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Backflow Preventer Relief Valve Discharge Calculator

Calculate the discharge flow rate from a reduced-pressure backflow preventer's relief valve, for air gap/drain sizing.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Relief valve discharge flow

2.3 L/s

Check your inputs

Confirm the actual relief valve orifice area and discharge coefficient from the specific backflow preventer manufacturer's data — this is a general orifice-flow estimate, not a substitute for the manufacturer's rated relief capacity.

Discharge velocity
4.6 m/s

Running these inputs gives 2.30 L/s as the relief valve discharge flow. Expect some drift against the real job; the calculation is solid but conditions on site are not. Currently reading for United States under IRC 2024 — pick a different market above and the figures re-cast accordingly.

Add the equipment this sizes

This result is a specification — 2.302 L/s — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Discharge flow = orifice area × discharge coefficient × √(2×g×head), the standard orifice discharge equation applied to a relief valve opening, used to size the air gap fitting or floor drain that must handle the preventer's full relief discharge

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Frequently asked questions

Why does this matter for air gap or drain sizing?
A reduced-pressure backflow preventer's relief valve can discharge its full rated flow instantaneously if the assembly senses a pressure fault. The air gap fitting or floor drain receiving that discharge must be able to handle this flow rate without flooding, so this figure is used to size that drainage path.
How accurate is the 0.6 discharge coefficient?
0.6 is a commonly used approximation for a sharp-edged orifice, but actual relief valve geometry varies by manufacturer and model. Confirm the manufacturer's actual orifice area and discharge coefficient (or a directly rated relief flow) rather than relying solely on this generic estimate.
Is this the same as the backflow preventer's rated flow capacity?
No — this estimates the relief valve's discharge flow during a relief event, which is a separate figure from the assembly's normal supply flow capacity. Always verify against the manufacturer's published relief discharge data for the specific model before final drain sizing.