Detention Pond Volume Calculator
Estimate stormwater detention storage from drainage area, design storm depth, runoff coefficient, basin geometry, side slopes, pond depth, top area, bottom area, and freeboard.
Load a common rural, nursery, barnyard, greenhouse, or access-lane drainage case, then adjust storage, basin geometry, side slopes, and freeboard for your site.
Detention Pond Storage Breakdown
Enter a drainage area, storm depth, runoff coefficient, basin depth, and pond geometry to compare required storage with available detention volume.
The cards compare the entered design against common planning signals. They update after each calculation.
| Surface or cover | Typical C range | Planning use | Field note |
|---|---|---|---|
| Pasture, meadow, orchard floor | 0.20 to 0.40 | Low runoff | Dense cover and good infiltration lower storage demand. |
| Row crop or tilled garden soil | 0.35 to 0.60 | Moderate runoff | Use higher values for compacted or bare soil. |
| Gravel yard or packed lane | 0.55 to 0.80 | High runoff | Compacted aggregate can behave close to pavement. |
| Roof, pavement, concrete apron | 0.85 to 0.95 | Very high runoff | Most rainfall becomes direct runoff. |
| Mixed farmstead drainage | 0.45 to 0.75 | Weighted value | Area-weight each cover type before entering C. |
| Rainfall depth | Runoff from 1 acre at C 0.50 | Runoff from 1 acre at C 0.80 | Planning signal |
|---|---|---|---|
| 1 inch | 0.042 acre-ft | 0.067 acre-ft | Small water quality or frequent storm check. |
| 2 inches | 0.083 acre-ft | 0.133 acre-ft | Common small-site detention screening. |
| 3 inches | 0.125 acre-ft | 0.200 acre-ft | Useful mid-range farmstead design test. |
| 4 inches | 0.167 acre-ft | 0.267 acre-ft | Higher storage demand for roofs and yards. |
| 6 inches | 0.250 acre-ft | 0.400 acre-ft | Large event screening; confirm local criteria. |
| Side slope | Horizontal widening per side at 4 ft depth | Maintenance note | Typical fit |
|---|---|---|---|
| 2:1 | 8 ft | Steep for mowing; may need stabilization. | Tight sites with controlled access. |
| 3:1 | 12 ft | Common practical farm pond embankment. | General detention basins. |
| 4:1 | 16 ft | Easier maintenance and safer side access. | Open farmstead and nursery sites. |
| 5:1 | 20 ft | Gentle slope with larger footprint. | Shallow basins, grassed areas. |
| Check item | Calculator value to review | Good sign | Follow-up |
|---|---|---|---|
| Required storage | Runoff volume plus allowance | Available pond volume is higher. | Confirm local method and return period. |
| Freeboard | Reserve above design water | At least 0.5 to 1.0 ft for small basins. | Use higher values where required. |
| Top area | Entered top area vs slope check | Values are reasonably close. | Survey irregular basins or adjust dimensions. |
| Outlet routing | Routing factor applied to requirement | Credit is documented and permitted. | Model outlet, drawdown, and emergency spillway. |
| Embankment height | Depth plus freeboard | Matches field grade constraints. | Check seepage, stability, and access. |
This calculator uses the simple volumetric runoff method, so confirm required storm depth, runoff coefficient, outlet routing credit, and freeboard with your local stormwater standard.
The top area, bottom area, side slopes, and freeboard should come from actual grades or a layout plan, especially when the pond is irregular or tied into an embankment.
Stormwater is the water that fall from the sky and flows across a surface, and stormwater must go somewhere when there is heavy rain that falls. When it rains, the stormwater will fall on a roof, barnyard, or gravel lane and flow toward the lowest point on the land due to the gravitational pull on the stormwater. A detention pond will be constructed to hold the stormwater for a specific amount of time.
The detention pond will hold stormwater that eventually percolates into the ground or drains away from the detention pond. These detention pond are constructed to ensure that stormwater dont flood areas like driveways or wash out ditches. The correct size for the detention pond must be determined prior to any storm occurs to ensure that the pond will effectively fulfill its purpose.
How to Size a Detention Pond
The size of the detention pond will depend upon three factors: the area that drains into the pond, the depth of the storm, and the runoff coefficient. The drainage area is the amount of land that contributes to the stormwater that drains into the detention pond. The storm depth is the amount of rain that falls during the storm.
The runoff coefficient is a number that measures the amount of precipitation that fall as stormwater runoff. For instance, areas like pastures and orchards typically have a low runoff coefficient because the rain percolates into the ground on these features. In contrast, areas like greenhouses or aprons of concrete will have a high runoff coefficient because the rain will fall onto these features and become stormwater runoff.
It is important to accurately calculate the runoff coefficient. For instance, if the surface of the land is to be changed, the runoff coefficient will change, potentially by half. Once you calculate the runoff coefficient, the runoff volume can be determined.
The detention pond will have a specific shape, with a flat bottom and slopes for the sides of the pond. The volume of the pond is the amount of space within the pond between the bottom and the top of the pond. The side slope of the pond will affect the volume of the pond.
Steeper sides will reduce the amount of space within the pond for the stormwater to accumulate; gentler slopes will allow for more stormwater to accumulate within the pond. However, gentler slopes will require more land to build the pond to the necessary size. Many individuals will choose a three-to-one or four-to-one slope for the sides of the pond.
Such slopes are stable enough to mow the grass on the slope. An essential component of detention ponds is the addition of freeboard. The freeboard is the height of the embankment of the pond that exists above the water level of the pond.
This feature prevents the pond from overflowing its embankment due to the volume of stormwater that runs into the pond. In addition to providing for freeboard, individuals can also add more freeboard if the outlet of the pond is small in size or if it is near a building. An outlet is the structure that allow for water to leave the pond.
An outlet can be used to reduce the required storage capacity of the pond if it allows for water to leave the pond during periods of rainfall. However, only if the local regulations for the site allow for outlets will the user use this component in the calculation of the pond size. Within the tool and its reference tables is important information about runoff coefficients, storm depths, and the effect of different slopes on the storage volume of the pond.
These tables can be used prior to calculation of the size of the pond. For instance, these tables can be used to determine if the top area of the pond matches that calculated from the slope of the pond. Additionally, the amount of freeboard can be verified from these tables.
Finally, the required volume of the pond can be compared with the volume that will be calculated. It is important to include a safety allowance in the calculations. For instance, a detention pond that is too small will fill with stormwater and not be able to manage the stormwater that falls on the land.
Thus, it is cheaper to build a detention pond that is too large to manage the stormwater at any given time rather than building an embankment that cant handle the stormwater that falls on the land. Ten or fifteen percent can be added to the required volume of the pond to allow for any measurement error in building the pond. Only if the outlet size is adequate will the user use the outlet to calculate the required storage volume for the pond.
For instance, if sediment blocks the outlet, water will not leave the pond. Therefore, the outlet will not be used unless it is modeled to allow for the water to leave the pond at the necessary rate. The calculator will calculate the size of the pond once the individual provides the dimensions of the land, the drainage area, storm depth, runoff coefficient, and the dimensions of the detention pond.
The individual must decide which number will go into which fields on the calculator. The construction site for the pond must match the plan for the pond. When build and constructed properly, the detention pond will help protect the land and the areas located downstream of the pond.
