Rain Garden Size Calculator
Rain garden area and storage from roof drainage with shape suggestion.
What it does
Rain gardens catch roof and drive runoff in a shallow planted bowl and let it soak in within a day. Size decides success: too small and storms blow past, too large and plants sit dry between rains.
The tool above takes drainage area with storm and depth. It returns target garden size with storage volume and a workable shape.
The default 1,000 sq ft roof wants 300 sq ft (200 minimum) holding 150 cu ft at 6 inches. The metric mirror of 92.9 sq m reads 27.9 sq m.
Sizing follows Maine and UConn extension guides with EPA Soak Up the Rain behind them. All three agree on the 20 to 30% band and 24 hour drain used here.
How to use
- Pick square feet or meters.
- Enter the drainage area.
- Enter the design storm.
- Enter ponding depth.
- Read garden size first.
- Shape it per the suggestion.
Formula
- drainage
- Roof and drive area draining in.
- storm
- Design storm in inches.
- depth
- Ponding depth in inches.
- 0.30
- Target fraction of drainage.
Metric converts to imperial first (sq m x 10.7639, mm / 25.4), then displays metric results. Areas round to whole units; the UConn 1.3 factor check lands inside the same 20 to 30 band.
Worked example
Example: 1,000 sq ft roof
Imperial defaults: 1,000 sq ft drainage, 1 in storm, 6 in depth.
- Drainage: 1,000 sq ft of roof.
- Target: 1,000 x 0.30 = 300 sq ft.
- Minimum: 1,000 x 0.20 = 200 sq ft.
- Storage: 300 x 6/12 = 150 cu ft.
- Storm check: 1 in over 1,000 sq ft = 83.3 cu ft, fits.
- Shape: 10 x 30 ft or 20 ft circle.
- Tool shows 300 sq ft target (200 min), 150 cu ft at 6 in.
Reference table
| Rule | Value | Note |
|---|---|---|
| Target fraction | 30% | 20% minimum |
| Depth | 6 in | 1 in/ft slopes |
| Drain time | 24 hr | Relocate if slower |
| Setback | 10 ft | From basements |
Limits
- Infiltration tests rule; clay may refuse.
- No septic, wells, or wet spots.
- Overflow path required.
- Plants need native selections.
Common mistakes
- Sizing under 20% of drainage.
- Ponding deeper than 8 inches.
- Amending with pure compost.
- Draining toward foundations.
- Skipping the infiltration test.
Try it yourself
Quick checks. Answers sit tucked under each question.
500 sq ft drainage?
Target 150 (100 min) at the same depth.
1,000 sq ft at 8 in deep?
Same 300 sq ft: depth sets storage, not footprint.
UConn factor check?
1.3 rate check: 1,000 x 1.3 / 6 = 216.7 sq ft, inside the band.
Questions
How big should a rain garden run?
About 30% of the drainage area, 20% minimum. A 1,000 sq ft roof wants 200 to 300 sq ft.
How deep is the garden?
About 6 inches below grade at center with 1 inch per foot side slopes.
How fast must it drain?
Within 24 hours. Standing water past a day means a new site or amended soil.
What soil mix works?
50 to 60% sand with native soil plus up to 20% compost, more for heavy clay.
Where can it sit?
At least 10 ft from basements, never over septic or near wells, overflow to lawn.
Can I split one roof into two gardens?
Yes: halve the footprint per garden when one roof drains two ways.