SettingsSettings for this calculationUS
The design flow rate through the backflow preventer.
Use the peak or design flow rate expected through this connection, in the unit shown.
The internal diameter of the pipe at the backflow preventer connection.
Use the actual internal diameter of the pipe connection, matching the backflow preventer's port size.
Flow velocity
484.1 ft/min
They open the calculator with your figures already in it
Backflow Preventer Discharge Velocity Calculator: 484 ft/min — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- Velocity = flow rate ÷ pipe cross-sectional area, the standard method for checking flow velocity through a backflow preventer assembly against the manufacturer's rated maximum velocity
Inputs used
- Flow Rate
- 79 gal/min
- Pipe Diameter
- 2 in
What this calculation does not cover
- The relief port is the flow that floods plant rooms, and it is not this one. A reduced-pressure assembly is built to dump water to atmosphere when a check fouls or downstream pressure rises, and the rate at full relief can be a large fraction of line flow — far more than a routine floor drain and air-gap fitting will swallow. That receptor is sized from the manufacturer's published relief discharge for the specific model and size, and it is what decides whether a failure is a puddle or a ceiling coming down on the floor below.
- The velocity limit that bites first usually belongs to the pipe, not the assembly. Copper thins by erosion-corrosion above roughly 8 ft/s (2.4 m/s) cold and about half that on hot recirculating lines, and it thins at the elbow and tee immediately downstream where the flow is still disturbed by the device — with plumbing codes capping velocity for noise on top of that. A figure comfortably inside the assembly's rating can still be outside what the pipe carrying it will survive.
Add the equipment this sizes
This result is a specification — 484.1 ft/min — 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
- Velocity = flow rate ÷ pipe cross-sectional area, the standard method for checking flow velocity through a backflow preventer assembly against the manufacturer's rated maximum velocity
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.