DrainageCalculators

Storage Volume Calculator

Calculate required detention storage volume using the Modified Rational Method, Baker Method, FAA Method, and simplified approaches. Professional stormwater management tool for civil engineers.

What This Solves

Calculates the required detention storage volume using multiple simplified methods (Modified Rational, Baker, FAA) without full hydrograph routing.

Best Used When

  • You need a preliminary estimate of detention storage volume for site planning
  • You want to compare storage estimates from several standard screening methods
  • You are at the conceptual design stage and full routing analysis is not yet warranted

Do NOT Use When

Key Assumptions

  • Simplified volume methods assume idealized inflow and outflow hydrograph shapes
  • The allowable release rate is known and constant
  • Each method has its own inherent assumptions about hydrograph shape and timing
  • Results are preliminary and should be verified with full routing for final design

Input Quality Notes

Allowable release rate is typically set by local regulations (e.g., pre-development peak flow for a specified return period). Verify this value with the reviewing authority before sizing.

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.

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

This disclaimer applies to the Storage Volume Calculator calculator and all associated calculations.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.