Rain Garden Calculator:

This tool is a reservoir-routing spreadsheet model that uses a standardized NRCS Type 1a 24-hour rainfall distribution (typical of Western and parts of Central Oregon) with variables input by the user to approximate the performance of various rain garden configurations. It is to be used for modeling runoff from impervious surfaces of less than 0.25 acres (approx. 11,000 sq. ft.).

Note: This size calculator is for swales with berms that function the same as a linear rain garden, and therefore is sized the same as a vegetated basin. The swale may be at a small slope (<5%), and the berms placed frequently enough to slow the water down and provide infiltration. Vegetated swales designed for conveyance must use a different procedure.

Choose type of rain garden:

Vegetated Basin
Stormwater Planter
Vegetated Swale with Berms

 Enter the 24-hour rainfall depth to be treated (inches):

 Enter the square footage of the impervious surface to be treated. This must be greater than 0 and 11,000 square feet or less:

 Enter the runoff coefficient. This ranges from 0.90-0.98 for impervious surfaces and represents the percentage of rainfall that runs off the surface (i.e. 0.90 = 90% runoff):

 Enter the soil's infiltration rate. This should be greater than 0 and be 20 inches per hour or less. Soil Infiltration Rate (in/hr):

 Select one: Calculate based on maximum raingarden size or calculate based on maximum target ponding depth:

Maximum Raingarden Size (Sq Ft)
Maximum Target Ponding Depth (in)

  If you selected maximum raingarden size above, then enter square footage of rain garden. This will calculate the maximum ponding depth necessary. If you selected maximum target ponding depth, then enter the max ponding depth. Note, ponding depth is typically between 6-12 inches:

  (Optional): If you would like to include a gravel layer, check the Gravel Underlayer Present box. You will be presented with additional options concerning the fill in the depth of rock you would like to use:

Gravel Underlayer Present

 (Optional):Enter depth of rock trench below rain garden (inches). Rock trenches are sometimes used to increase the amount of water the garden can hold while it infiltrates the underlying soil layers. This should be between 0 and 2 feet:

 (Optional): Enter the void ratio of rock trench (percentage). This is typically 30-40%:

 (Optional): Enter the Rock Trench Storage Volume (Cubic Feet):

 To view your Raingarden Results, click on the Size Raingarden button below:

Raingarden Results
 Raingarden Type: Stormwater Basin
 Gravel underlayer present:
 Gravel layer ponding level (in):
 Raingarden max ponding depth (in):
 Raingarden Size (Sq Ft):
 Peak Rainfall (in):

Addtional Optional Steps: Specify an average width and allow the model to calculate the rain garden length. Note: basins and swales should have at least a 5 foot width to compensate for the gradual side slopes (3 feet horizontal to 1 foot vertical, i.e. 3:1 side slopes).

 To run the simulation below, press the ENTER key on your keyboard after specifying calculation type and either length or width variables:

Calculate width using a length for the above raingarden:
Calculate length using a width for the above raingarden:

Enter Length:

Width (ft):