Detention Basin Volume Calculator
Get a planning-level detention storage volume for a development: the extra runoff volume the project adds is the storage the basin must hold.
Required Detention Storage (Planning Estimate)
| Item | Value |
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How the calculation works
Runoff-volume-difference method (planning level)
Added runoff volume V (ft³) = (Cpost − Cpre) × P (in) × A (acres) × 3,630
1 acre-in = 43,560 ft² × (1/12 ft) = 3,630 ft³; acre-ft = ft³ ÷ 43,560
Drawdown time (hr) = V (ft³) ÷ (release (cfs) × 3,600) — approximate, assumes the basin drains at the release rate
Added runoff volume V (ft³) = (Cpost − Cpre) × P (in) × A (acres) × 3,630
1 acre-in = 43,560 ft² × (1/12 ft) = 3,630 ft³; acre-ft = ft³ ÷ 43,560
Drawdown time (hr) = V (ft³) ÷ (release (cfs) × 3,600) — approximate, assumes the basin drains at the release rate
Notes & limitations
This volume-difference approach is a standard planning-level method found in state stormwater design manuals. It estimates the extra runoff volume the development creates, which the basin must store and release at the pre-development rate.
- Planning estimate only — final basin design requires hydrograph routing (e.g., SCS TR-55/TR-20, SWMM, or HEC-HMS) with the outlet structure sized for the required release rates at each design storm.
- Most regulations require controlling multiple storms (e.g., 2-, 10-, and 100-year) and water-quality volume — this tool covers one storm at a time.
- The drawdown check is approximate; many agencies require extended detention (24–48 hr drawdown) for channel protection — verify against your local manual.
- Post-development C must exceed pre-development C; a redevelopment that reduces imperviousness needs no added storage by this method.
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