How to Calculate Pressure Drop Across a Valve Using Cv (Step-by-Step Guide)

The Cv value (flow coefficient) is the most widely used method to calculate the pressure drop (ΔP) across a valve. Valve manufacturers publish Cv for every valve size and model, making it the standard approach for sizing and system design.

This guide explains how Cv works and how to use it to calculate the exact pressure drop.

Table of Contents

What Is Cv?

Cv = the flow rate (in GPM) that produces a 1 psi pressure drop across the valve for water at 60°F.

So with Cv, you can easily estimate how much pressure will be lost at any flow rate.

Pressure Drop Formula Using Cv

For water or water-like liquids:

ΔP = (Q / Cv)²

Where:

ΔP = pressure drop (psi)
Q = flow rate in GPM
Cv = valve coefficient from the manufacturer

This is the official industry-standard equation.

Step-by-Step Example

Goal: Calculate pressure drop when water flows at 8 GPM through a valve with Cv = 4.

Step 1 — Divide flow rate by Cv

8 ÷ 4 = 2

Step 2 — Square the result

2² = 4

✅ Pressure drop = 4 psi

Calculate Pressure Drop for Fluids Other Than Water

If the fluid has a different density than water, use:

ΔP = (Q / Cv)² × SG

Where:

SG = specific gravity (water = 1.0)

Example:
Flow = 8 GPM
Cv = 4
SG = 1.2

ΔP = (8/4)² × 1.2
= 4 × 1.2
= 4.8 psi

How to Calculate Flow Rate When Pressure Drop Is Known

Sometimes you know the allowable pressure drop and need to find the flow:

Q = Cv × √ΔP

Example:
Cv = 6
ΔP = 3 psi

Q = 6 × √3
= 6 × 1.732
= 10.39 GPM

Common Mistakes to Avoid

❌ Using Cv from a fully open valve when the valve is throttled
❌ Mixing up units (Cv uses GPM + psi only)
❌ Forgetting to include specific gravity for oils, chemicals, or hot water
❌ Using published Cv for a different valve size or end connection