Make a quick planning-level estimate of the storage volume a detention basin needs to attenuate a peak inflow down to an allowable outflow. The simplified modified-rational estimate, Vs = (Q_in − Q_out)·t, captures the volume of water that must be temporarily held back during the critical storm.
A detention basin temporarily stores stormwater during a storm and releases it slowly afterward, shaving the peak discharge down to a rate the downstream system can safely accept. The required storage is, fundamentally, the volume of water that arrives faster than it is allowed to leave. This calculator gives a fast, planning-level estimate of that volume using the simplified modified-rational relationship Vs = (Q_in − Q_out)·t — useful for early sizing before a full hydrograph routing is performed.
Vs = max(0, Q_in − Q_out) · t.
Q_in is the peak inflow to the basin (m³/s), Q_out is the maximum allowable outflow (m³/s) set by downstream capacity or regulation, and t is the critical storm duration converted to seconds (the calculator multiplies your minutes by 60). The product is the excess volume — water entering faster than it leaves — that the basin must hold. If the allowable outflow equals or exceeds the inflow, no storage is needed and Vs is zero.
Conservation of mass governs a basin: the rate of change of stored volume equals inflow minus outflow. Whenever inflow exceeds the controlled outflow, water accumulates; the basin fills until inflow drops back below the release rate, then it drains. The peak storage required is the largest cumulative difference between the inflow and outflow hydrographs. This calculator approximates that by assuming a roughly constant excess (Q_in − Q_out) sustained over the critical duration — a deliberately conservative simplification.
A detention basin (dry pond) is designed to empty completely between storms; it controls the rate of runoff but not its total volume, and its main job is flood-peak attenuation. A retention basin (wet pond) maintains a permanent pool, capturing a volume of runoff that leaves only by infiltration and evaporation; it provides water-quality treatment and volume reduction as well as peak control. The storage estimated here is the active (live) storage above the permanent pool that does the peak-shaving.
This is a screening tool, not a final design. Rigorous detention design routes the entire inflow hydrograph through the basin using the storage-indication (modified Puls) method, balancing inflow, the basin's stage-storage curve, and the outlet structure's stage-discharge relationship at each time step. The outlet — typically an orifice, weir, or multi-stage riser — is sized so the peak outflow never exceeds the allowable release rate for each design return period (commonly the 2-, 10-, and 100-year storms checked together). Always confirm the final volume with full routing.
It is a conservative first-cut estimate, generally on the high side, because it assumes the full excess flow persists for the whole critical duration rather than following the rise-and-fall of a real hydrograph. It is excellent for early feasibility and footprint planning, but the basin should be finalised with hydrograph routing, which usually yields a somewhat smaller required volume.
Use the critical storm duration — the duration that produces the maximum required storage, which is often longer than the catchment's time of concentration. In practice you test several durations from the IDF curves and select the one giving the largest Vs. For a quick estimate, a duration near the time of concentration or a standard design storm length is a reasonable starting point.
It is usually dictated by regulation or downstream capacity — commonly the pre-development peak runoff for the same return-period storm, so the site does not increase flooding downstream. Sometimes it is the capacity of the receiving channel, culvert, or storm sewer. The outlet structure is then sized to physically limit the release to that rate.
Then the controlled release can handle the inflow as fast as it arrives, so no water accumulates and no detention storage is required — the calculator reports zero volume. In real projects this means the downstream system already has enough capacity for that storm and a basin is unnecessary for peak control (though one might still be wanted for water quality).
No — it estimates only the storage volume. Sizing the outlet (orifice diameter, weir length, multi-stage riser openings) requires the basin's stage-storage and the target stage-discharge relationship, solved through routing so that the peak outflow stays at or below the allowable rate across all design storms. The outlet design and the storage volume are determined together in the full routing analysis.