Detention Volume Calculator
Modified Rational Method — Storage Sizing
Size required underground detention storage from post-development inflow and your permitted release rate. Results in cubic feet, gallons, and acre-feet.
1Site & Storm Parameters
1 acre = 43,560 ft².
Use the IDF intensity that matches your storm duration. The runoff calculator can look this up by ZIP from NOAA data.
Test several durations with their matching intensities; the largest required volume is the critical (controlling) case.
Often set to the pre-development peak (Q = C·I·A) by your jurisdiction. Size the outlet with the weir & orifice calculator.
2Required Detention Storage
Required Storage Volume
10,260ft³
In Gallons
76,750
gal
In Acre-Feet
0.236
ac-ft
Peak Inflow (Q = CIA)
7.20
CFS
Inflow Volume
12,960
ft³
About the Modified Rational Method
Peak inflow is held constant over the storm duration to approximate the runoff volume, then the volume released at the allowable rate is subtracted. The remainder is the storage you must provide. This is a screening method for small sites — large or complex sites should be routed with a full hydrograph model.
V = (C·I·A − Qallow) × td
- V = Required storage (ft³, with td in seconds)
- C·I·A = Peak inflow Qin (cfs)
- Qallow = Allowable release rate (cfs)
- td = Storm duration (s)
Related Calculators & Products
Complete the Design
- Runoff Calculator — get peak inflow and the matching IDF intensity by ZIP.
- Weir & Orifice Calculator — size the outlet structure that sets the allowable release.
- Pipe Sizing Calculator — size the inflow and outflow pipes for the system.
- Open Channel Flow Calculator — check conveyance to and from storage.
Specify the Storage
- Detention & retention systems — subsurface chambers sized to the volume above.
- ACO StormBrixx and NDS StormChamber modular storage systems.
- Permeable pavers for surface storage and infiltration credit.
- Catch basins as inflow inlets to the detention system.