# Stormwater Infiltration Planning Guide

> Plan runoff capture, soil infiltration, storage volume, overflow and maintenance for onsite stormwater control.

- Canonical URL: https://buildmeter.net/guides/stormwater-infiltration-planning-guide/
- Category: Excavation & Drainage
- Author: Liron Elimeleh
- Publisher: BuildMeter, developed by ELStudios
- Published: July 30, 2026
- Last reviewed: August 19, 2026

## Direct answer

A reliable plan separates measured quantity from design decisions, allowances, package rounding, accessories, labor and field verification. Record each assumption so estimates and supplier quotes can be compared consistently.

## Primary calculator

- [Dry Well Size Calculator](https://buildmeter.net/dry-well-size-calculator/)

## Start with runoff volume, then test whether the site can infiltrate it

Stormwater infiltration planning combines hydrology and soil/site constraints. A rain garden or dry well must receive a defined runoff volume, temporarily store that volume and release it by infiltration or controlled overflow. The calculation is not complete until setbacks, seasonal groundwater, soil infiltration and an emergency overflow path are considered.

### Stormwater infiltration planning inputs

| Input | Purpose | Typical source |
| --- | --- | --- |
| Drainage area | Area contributing runoff | Roof/impervious plan |
| Design rainfall depth | Event volume | Local stormwater criteria |
| Runoff coefficient | Loss adjustment | Surface type/design method |
| Infiltration rate | How fast stored water can enter soil | Field test/design data |
| Storage void ratio | Usable volume in stone/media | Material/design data |

## Worked runoff example

A 1,200 ft² roof receiving 1 inch of rain sees 100 ft³ of rainfall volume because 1,200 × 1/12 = 100. Using a 0.95 runoff coefficient gives about 95 ft³, or roughly 710 gallons, directed toward the infiltration system. A dry well filled with stone cannot use its entire excavation volume as water storage because the stone occupies most of that space; the [Dry Well Size Calculator](https://buildmeter.net/dry-well-size-calculator/) therefore needs a realistic void ratio.

## Recommended workflow

1. Map the actual contributing roof or paved area.
2. Select the design rainfall depth required for the project objective.
3. Calculate runoff volume with the [Rainwater Runoff Calculator](https://buildmeter.net/rainwater-runoff-calculator/).
4. Compare storage volume with the [Dry Well Size Calculator](https://buildmeter.net/dry-well-size-calculator/) or [Rain Garden Size Calculator](https://buildmeter.net/rain-garden-size-calculator/).
5. Verify infiltration rate, drawdown time, groundwater separation, setbacks and overflow path before construction.

## Common mistakes

- Using excavation volume as though it were fully available water storage.
- Ignoring roof areas that drain to the same downspout.
- Assuming sandy-looking surface soil represents the infiltration rate at the bottom of the facility.
- Providing no safe overflow route for storms larger than the design event.

## Professional limits

Local stormwater rules, soils, groundwater, slopes and building setbacks can control whether infiltration is appropriate. Do not infiltrate runoff where it can threaten foundations, slopes, wells or contaminated soil.

## Sources

- [U.S. EPA — Green Infrastructure](https://www.epa.gov/green-infrastructure)

## Related calculators

- [Dry Well Size Calculator](https://buildmeter.net/dry-well-size-calculator/)

## Related guides

- See the BuildMeter Guides directory.