Valve Flow Rate Calculator (Cv & ΔP → Flow)
Know your valve’s Cv (or Kv) and the pressure drop, and want to know how much will actually flow? This calculator solves the flow-capacity question directly — for liquid, gas, and steam — and warns you when the valve is choked. It is the inverse of valve sizing: use it to verify capacity, debottleneck a line, or check an existing valve. To size a new valve from a target flow, use a Cv sizing calculator instead.
The flow equations
If you enter Kv, it is converted to Cv first (Cv = 1.156 × Kv). Use absolute pressure for gas and steam.
Typical Cv by valve type & size
| Size | Globe | Ball | Butterfly | Gate |
|---|---|---|---|---|
| 1/2" | 4–6 | 15–20 | – | 10–15 |
| 1" | 10–15 | 30–40 | 35–45 | 20–30 |
| 2" | 40–50 | 100–125 | 100–120 | ~90 |
Cv = gpm of 60 °F water at 1 psi drop. Kv = m³/h at 1 bar. Kv = 0.865·Cv.
Frequently Asked Questions
How do I find the flow rate through a valve?
Use Q = Cv × √(ΔP / SG) for liquids, where Cv is the valve’s flow coefficient, ΔP is the pressure drop in psi, and SG is specific gravity. The calculator also handles gas and steam with the appropriate compressible-flow equations.
What is the difference between this and a Cv sizing calculator?
A Cv sizing calculator starts from a target flow and tells you the Cv you need. This calculator does the reverse: you already have a valve with a known Cv and want to know how much it will actually pass at a given pressure drop — useful for capacity checks and troubleshooting.
Why is my measured flow lower than calculated?
Common causes are choked flow, a higher-than-assumed specific gravity, partially open or worn trim, upstream/downstream fittings reducing effective Cv, or gauge-vs-absolute pressure errors. The calculator flags choked flow automatically.
Can I enter Kv instead of Cv?
Yes. Switch the coefficient selector to Kv and it is converted to Cv internally (Cv = 1.156 × Kv) before the flow is computed.
Does the calculator handle steam?
Yes. Steam uses a mass-flow form, W = 2.1 × Cv × √(ΔP × (P1+P2)), giving flow in lb/hr and kg/hr. It is a saturated-steam estimate; superheated steam flows slightly less for the same Cv.
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