Backflow Preventer Calculator

Calculator Inputs

Enter project values below. Results appear above this form after submission.

Flow Rate

Flow Unit

Supply Pressure

Supply Pressure Unit

Required Downstream Pressure

Required Pressure Unit

Elevation Rise

Elevation Unit

Equivalent Pipe Length

Length Unit

Internal Pipe Diameter

Diameter Unit

Hazen-Williams C Factor

Device Type

Custom Cv

Safety Factor (%)

Reset

Formula Used

1) Adjusted design flow: Q_adj = Q × (1 + Safety Factor)

2) Device pressure loss: ΔP_device = (Q / Cv)² for water, with flow in gpm and loss in psi.

3) Pipe velocity: V = Q / A, where area is based on internal pipe diameter.

4) Pipe friction loss: Hazen-Williams form using equivalent straight length: h_f = 4.52 × Q^1.85 / (C^1.85 × d^4.87) × (L / 100)

5) Elevation loss: ΔP_elev ≈ 0.433 × Elevation(ft)

6) Residual pressure: P_res = P_supply - (ΔP_device + ΔP_friction + ΔP_elev)

7) Recommended size: the smallest catalog size whose flow limit is at least the adjusted design flow.

How to Use This Calculator

Enter the design flow and choose the correct unit.
Provide the available supply pressure at the device location.
Enter the minimum downstream pressure needed by the system.
Add elevation rise to the highest served point or equivalent critical outlet.
Enter equivalent pipe length, including fittings if needed.
Input the actual internal pipe diameter and Hazen-Williams C value.
Select a device type, or use Custom Cv from manufacturer data.
Set a safety factor, then calculate and review size, losses, and margin.

Example Data Table

Flow	Supply Pressure	Pipe Diameter	Device Type	Total Loss	Residual Pressure	Recommended Size
65 gpm	78 psi	2.0 in	RPZ	11.85 psi	66.15 psi	2 in
120 gpm	82 psi	2.5 in	DCVA	14.22 psi	67.78 psi	2.5 in
38 gpm	60 psi	1.5 in	PVB	9.31 psi	50.69 psi	1.25 in

FAQs

1) What does this calculator estimate?

It estimates backflow preventer size, device loss, pipe velocity, friction loss, elevation loss, residual pressure, and pressure margin. Use it for planning, screening, and comparing options before final design approval.

2) Why is Cv important?

Cv links water flow to pressure drop through the selected device. A higher Cv usually means lower loss at the same flow, which helps preserve downstream pressure.

3) Why does elevation matter?

Water pressure falls as it rises. Each foot of elevation costs roughly 0.433 psi, so tall buildings or raised outlets can change whether a device arrangement is workable.

4) What pipe velocity is usually preferred?

Many designers aim to keep service velocity moderate, often below about 8 ft/s. Lower velocity reduces noise, wear, and friction loss while improving operating stability.

5) Is this enough for final device selection?

No. Final selection must follow plumbing code, hazard classification, manufacturer data, orientation limits, clearances, testing rules, and local authority requirements.

6) Why can residual pressure become too low?

The device may be undersized, the line may be too small, the equivalent length may be high, or the supply pressure may be too low for the chosen arrangement.

7) When should I use Custom Cv?

Use Custom Cv when you already have a manufacturer coefficient from a submittal or catalog. It lets you compare a real product against the default screening values.

8) Which assembly type should I choose?

That depends on hazard level, code rules, and site conditions. High hazard systems often require RPZ assemblies, while lower hazard cases may allow other approved devices.