PONDCALC
Detention Pond Sizing Estimator
ModeQuick Estimate
StandardASCE / TR-55 / Local Manual
UnitsUS Customary
⚠ NOT FOR DESIGN OR CONSTRUCTION — This tool provides preliminary estimates only. Results must be verified by a licensed professional engineer using site-specific survey data, local IDF curves, and applicable municipal/state stormwater standards before use in any engineering design, permitting, or construction activity.
01
Project Information
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02
Watershed Parameters
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Typical C: Pasture 0.10–0.30 · Residential 0.35–0.55 · Commercial 0.70–0.90
03
Storm Events & Design Criteria
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Verify against NOAA Atlas 14 or local IDF curves.
04
Pond Geometry
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05
Wet Pond / Water Quality
OPTIONAL▶
Enable Wet Pond / Water Quality Sizing
Permanent pool provides sedimentation and nutrient uptake. Sized per EPA/State stormwater manual.
06
Freeboard Requirements
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Total = min required + wave runup + settlement. NRCS requires 2.0 ft for high-hazard dams.
⟐ View equations & methodology
◈
Enter parameters and
click Calculate
⚡
Quick Pond Estimate
No IDF curves, no C values needed. Just your site area, land cover breakdown, and state — Tc and intensity are automatically scaled to your drainage area size using TR-55 guidelines.
📐 Site Information
Select a state to see regional rainfall assumptions.
🌾 Land Cover — PRE-DEVELOPMENT
Existing conditions before your project. Must total 100%.
Grass / Lawn / Pasture 70%
Bare Earth / Disturbed 20%
Impervious (roof, paving) 10%
Total 100%
Pre-Dev Composite C: —
🏗 Land Cover — POST-DEVELOPMENT
Proposed / built-out conditions after your project. Must total 100%.
Grass / Lawn / Pasture 25%
Bare Earth / Disturbed 10%
Impervious (roof, paving) 65%
Total 100%
Post-Dev Composite C: —
Grass C≈0.25 · Bare earth C≈0.55 · Impervious C≈0.85
⟐ View equations & methodology
Runoff Contribution by Cover Type
Pond Geometry Rough Estimates (Conservative Assumptions)
⚠ Preliminary Estimate Only — Not for Design
  • Regional rainfall from conservative NOAA Atlas 14 representative values for selected state grouping
  • Design storm:
  • Pre- and post-development composite C values calculated from user-entered land cover percentages
  • Time of concentration (Tc) and rainfall intensity automatically scaled to drainage area size per TR-55 / typical municipal standards — larger sites use longer Tc and lower intensity
  • Pond geometry estimated at 3:1 side slopes, 5 ft active depth, 1 ft freeboard
  • A full site-specific analysis with IDF curves, actual Tc, and surveyed geometry is required for design