Valve Cv Calculator
Flow coefficient from flow, gravity, and drop: required Cv for liquid service, plus the reverse check for a valve you already own.
Calculator
About this calculator
This valve Cv calculator sizes liquid service: the flow coefficient a valve needs to pass your flow at your allowed pressure drop. The reverse direction — flow through a valve you already have — is the same formula turned around. Pipe-side losses are on the pipe pressure drop calculator; pump-side head is on the pump sizing calculator.
Calculation Results
Cv = 5.0
Required flow coefficient · Kv 5.8 · reverse check: Cv 5 valve at 4 psi passes 10 gpm
Formula: Cv = Q × √(SG / ΔP)
10 × √(1/4) = 5.0. Pick a valve with catalog Cv comfortably above 5 at full open.
How to use this result
Aim for the valve working 60–80% open at normal flow with catalog Cv above the calculated value at full open — a valve barely bigger than the number, not three sizes up. For control service, check the valve still has authority: it should see roughly a third of the system's dynamic drop at design flow. Choked-flow checks (cavitation, flashing) are beyond this calculator — verify ΔP against the liquid's vapor pressure for hot or volatile fluids.
Continue Your Calculation
The pressure drop you just spent comes from the pipe run — size it next.
Required Cv chart (water, SG = 1)
Cv = Q × √(1/ΔP). For other liquids multiply by √SG.
| Flow (gpm) | 1 psi | 2 psi | 4 psi | 8 psi | 16 psi |
|---|---|---|---|---|---|
| 2 | 2.00 | 1.41 | 1.00 | 0.71 | 0.50 |
| 5 | 5.00 | 3.54 | 2.50 | 1.77 | 1.25 |
| 10 | 10.00 | 7.07 | 5.00 | 3.54 | 2.50 |
| 20 | 20.00 | 14.14 | 10.00 | 7.07 | 5.00 |
| 50 | 50.00 | 35.36 | 25.00 | 17.68 | 12.50 |
Valve Cv formula
Sizing (find Cv): Cv = Q × √(SG / ΔP). For 10 gpm of water at 4 psi: 10 × √(1/4) = Cv 5.0.
Reverse (find flow): Q = Cv × √(ΔP / SG). A Cv-5 valve at 4 psi of water passes 5 × √4 = 10 gpm. At 1 psi the same valve passes only 5 gpm — flow scales with the square root of drop, not linearly.
Unit conversion: Kv = 1.156 × Cv (m³/h at 1 bar). A Cv-5 valve is a Kv-5.8 valve.
Where the drop comes from: pipe pressure drop calculator. Pump head to drive the loop: pump sizing calculator.
Which valve task this is
| Question | Use |
|---|---|
| Liquid valve Cv from flow and drop | This page |
| Straight pipe friction loss | Pipe pressure drop calculator |
| Pump head for the whole loop | Pump sizing calculator |
| Hydraulic (oil) pump power | Hydraulic pump size calculator |
Frequently Asked Questions
How do you calculate valve Cv?
Liquid service: Cv = Q × √(SG / ΔP). Example: 10 gpm of water with a 4 psi drop: 10 × √(1/4) = Cv 5.0.
What is the difference between Cv and Kv?
Same quantity, different units: Kv = 1.156 × Cv (Kv: m³/h water at 1 bar; Cv: US gpm at 1 psi). A Cv-5 valve is Kv-5.8. European catalogs print Kv, US catalogs print Cv.
Is a higher Cv always better?
No. An oversized control valve sits nearly closed: plugs erode, control hunts. Size for 60–80% open at normal flow with full-open Cv above the requirement — one valve size up from the number, not three.
Does this Cv formula work for gases and steam?
No — liquid only. Gas and steam sizing involves upstream pressure, temperature, and expansion factors; the same valve needs a different effective Cv per condition. Use the manufacturer's compressible-flow method before buying for gas or steam.
How much pressure drop should the valve take?
Control service rule of thumb: about one third of the system's dynamic drop at design flow — enough authority to control without wasting pump head. Isolation valves just need wide-open drop small vs the system.
How do I find flow through a valve I already have?
Reverse it: Q = Cv × √(ΔP / SG). Cv 5 at 4 psi water = 10 gpm; at 1 psi only 5 gpm — flow follows the square root of drop. Part-stroke Cv comes from catalog tables (25/50/75% open).
How does Cv relate to pipe size?
Loosely. A full-port 1/2 in ball valve dwarfs a 1/2 in globe valve's Cv because Cv measures the internal flow path, not the thread. Never infer Cv from pipe size — read the catalog.
What pressure drop units does the formula use?
US convention: Q in gpm, ΔP in psi → Cv in gpm/√psi. Metric Kv (m³/h, bar) converts at Kv = 1.156 × Cv. The square-root structure is identical.
Review & sources
Reviewed: CalcPanel Fluid Systems Team
Updated: 2026-11-10