Step 1: Impervious Drainage Area

Impervious areas are hard surfaces that don’t allow water to soak through (e.g. roofs, decks, walkways and driveways). Measure the impervious area(s) that will drain into your garden. To do this:

Open the Google Earth website.
Search for your property address.
Measure the length and width of your roof and/or other hard surfaces.
1. On Google Earth, select the ruler tool.
2. Click on the edges of your roof to create an outline of it.
3. This will give you the roof measurement in square feet (ft2). (Make sure square feet are selected, not meters.)
Do the same steps in the mapping tool if you are measuring a driveway, patio, or other hard surfaces that will drain into the garden.

Step 2: Non-Impervious Drainage Area

Non-impervious area refers to the natural surfaces like lawns, gardens, or forested areas that will contribute runoff water to your garden during a rain event.

If your garden is going to accept water from any non-impervious areas, measure the amount of non-impervious drainage area and enter it below. Use Google Earth and the same steps above to find the square footage of the non-impervious area.

For a rain garden only capturing runoff from your roof or driveway, you would enter 0.

Step 3: Inches of Rain to Be Treated

Residential rain gardens should be sized for handling 1.2 to 2.7 inches of rain from a 24-hour storm event. The minimum rain capture goal for a RainScapes Reward Rebate is 1.2 inches. Treating as much as 2.7 inches is encouraged.

1. Enter your total amount of impervious surface area (in square feet).*

Surfaces that don’t let water soak in like roofs, driveways, patios, or walkways where rain runs off instead of absorbing into the ground.

2. Enter your total amount of non-impervious drainage area (in square feet).*

The parts of the property where water can soak into the ground like lawns, gardens, gravel areas, or natural soil rather than running off hard surfaces

3. Enter inches of rain to be treated (1.2 minimum).*

The total drainage area cannot exceed 2,000 square feet. Consider a conservation landscape with 0" of ponding instead.

Validation

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Required Garden Size (in Square Feet) for Rain Garden with 6" ponding

Garden size can’t go below 100 sq ft

Garden size can’t go below 100 sq ft.

This is a small rain garden.

This is a medium-size rain garden.

This is a large rain garden.

This is a very large rain garden.

This number indicates the ponding footprint (total planting area) for your rain garden. This is an estimate. The final sizing for RainScapes Rebate-eligible projects will be verified by your RainScapes planner after you apply.

If you are not doing a rain garden but instead plan to make a more shallow garden to handle 3 inches of ponding, this is your estimate:

Required Garden Size (in Square Feet) for Conservation Landscape with 3" ponding

Next, find out how many plants you will need to cover your garden.

For rain gardens, 40-50% of your plants should be ones that are wet-soil tolerant. The rest can handle drier conditions on the slopes of the garden.

Rain Garden Sizing Estimator

Step 1: Impervious Drainage Area

Step 2: Non-Impervious Drainage Area

Step 3: Inches of Rain to Be Treated