Backflow Preventer Calculator

Inputs

Device type Reduced Pressure Zone (RPZ) Double Check (DCVA) Pressure Vacuum Breaker (PVB) Spill-Resistant Vacuum Breaker (SVB)

Type selection affects typical Cv values.

Flow rate

gpm L/s

Use peak or combined demand flow.

Safety factor

Typical: 1.05–1.25 for planning.

Allowable pressure loss

psi kPa

Use available residual pressure constraints.

Max velocity limit

ft/s m/s

Common planning range: 6–10 ft/s.

Specific gravity

Water ≈ 1.0. Higher SG increases pressure loss.

Custom Cv (optional)

Enter a model-specific Cv if known.

Project notes (optional)

Included in downloads for documentation.

Calculate Reset

Example data table

Outcomes vary by manufacturer Cv, piping constraints, and local requirements.

Formula used

The recommended size is the smallest nominal size that meets both limits.

How to use this calculator

1. Choose a device type that matches the hazard classification and code requirements.
2. Enter the expected peak flow, then apply a safety factor for uncertainty.
3. Set an allowable pressure loss based on your available residual pressure.
4. Pick a velocity limit consistent with your design standards.
5. Optionally enter a manufacturer Cv to match a specific model.
6. Press Calculate, review results, then export CSV or PDF for documentation.

Backflow prevention sizing guidance

1) Why sizing matters on site

Backflow preventers protect potable systems from contamination caused by backpressure or backsiphonage. The required device type is set by hazard level and authority requirements, but correct sizing controls pressure loss and service performance. Undersizing can reduce downstream pressure and complicate startup. Oversizing can add cost and maintenance without improving protection.

2) Inputs used for practical selection

Start with a peak flow from fixture units, irrigation demand, or equipment schedules. Apply a safety factor for uncertainty and future changes. Then set an allowable pressure loss so residual pressure remains acceptable at the most demanding outlet.

3) How the calculator estimates performance

The tool checks two limits for each nominal size: line velocity and estimated assembly pressure loss. Velocity comes from design flow and pipe area. Pressure loss is estimated with a Cv relationship; if you have a submittal, enter the published Cv to match a specific model more closely.

4) Example sizing snapshot

Use the examples below as planning references only. Results can change with different Cv values, available site pressure, and total losses from meters, strainers, and fittings.

5) Installation and commissioning

Provide access for testing, drainage for relief discharge where applicable, and isolation valves that allow certification. Protect outdoor assemblies from freezing and flooding. During startup, confirm inlet pressure and verify the measured pressure drop aligns with the design budget.

6) Interpreting the output

If the recommended size is larger than expected, review your pressure-loss budget and confirm whether upstream equipment already consumes most of the available head. If velocity is the limiting factor, a modest increase in nominal size can stabilize performance. If pressure loss is limiting, consider using a higher-Cv listed model or rebalancing total losses in the service assembly.

7) Documentation for submittals

Record device type, nominal size, approvals, and expected pressure loss at design flow. Attach the model data sheet that supports the Cv used. Use the CSV or PDF export to keep calculations consistent across packages and inspections.

8) Professional note

This page supports early comparison and sizing. Final device selection and installation must follow code, authority direction, and verified manufacturer performance data for the chosen model.

FAQs

1) What flow should I enter?

Use the expected peak demand for the protected line. For mixed-use services, use the combined peak or the governing case from your load calculations and schedules.

2) Why use a safety factor?

A safety factor covers uncertainty, future tenant changes, and data gaps. It prevents repeated resizing when actual site demand is slightly higher than early design estimates.

3) How do I choose allowable pressure loss?

Start with available residual pressure at the most demanding outlet, then subtract piping, meter, and fitting losses. The remaining budget is the allowable loss for the assembly.

4) Can I rely on the default Cv values?

Defaults are planning estimates only. Cv varies by manufacturer and model. For final selection, use the published Cv from the specific submittal that will be installed.

5) Why does the calculator check velocity?

Higher velocity increases noise, erosion risk, and pressure loss sensitivity. A velocity limit helps keep service performance stable and aligns sizing with common design standards.

6) What if no size meets my limits?

The tool will show the largest size for comparison and note the issue. Increase allowable loss, reduce design flow, or use a higher-Cv model to meet constraints.

7) Does this replace code or authority decisions?

No. Device type and installation details must follow local code and the authority having jurisdiction. Use this tool to support sizing and documentation, not compliance decisions.