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 offset

Your Results

Calculated
Runoff Volume
-
Total runoff volume
Captured Volume
-
Captured runoff
Garden Capacity
-
Infiltration capacity
Offset Percent
-
Percent offset

Runoff Plan

Your defaults show meaningful runoff offset.

What This Calculator Measures

Estimate rain garden runoff offset using roof area, rainfall, and infiltration capacity.

By combining practical inputs into a structured model, this calculator helps you move from vague estimation to clear planning actions you can execute consistently.

This calculator estimates runoff offset from a rain garden design.

How to Use This Well

  1. Enter roof area and rainfall.
  2. Set capture percent and garden area.
  3. Add infiltration rate and hours.
  4. Review runoff offset.
  5. Adjust capture or capacity.

Formula Breakdown

Runoff = roof x rainfall x 0.623
Captured: runoff x capture %.
Capacity: garden x infiltration x hours x 0.623.
Offset: min(captured, capacity) / runoff.

Worked Example

  • 1,800 sq ft x 1.2 in = 1,345 gal.
  • Capture 45% = 605 gal.
  • Capacity around 269 gal.

Interpretation Guide

RangeMeaningAction
80%+High.Great offset.
50-80%Strong.Solid reduction.
30-50%Moderate.Increase capacity.
Below 30%Low.Expand garden.

Optimization Playbook

  • Increase garden area: raise capacity.
  • Improve soil: boost infiltration rate.
  • Add storage: capture more water.
  • Track storms: size for typical events.

Scenario Planning

  • Baseline: current capture percent.
  • More area: add 50 sq ft.
  • Higher rainfall: add 0.5 in.
  • Decision rule: keep offset above 50%.

Common Mistakes to Avoid

  • Overestimating capture percent.
  • Ignoring infiltration limits.
  • Skipping storm sizing.
  • Not measuring roof area.

Implementation Checklist

  1. Measure roof area.
  2. Estimate rainfall.
  3. Test infiltration.
  4. Plan garden size.

Measurement Notes

Treat this calculator as a directional planning instrument. Output quality improves when your inputs are anchored to recent real data instead of one-off assumptions.

Run multiple scenarios, document what changed, and keep the decision tied to trends, not a single result snapshot.

FAQ

What is the 0.623 factor?

It converts inches of rain to gallons per sq ft.

How do I estimate infiltration?

Use soil tests or local guidance.

Should I size for big storms?

Focus on frequent storms for best impact.

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