Valve Flow Coefficient Calculator — Cv & Kv, Liquid & Gas

Cv is the flow capacity rating of a valve: the number of US gallons per minute of 60°F water that flows through the valve with exactly a 1 psi pressure drop. A higher Cv means a less restrictive valve. Use tab 1 above to compute it from a measured flow and pressure drop.

For liquids, Cv = Q × √(SG / ΔP), where Q is flow in gpm, ΔP is pressure drop in psi, and SG is specific gravity relative to water. Example: 100 gpm of water (SG = 1) with a 4 psi drop gives Cv = 100 × √(1/4) = 50.0. Enter your numbers in tab 1 — gpm, L/min and m³/h are all accepted.

Rearrange the definition: Q = Cv × √(ΔP / SG). Example: a valve with Cv = 50 and a 4 psi drop on water passes Q = 50 × √(4/1) = 100 gpm. Tab 2 solves this and reports the answer in gpm, L/min and m³/h.

For liquids, ΔP = SG × (Q / Cv)². Example: Cv = 50 with 100 gpm of water gives ΔP = 1 × (100/50)² = 4.0 psi. Note the squared relationship: doubling the flow quadruples the pressure drop. Tab 3 solves this and converts the answer to psi, bar and kPa.

Multiply by 0.865: Kv = 0.865 × Cv. Example: a valve rated Cv = 50 has Kv = 43.25. Tab 5 does the conversion both ways.

Divide by 0.865: Cv = Kv / 0.865. Example: Kv = 43.25 corresponds to Cv = 50.0. Tab 5 does the conversion both ways.

They measure the same thing in different unit systems. Cv is the US coefficient: gpm of 60°F water at a 1 psi drop. Kv is the metric coefficient: m³/h of water at a 1 bar drop. They are related by Kv = 0.865 × Cv (equivalently Cv ≈ 1.156 × Kv).

Gas is compressible, so the published Cv gas sizing equation is used: with flow Q in SCFH and pressures P1, P2 in psia, Cv = Q / (1360 × P1 × √(ΔP/P1) × √(520/(SG·T·Z))) for subcritical flow. Example: 100,000 SCFH of natural gas (SG 0.6) at P1 = 100 psia, P2 = 80 psia and 60°F gives Cv ≈ 1.273 (subcritical). Tab 4 runs this automatically.

When the pressure drop reaches half the inlet pressure (ΔP ≥ P1/2), gas velocity at the vena contracta hits sonic speed and lowering the outlet pressure further cannot increase flow — the flow is choked. The equation then switches to Cv = Q / (660 × P1 × √(520/(SG·T·Z))). Example: 50,000 SCFH of natural gas at P1 = 100 psia, P2 = 40 psia and 60°F gives Cv ≈ 0.587 in the critical regime. Tab 4 detects the regime for you and labels it.

Yes. Tabs 1–3 accept psi, bar and kPa and convert to psi internally (1 bar = 14.5038 psi, 1 kPa = 0.145038 psi) before applying the formulas. Tab 4 accepts psi, bar or kPa for P1/P2 with an absolute/gauge selector, then converts to psia for the gas equation.

SG is relative to water for liquids (water = 1.0) and relative to air for gases (air = 1.0; natural gas ≈ 0.6, propane ≈ 1.52). Tab 4 offers presets for common gases. For liquids denser than water the same flow needs a larger Cv — for example 60 gpm of a SG 1.2 fluid at 9 psi drop needs Cv ≈ 21.9 instead of 20 for water.

Cv itself is defined at 60°F water, so liquid temperature enters only through density (SG). For gases, temperature matters directly: the gas equation uses absolute temperature in Rankine (°R = °F + 459.67) plus the compressibility factor Z. Tab 4 takes °F and Z as inputs.

It is a solid first step for sizing estimates — but always confirm against the manufacturer's published Cv data and valve curves before buying or installing. Real valves deviate by trim style, and critical-service applications need proper engineering review.

Yes. This tool runs 100% in your browser — your flow rates, pressures and valve data never leave your device. There are no accounts, no uploads and no external requests.