SettingsSettings for this calculationUS
The design (peak) flow rate the valve must pass.
Use the peak demand flow rate expected through the valve, not just an average — undersizing for peak flow can cause excessive pressure drop or noise.
The target pressure drop across the valve at design flow.
This is the difference between the upstream (inlet) pressure and the desired downstream (reduced) pressure at the design flow rate.
Required flow coefficient (Cv)
11.2 Cv
This is the standard simplified Cv equation for water service — always confirm the selected valve's actual rated Cv from the manufacturer's catalog at your specific operating conditions before final selection.
They open the calculator with your figures already in it
Pressure Reducing Valve (PRV) Sizing Calculator: 11.18 Cv — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Simplified valve flow coefficient equation for water: Cv = Q ÷ √(ΔP), where Q is flow rate (gpm) and ΔP is the pressure drop across the valve (psi) — the standard method used in valve manufacturers' catalogs to select a valve size for a target flow and pressure drop
Inputs used
- Design Flow Rate
- 50 gal/min
- Pressure Drop Across Valve
- 20 psi
Confidence note: This is the standard simplified Cv equation for water service — always confirm the selected valve's actual rated Cv from the manufacturer's catalog at your specific operating conditions before final selection.
What this calculation does not cover
- Cv answers flow, and what destroys pressure reducing valves is cavitation. Whether the valve survives its own drop is a separate check on the RATIO of the drop to the inlet pressure above vapor pressure, judged against that valve's own pressure recovery coefficient - a globe-style PRV taking most of its inlet pressure across one seat will flash and collapse bubbles on the downstream side, chewing out the trim and making a noise like gravel in the pipe. Large reductions are split across two valves in series or given anti-cavitation trim, and the Cv from this page will look entirely correct on the valve that fails.
- Sized at one flow, and buildings run at all of them. A direct-acting PRV droops in outlet pressure as flow rises and creeps back up as flow falls away, so a valve chosen for peak demand sits almost shut at three in the morning and can let downstream pressure drift toward inlet at no flow. The standard answer on a riser is a parallel pair - a small valve carrying night and trickle flows, a large one that opens at peak - which is a decision a single-point Cv never puts in front of you.
- A PRV turns everything downstream into a closed system. Water heated behind a valve that will not pass flow backward expands with nowhere to go, and the pressure climbs quickly enough to lift the relief valve on the heater, which is why a thermal expansion tank on the cold feed is required wherever a PRV, check valve or backflow preventer closes the system. That tank is sized from the heater volume and the pressure settings, and has nothing to do with Cv.
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This result is a specification — 11.2 Cv — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Simplified valve flow coefficient equation for water: Cv = Q ÷ √(ΔP), where Q is flow rate (gpm) and ΔP is the pressure drop across the valve (psi) — the standard method used in valve manufacturers' catalogs to select a valve size for a target flow and pressure drop
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