DrainageCalculators

Pond Sizing Calculator

Calculate preliminary detention pond storage requirements using the Modified Rational Method. Size orifice and weir outlets for stormwater management facilities. Professional-grade hydrology tool for civil engineers.

What This Solves

Calculates the required storage volume and dimensions for a detention pond that temporarily stores stormwater runoff and releases it at a controlled rate.

Best Used When

  • You need to size a surface detention basin to meet peak flow reduction or water quality requirements
  • You want to estimate preliminary pond dimensions and outlet structure sizing
  • You are comparing different pond shapes and outlet configurations for site planning

Do NOT Use When

Key Assumptions

  • Pond shape can be approximated by simple geometric forms (rectangular, circular, trapezoidal)
  • The stage-storage relationship is based on uniform side slopes or cross-sections
  • Outlet discharge is governed by orifice or weir equations with standard coefficients
  • Tailwater elevation does not significantly affect outlet discharge
  • Infiltration and evaporation losses are negligible during the design storm

Input Quality Notes

Outlet structure sizing is preliminary and should be verified with detailed hydraulic routing. Field surveys are required to confirm pond geometry and outlet invert elevations.

Calculation Mode

Pond Storage: Calculate preliminary detention storage volume using the Modified Rational Method.

Input Parameters

Peak Flow Data

Enter pre- and post-development peak flows

cfs

Peak flow before development

cfs

Peak flow after development

Storm Parameters

Enter watershed timing and rainfall

min

Watershed time of concentration

in/hr

Design storm intensity for Tc duration

Target Release (Optional)

Override default pre-development release rate

cfs

Leave blank to use pre-development peak

Detention Pond Sizing Overview

This calculator provides preliminary sizing for detention ponds and outlet structures. The Modified Rational Method estimates required storage volume based on the difference between post-development and pre-development peak flows.

Key equations:

  • Storage: Vs = 0.5 x (Qi - Qo) x (tc + td)
  • Orifice: Q = Cd x A x sqrt(2gh)
  • Weir: Q = Cw x L x H1.5

Typical Coefficients

Structure TypeCoefficientNotes
Sharp-crested orifice0.60Standard value
Short tube orifice0.80L/D = 2-3
Rectangular weir (US)3.33Sharp-crested
Cipolletti weir (US)3.371:4 side slopes
90-degree V-notch2.50Q = C x H^2.5

Source: FHWA HEC-22 (2009), Chapter 7-8.

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 Pond Sizing 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.