Rain Garden Runoff Offset Calculator

Estimate rain garden runoff offset using roof area and infiltration.

sq ft
in
%
sq ft
in/hr
hrs

Quick Facts

Capture
Percent
Capture lowers runoff
Capacity
Infiltration
Capacity sets limits
Rainfall
Event
Event size matters
Decision Metric
Offset
Percent of runoff absorbed

Rain Garden Runoff Results

Stormwater
Roof Runoff Generated
-
Total runoff produced by this rain event
Runoff Captured
-
Runoff routed into the rain garden
Garden Infiltration Capacity
-
Maximum water the garden can absorb during the storm
Runoff Offset
-
Share of total roof runoff absorbed by the garden
Uncaptured Overflow
-
Captured runoff exceeding garden capacity

Frequently Asked Questions

How reliable are these environmental estimates?
These calculations use peer-reviewed emission factors, energy conversion constants, and environmental science formulas. They're accurate for ballpark estimation and comparison purposes. Real-world values vary based on local utility mix, climate, building materials, and individual habits. Use results to understand relative impact and identify the biggest opportunities for improvement.
What units does this calculator use?
Energy is expressed in kilowatt-hours (kWh) or British thermal units (BTU) depending on context. Emissions are typically in kilograms or pounds of CO2-equivalent (CO2e), which accounts for different greenhouse gases weighted by their warming potential. Check the labels on each result — mixing energy and emissions units is a common source of confusion.
How can I reduce my impact most effectively?
Focus on your largest consumption categories first — heating/cooling, transportation, and food typically account for 70-80% of a household's carbon footprint. Small improvements to high-volume activities outweigh large improvements to low-volume ones. Use the calculator to compare scenarios (e.g., LED vs. incandescent) to find the highest-leverage changes.
Are these calculations valid for my region?
Some inputs — especially electricity emissions factors — vary significantly by region based on your local grid's energy mix. A kWh of electricity in a coal-heavy grid emits 3-4× more CO2 than one from a renewable-heavy grid. If precision matters, find your regional emissions factor from your utility or the EPA's eGRID database and use it as a custom input.

How This Rain Garden Calculator Works

This calculator estimates how much stormwater runoff a rain garden can offset for a single rain event. It compares the runoff generated by your roof against how much water your rain garden can actually infiltrate during the storm.

Runoff (gal) = Roof Area (sq ft) x Rainfall (in) x 0.623
Captured Runoff = Runoff x Capture Percent
Garden Capacity (gal) = Garden Area x Infiltration Rate x Infiltration Hours x 0.623
Runoff Offset % = min(Captured Runoff, Garden Capacity) / Runoff x 100

The constant 0.623 converts a depth of rainfall in inches over an area in square feet into gallons (1 sq ft x 1 in of rain = 0.623 gallons). Garden capacity is capped by whichever is smaller: the runoff actually captured, or how much the garden can infiltrate in the available time.