Rain Garden Sizing Calculator

Calculator Inputs

Use measured values where possible. Conservative inputs improve robustness.

Unit system

Switching units keeps meaning, not exact numbers.

Catchment area (ft²)

Total contributing roof/paving to the rain garden.

Runoff coefficient (0–1)

Impervious surfaces often range 0.85–0.95.

Rainfall depth (in)

Design depth for water quality or target storm.

Ponding depth (in)

Temporary surface storage before infiltration.

Infiltration rate (in/hr)

Use field-tested rate; reduce for safety if unsure.

Drain time (hours)

Common targets: 24–48 hours.

Media depth (in) (optional)

Soil media depth used for void storage.

Media void ratio (0–0.6)

Typical engineered media: 0.20–0.35.

Safety factor (1.0–2.0)

Accounts for clogging, compaction, and uncertainty.

Preferred shape

Rectangle provides easier grading and planting zones.

Aspect ratio L:W (rectangle)

Used only when rectangle is selected.

Reset Results display above the form after submission.

Example Data

Sample inputs for a typical residential driveway + roof.

Parameter	Example	Notes
Catchment area	1200 ft²	Contributing impervious surfaces
Runoff coefficient	0.90	High for paved/roof areas
Rainfall depth	1.0 in	Water quality design storm
Ponding depth	6.0 in	Temporary surface storage
Infiltration rate	0.5 in/hr	Field tested, conservative
Drain time	24 hr	Drains within one day

Quick interpretation

Use higher safety factor if sediment is expected. Confirm overflow path to a safe outlet for larger storms.

Formula Used

This calculator sizes rain garden area using runoff volume and per-area storage plus infiltration during a chosen drain time.

Runoff volume	V = A × C × P A = catchment area, C = runoff coefficient, P = rainfall depth.
Capacity per area	Cap = D_pond + (D_media × n) + (f × t) D_pond = ponding depth, D_media = media depth, n = void ratio, f = infiltration rate, t = drain time.
Required area	A_rg = (V × SF) / Cap SF = safety factor for uncertainty and long-term performance.

The infiltration term assumes sustained infiltration over the drain time. If soils clog easily, reduce infiltration rate or increase safety factor.

How to Use This Calculator

Measure total contributing roof and paving area.
Select unit system and enter catchment area.
Choose a design rainfall depth for your target storm.
Enter a realistic runoff coefficient for surfaces.
Input ponding depth based on grading and safety.
Use a tested infiltration rate and choose drain time.
Optionally include media depth and void ratio storage.
Apply a safety factor, then calculate and review notes.
Download CSV or PDF for records and design review.

For permitting, also document soil test method, groundwater separation, overflow elevation, and maintenance plan.

Rain Garden Sizing Guide

1) Purpose and design storm

A rain garden is a shallow landscaped basin that captures runoff, stores it briefly, and allows it to soak into the soil. Many programs size to a small “water quality” storm depth. Common targets are around 25 mm (1 in), but your local standard may differ. The calculator converts that depth into runoff volume using the drainage area and runoff coefficient.

2) Estimating runoff volume

Runoff volume is computed as V = A × C × P. For roofs and paving, C frequently falls between 0.85 and 0.95. Example: a 1200 ft² catchment, C=0.90, and P=1.0 in produces about 90 ft³ of runoff (≈2.55 m³). This aligns with a quick planning check before detailed drainage modeling.

3) Storage and infiltration capacity

Capacity per garden area includes ponding storage, optional media void storage, and infiltration during the selected drain time: Cap = Dpond + (Dmedia × n) + (f × t). Typical ponding depths are 100–200 mm (4–8 in). Engineered media void ratios often range 0.20–0.35. A conservative infiltration rate and a drain time of 24–48 hours help limit plant stress and mosquito habitat.

4) Footprint sizing and layout

Required area is Arg = (V × SF) / Cap, where SF accounts for clogging, compaction, and uncertainty. Many sites end up in a practical range of roughly 5–20% of the contributing area. The calculator also suggests rectangle or circle dimensions to translate the computed footprint into a buildable plan.

5) Using results for decisions

Use the Notes output to flag low infiltration or unusually large footprint needs. If the area ratio exceeds typical ranges, consider splitting the garden into multiple cells, adding a pretreatment strip, or using an underdrain. Always confirm separation to groundwater, overflow routing, and maintenance access before installation.

FAQs

1) What runoff coefficient should I use?

Roofs and concrete often use 0.85–0.95. Mixed surfaces can be weighted by area. When unsure, choose a higher value to size conservatively.

2) How do I get an infiltration rate?

Use a field infiltration test or a qualified soil report. For planning, reduce optimistic values to reflect compaction and clogging risk.

3) Why does drain time matter?

Longer drain times increase the modeled infiltration contribution, but prolonged ponding can harm plants and attract mosquitoes. Many criteria aim for full drawdown within 24–48 hours.

4) Should I include media void storage?

Include it if you have a defined soil media depth and expect reliable void space. If the garden may compact or clog, lower the void ratio or increase the safety factor.

5) What safety factor is reasonable?

Values from 1.1 to 1.5 are common for preliminary sizing. Use higher factors where sediment loads are high, maintenance is uncertain, or infiltration data is limited.

6) What if my required area is very large?

Consider splitting into multiple basins, improving pretreatment, revising ponding depth, or adding an underdrain. Also confirm that the chosen design storm depth matches your local requirement.

7) Do I still need an overflow?

Yes. Provide a stable overflow route to a safe outlet for larger storms. Overflow protection reduces erosion and protects nearby foundations and planting areas.