
Input: m³/h for flow, bar for pressure drop, specific gravity as a ratio. Output: both Kv and Cv, plus a recommended DN for the valve type you select.
Can I use it for steam or gas?
No. Compressible fluids need the expansion factor and pressure-drop-ratio corrections of IEC 60534-2-1. The liquid formula will mis-size the valve. Contact us with the operating data instead.
Why do you recommend a 15–25% margin?
So the valve is not asked to run near its maximum capacity. A valve operated at its choked limit loses control authority, becomes noisy, and cannot be opened further if plant throughput later increases. The margin is what keeps the valve controllable.
My fluid is viscous — does that matter?
Yes. The standard equation assumes turbulent flow. For viscous liquids (heavy oils, syrups, slurries) a Reynolds-number correction factor must be applied — the required Cv is higher than the formula suggests. Tell us the viscosity and we will correct for it.
How accurate is the recommended size?
The capacity table is indicative for centric / wafer butterfly valves and varies with seat and disc design. Treat the recommendation as first-pass selection. Confirm against the actual valve datasheet, or send us the duty and we will confirm the model.