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
- You need an underground detention system instead of a surface pond — Use Underground Detention Calculator
- You need to route a full inflow hydrograph through the pond to get the outflow hydrograph — Use Level Pool Routing Calculator
- You need only the storage volume without pond geometry or outlet details — Use Storage Volume Calculator
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.
Estimate the detention storage volume needed to hold a developed site's peak runoff back to its pre-development rate, then size the orifice or weir outlet that controls the release. Sizing uses the Modified Rational Method with standard outlet hydraulics for preliminary detention basin design.
Calculation Mode
Pond Storage: Calculate preliminary detention storage volume using the Modified Rational Method.
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 Type | Coefficient | Notes |
|---|---|---|
| Sharp-crested orifice | 0.60 | Standard value |
| Short tube orifice | 0.80 | L/D = 2-3 |
| Rectangular weir (US) | 3.33 | Sharp-crested |
| Cipolletti weir (US) | 3.37 | 1:4 side slopes |
| 90-degree V-notch | 2.50 | Q = C x H^2.5 |
Source: FHWA HEC-22 (2009), Chapter 7-8.
For educational purposes only. Not a substitute for professional engineering judgment.
How pond sizing works
Detention works by storing the extra runoff that development creates and letting it out slowly so the downstream peak is no higher than before. This tool covers the two core design steps: estimating the storage volume, and sizing the outlet structure that meters the flow.
1. Required storage — Modified Rational Method
The Modified Rational Method approximates the inflow as a triangular hydrograph and the outflow as a constant release. The required storage is the maximum difference between inflow and outflow volume over a range of storm durations:
Vs = Qi2 × tc ÷ (4 × Qo)
- Vs = required detention storage (acre-ft or m³)
- Qi = post-development peak inflow (cfs or m³/s)
- Qo = target release rate, defaults to the pre-development peak (cfs or m³/s)
- tc = time of concentration (min)
Two intermediate values support this. The routing coefficient is the ratio of outflow to inflow, r = Qo ÷ Qi. The critical storm duration — the duration that produces the largest storage requirement — is estimated as t* = tc × (1 + r) ÷ (1 − r), capped at 10×tc. When r ≥ 1 the outlet can already pass the inflow and no detention is required.
2. Outlet sizing — orifice and weir equations
The outlet structure controls the release rate. The calculator sizes three common types:
- Orifice: Q = Cd × A × √(2gh). Solving for area gives A = Q ÷ (Cd × √(2gh)), and the circular diameter is D = √(4A ÷ π). Here g = 32.2 ft/s² (US) or 9.81 m/s² (SI), and h is the head above the orifice centerline.
- Rectangular / Cipolletti weir: Q = C × L × H1.5, so the required crest length is L = Q ÷ (C × H1.5), where H is the head over the crest.
- V-notch weir: Q = C × tan(θ/2) × H2.5, solved for the notch angle θ that passes the target discharge.
Methods and coefficients follow FHWA HEC-22 (Urban Drainage Design Manual), McCuen (2005), NRCS TR-55, the USBR Water Measurement Manual, and ASCE MOP 77.
Outlet discharge coefficients
Default discharge coefficients used by the outlet sizing tabs. US values are for use with feet and cfs; SI values are for use with meters and m³/s. Enter a custom coefficient if your structure differs from these standard sharp-crested cases.
| Outlet type | Equation | Coefficient (US) | Coefficient (SI) | Source |
|---|---|---|---|---|
| Sharp-crested orifice (Cd) | Q = Cd·A·√(2gh) | 0.60 | 0.60 | Mays (2011); HEC-22 |
| Rectangular weir | Q = C·L·H1.5 | 3.33 | 1.84 | HEC-22; Chow (1959) |
| Cipolletti (trapezoidal) weir | Q = C·L·H1.5 | 3.367 | 1.859 | USBR Water Measurement Manual |
| 90° V-notch weir | Q = C·tan(θ/2)·H2.5 | ~2.5 | ~1.38 | Mays (2011) |
Minimum recommended orifice diameter to limit clogging: 4 in (100 mm), per ASCE MOP 77. Coefficients assume free (unsubmerged), fully ventilated flow with negligible approach velocity.
Inputs and assumptions
What you need to enter
- Pre- and post-development peak flow (cfs or m³/s), typically from the Rational Method
- Time of concentration of the watershed (minutes)
- Rainfall intensity at the time of concentration (in/hr or mm/hr)
- An optional target release rate if your ordinance is stricter than the pre-development peak
Method assumptions and limits
- Triangular inflow hydrograph and constant outflow rate
- Standard power-function IDF relationship for intensity
- Best for small to medium watersheds (under ~200 acres)
- Preliminary sizing only — verify with level-pool routing
- Does not size multi-stage outlets or account for tailwater
Frequently asked questions
What method does this detention pond calculator use?
The Pond Storage tab uses the Modified Rational Method, which estimates required storage from the difference between the triangular post-development inflow hydrograph and a constant outflow rate, evaluated at the critical storm duration. It is a recognized preliminary sizing method (McCuen 2005; FHWA HEC-22) intended for small to medium watersheds (generally under about 200 acres). It is for feasibility and order-of-magnitude sizing — final designs should be confirmed with reservoir (level-pool) routing of a full design hydrograph.
How is the required detention storage volume calculated?
Storage is based on the Modified Rational storage relationship Vs = Qi² · tc / (4 · Qo), where Qi is the post-development peak inflow, Qo is the target release rate, and tc is the time of concentration. Because the storage is inversely proportional to Qo, a lower allowable release rate requires more storage. The result is converted to acre-feet (US) or cubic meters (SI) for display.
What is the target release rate, and what should I use?
The target release rate (Qo) is the maximum discharge the pond outlet is allowed to pass. Most stormwater regulations require the post-development peak to be held to the pre-development peak for one or more design storms, so the calculator defaults Qo to the pre-development peak flow if you leave the override blank. Always check your local ordinance — some jurisdictions require matching multiple return periods (for example the 2-, 10-, and 100-year storms).
What orifice and weir coefficients does the calculator use?
The outlet sizing tabs use standard discharge coefficients: a sharp-crested orifice coefficient Cd of 0.60 in Q = Cd·A·√(2gh); a sharp-crested rectangular weir coefficient of 3.33 (US) / 1.84 (SI) in Q = C·L·H^1.5; a Cipolletti (trapezoidal) coefficient of 3.367 (US) / 1.859 (SI); and a 90-degree V-notch coefficient of about 2.5 (US) / 1.38 (SI) in Q = C·tan(θ/2)·H^2.5. These follow FHWA HEC-22, Mays (2011), and the USBR Water Measurement Manual.
Why does the calculator recommend a minimum orifice size?
Small orifices clog easily with leaves, sediment, and trash. Following ASCE Manual of Practice 77, the calculator flags any computed orifice smaller than 4 inches (100 mm) in diameter. If your hydraulic calculation calls for a smaller opening, use at least the minimum size with a flow-control plate, or add a trash rack or debris guard upstream.
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Last verified: February 2026