Storage Volume Calculator

Estimate preliminary detention storage with either a Modified Rational peak-flow approximation or one selected quick method (Baker, FAA, or simplified) using that method's inputs.

Calculation Method

Modified Rational Method: Calculate storage from peak flows using triangular hydrograph approximation. Best for preliminary sizing.

Input Parameters

Peak Flow Data

Enter pre- and post-development peak discharge rates

cfs

Peak flow before development

cfs

Peak flow after development

Time of Concentration

Watershed timing characteristics

Time for runoff to reach outlet

Select time unit for Tc

Optional Parameters

Override defaults if needed

cfs

Leave blank to use pre-development peak

hours

Leave blank to calculate automatically

Storage Volume Overview

Detention storage volume is the amount of stormwater that must be temporarily held in a detention basin to reduce peak discharge rates to allowable levels.

The Modified Rational Method calculates storage as:

Vs = 0.5 x (Qi - Qo) x (tc + td) x 3600

Where:

  • Vs = Required storage volume
  • Qi = Peak inflow (post-development)
  • Qo = Allowable outflow (pre-development)
  • tc = Time of concentration
  • td = Critical storm duration

Method Comparison

MethodBest ForAccuracy
Modified RationalPreliminary sizingGood for small watersheds
Baker MethodQuick estimatesApproximate
FAA MethodAirport drainageConservative
SimplifiedScreening analysisOrder of magnitude

For final design, use level pool routing with detailed stage-storage-discharge relationships.

For educational purposes only. Not a substitute for professional engineering judgment.

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