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Calculate French Drain Capacity
For educational purposes only. Not a substitute for professional engineering judgment.
French Drain Design Overview
French drains provide subsurface drainage by combining a perforated pipe for conveyance with aggregate storage for detention. The total capacity includes both pipe flow and infiltration into surrounding soil.
- Pipe Capacity - Calculated using Manning's equation for pipe flow
- Infiltration - Calculated using Darcy's Law with soil hydraulic conductivity
- Storage - Available volume in aggregate voids (based on porosity)
- Drawdown Time - Time to empty storage through infiltration
Typical Hydraulic Conductivity Values
| Soil Type | K Min (ft/day) | K Typical (ft/day) | K Max (ft/day) |
|---|---|---|---|
| Gravel | 2,800 | 14,000 | 28,000 |
| Coarse Sand | 140 | 280 | 560 |
| Medium Sand | 28 | 56 | 140 |
| Fine Sand | 2.8 | 7 | 28 |
| Silty Sand | 0.28 | 2.8 | 7 |
| Clay | 0.00028 | 0.0028 | 0.028 |
Source: Mays, L.W. (2011), Water Resources Engineering, Table 3.4
Typical Aggregate Porosity Values
| Aggregate Type | Porosity Min | Porosity Typical | Porosity Max |
|---|---|---|---|
| Clean Gravel | 0.30 | 0.35 | 0.40 |
| Crushed Stone (#57) | 0.35 | 0.40 | 0.45 |
| Washed Stone (#2) | 0.38 | 0.42 | 0.48 |
| River Rock | 0.30 | 0.35 | 0.40 |
| Pea Gravel (3/8") | 0.30 | 0.33 | 0.38 |
Source: ASCE MOP 77 (2006), Design and Construction of Urban Stormwater Management Systems
Interactive model
French drain cutaway
Explore how trench dimensions, pipe size, slope, and inflow affect pipe capacity and water stored in the gravel envelope.
Open full model (opens in a new tab)About French Drains
A French drain is a gravel-filled trench containing a perforated pipe that redirects surface water and groundwater away from an area. French drains are commonly used for foundation drainage, yard drainage, and roadway edge drains.
How French Drains Work
French drains operate through two primary mechanisms:
- Pipe Conveyance - Water enters through pipe perforations and flows by gravity to an outlet
- Infiltration - Water seeps through the aggregate and infiltrates into surrounding soil
Key Design Equations
Manning's Equation (Pipe Flow): Q = (k/n) * A * R2/3 * S1/2
Darcy's Law (Infiltration): Q = K * i * A
Where:
- Q = Flow rate
- K = Hydraulic conductivity of surrounding soil
- i = Hydraulic gradient (typically assumed as 1.0)
- A = Infiltration area (trench bottom and sides)
- n = Manning's roughness coefficient for pipe
- S = Trench slope
Design Considerations
- Trench Depth - Typically 18" to 36" deep, below frost line for cold climates
- Trench Width - Usually 12" to 24" wide to provide adequate storage
- Pipe Size - 4" diameter is common for residential; 6" for commercial
- Slope - Minimum 0.5% (1/4" per foot) to ensure positive drainage
- Aggregate - Clean washed gravel, typically 3/4" to 1-1/2" size
- Filter Fabric - Non-woven geotextile to prevent sediment clogging
French drain sizing table
Preliminary quantities for a 10 ft trench segment with clean gravel and smooth-wall perforated PVC at 1% slope. Pipe capacity is full-flow Manning capacity; aggregate quantity excludes the pipe volume. Use the calculator above for your actual flow, soil, and geometry.
| Pipe | Example trench W × D | Pipe capacity | Aggregate / 10 ft | Void storage / 10 ft |
|---|---|---|---|---|
| 4 in | 12 × 24 in | 0.247 cfs (111 gpm) | 0.71 yd³ | 6.7 ft³ |
| 6 in | 18 × 30 in | 0.729 cfs (327 gpm) | 1.32 yd³ | 12.4 ft³ |
| 8 in | 24 × 36 in | 1.571 cfs (705 gpm) | 2.09 yd³ | 19.8 ft³ |
| 12 in | 30 × 48 in | 4.632 cfs (2079 gpm) | 3.41 yd³ | 32.3 ft³ |
These are hydraulic and material-planning values, not minimum installation dimensions. Allow additional excavation and order contingency for compaction, waste, and field variation.
Reference & standards
What This Solves
Sizes a French drain trench for subsurface drainage by combining perforated pipe capacity with aggregate storage and soil infiltration using Darcy's Law.
Best Used When
- You need to drain groundwater or surface water away from a foundation, yard, or retaining wall
- You want to size the trench dimensions, pipe, and aggregate for a known design flow
- You are designing a linear subsurface drain and need to estimate drawdown time
Do NOT Use When
- You need a vertical infiltration system rather than a linear drain — Use Dry Well Calculator
- You are designing a surface-level bioretention or rain garden facility — Use Rain Garden Calculator
- You need a surface trench drain (channel drain with grate) instead of a buried drain — Use Trench Drain Calculator
Key Assumptions
- Soil conditions are uniform along the full trench length
- Aggregate is clean with the specified porosity (no fines or clogging)
- The perforated pipe is properly bedded with adequate perforations
- Infiltration uses a unit hydraulic gradient (conservative estimate)
- Steady-state flow conditions apply
- No clogging of aggregate or pipe perforations over time
Input Quality Notes
Hydraulic conductivity of the surrounding soil is the most uncertain input. Field-measured percolation rates are far more reliable than table lookups. Consider using the low end of the published range for conservative design.
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Last verified: February 2026