Stormwater Detention Volume Calculator
Estimates the on-site detention (OSD) storage volume needed to hold post-development stormwater back to the permitted pre-development discharge. A quick constant-flow check for developers, civil designers and council reviewers before a full OSD routing design.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the post-development peak flow Qpost (L/s) — the unattenuated flow leaving the developed site.
- Enter the permitted / pre-development discharge Qpre (L/s) set by the council or catchment authority.
- Enter the critical storm duration in minutes; the tool returns the required detention volume in m³.
How it works
On-site detention temporarily stores the extra runoff created by development and releases it slowly so the site's discharge does not exceed the pre-development rate. In the simplified constant-flow method, both inflow and controlled outflow are treated as steady over the storm, so the storage needed is the net flow times the storm length.
V (m³) = (Qpost − Qpre) × duration × 60 / 1000: the net detained flow (Qpost − Qpre) in L/s is multiplied by the duration in minutes and 60 s/min to get litres, then divided by 1000 for cubic metres. If Qpost does not exceed Qpre, no detention is required.
Worked example
150 L/s net over a 30-minute storm. With Qpost = 200 L/s, Qpre = 50 L/s and duration = 30 min: net flow = 150 L/s, so V = 150 × 30 × 60 / 1000 = 270 m³ of detention storage.
Common mistakes
- Mixing units — Qpost and Qpre must both be in L/s and the duration in minutes for the 60/1000 factors to work.
- Testing only one storm duration; the critical (governing) duration is the one that maximises required storage and must be found by checking several.
- Treating this constant-flow estimate as a final OSD design instead of routing the full storm hydrograph through the outlet.
Frequently asked questions
What is on-site detention (OSD)?
A storage (tank, basin or oversized pipe) that holds back the extra runoff from development and releases it at a controlled rate so the site's peak discharge stays at or below the permitted pre-development flow.
What is the permitted discharge Qpre?
The maximum flow the site is allowed to release, usually set by the council or catchment authority — often the pre-development peak for the design storm (the permissible site discharge).
Why does duration matter so much?
Storage depends on both the net flow and how long it lasts. Different storm durations give different volumes, so you must test a range and adopt the critical duration that needs the most storage.
Does this replace a proper OSD design?
No. It is a first-pass estimate assuming constant inflow and outflow. A real design routes the full storm hydrograph through the outlet's storage–discharge behaviour and checks multiple durations.
What if Qpost is less than or equal to Qpre?
Then the developed flow already meets the permitted rate and no detention is required — the calculator flags this instead of returning a volume.
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Tip: Enter any known values to calculate the remaining results.
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