GPM to Required Cv Converter (Water, ISA 75.01)
Per ISA 75.01-2012, the required Cv for a target water flow is: Cv = GPM / √(ΔP / SG). This is the inverse of the Cv-to-GPM formula. The engineer specifies a required flow (e.g.,...
Formula
Source: ISA 75.01-2012; Fisher Control Valve Handbook (5th ed.) | Last reviewed: July 3, 2026
Examples
100 GPM (US)
= 25.8 cv
- delta_p = 15
- sg = 1
100 GPM, 15 psi ΔP water → Cv 25.8
250 GPM (US)
= 72.2 cv
- delta_p = 12
- sg = 1
250 GPM, 12 psi ΔP water → Cv 72.2
500 GPM (US)
= 100 cv
- delta_p = 25
- sg = 1
500 GPM, 25 psi ΔP water → Cv 100 (large chiller)
1000 GPM (US)
= 141.4 cv
- delta_p = 50
- sg = 1
1000 GPM, 50 psi ΔP water → Cv 141 (cooling tower)
Quick Reference Table
| Flow (GPM) | Required Cv (15 psi ΔP) | Recommended Valve Size | Typical Application |
|---|---|---|---|
| 10 | 2.6 | 1/2" globe | Lab, small injection |
| 25 | 6.5 | 1/2" or 3/4" | Sample line, small process |
| 50 | 12.9 | 1" globe | Instrument air, small cooling |
| 100 | 25.8 | 1.5" or 2" | Process cooling water |
| 250 | 72.2 | 2" globe | Heat exchanger, chiller |
| 500 | 100 | 3" globe | Large chiller, condenser |
| 1000 | 141.4 | 4" globe | Cooling tower |
| 2000 | 258.2 | 6" globe | Main header |
Where is this used?
Used to specify the required Cv for: cooling water control valves, chemical injection valves, pump recirculation valves, tank level control valves, heat exchanger control valves, and boiler feedwater control valves.
The selection process: 1) Calculate required Cv from GPM and ΔP, 2) Add 20-30% margin (for control range and authority), 3) Select a valve body with max Cv ≥ 1.3 × required Cv, 4) Verify valve authority β = ΔP_valve / ΔP_system ≥ 0.5.
Real-World Usage Scenarios
Cooling Water Valve Sizing Example
A brewery's heat exchanger requires 200 GPM of cooling water with a system pressure drop of 18 psi (10 psi across heat exchanger, 5 psi in piping, 3 psi design for the valve). Required Cv = 200/√3 = 115.5. Selecting a 2-inch globe valve with a Cv range of 30-120 (max Cv 120). Adding 20% margin, the valve can modulate from Cv 24 to 96 (factor of 4) — adequate for normal load variations. Valve authority = 3/18 = 0.17 — BELOW the 0.5 rule of thumb. The engineer selects a smaller trim (max Cv 80) and reduces valve ΔP to 5 psi by closing a manual balancing valve downstream, achieving β = 5/(10+5+5) = 0.25. Still below 0.5 — the system needs a redesign with a dedicated control valve circuit.
Common Mistakes to Avoid
Sizing the valve for 100% of system pressure drop
A common error: assuming ΔP_valve = ΔP_system. In practice, ΔP_valve is only a fraction (sometimes a small fraction) of the system drop. If you size a valve for 100% ΔP and the actual system has only 20% across the valve, your authority is 0.20 — well below the 0.5 threshold, and the control will be poor. Always use the actual ΔP across the valve, not the total system.
No margin for control rangeability
A valve selected with max Cv exactly equal to required Cv will be fully open at design flow, with no control range. Always select max Cv ≥ 1.3 × required Cv (30% margin). This gives a 1.3:1 turndown in the upper range. For wider turndown, select larger margins (e.g., 2× required for 50% turndown).
Industry Standards Referenced
Frequently Asked Questions
What is the required Cv for 100 GPM at 15 psi?
Cv = 100 / √(15/1.0) = 100 / 3.87 = 25.8. This is the textbook example: a valve with Cv = 25.8 passes 100 GPM of water at 15 psi ΔP.
How much margin should I add when sizing?
Per ISA 75.01 best practice, the maximum valve Cv should be 1.2-1.5× the required Cv. The 1.2× gives modest margin for variations; 1.5× gives better control range. Avoid selecting the smallest valve body that meets the maximum Cv — a larger body with smaller trim is often better for control rangeability.
Does this formula work for gas or steam?
No. For gases, vapors, and steam, the ISA 75.01 formulas are different and include an expansion factor Y, the molecular weight, the upstream pressure, and the temperature. Use valve-cv-calculator-gas or valve-cv-calculator-steam for compressible service.
Reviewed for accuracy
Cross-referenced against ISA 75.01-2012 and Fisher worked examples · Last reviewed: July 3, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.