French Drain Calculator

What is a French Drain?

A French drain is a gravel-filled trench containing a perforated pipe that redirects surface water and groundwater away from an area. Named after Henry Flagg French (a lawyer and farmer from Concord, Massachusetts), the French drain is one of the most effective and widely used subsurface drainage solutions for residential and commercial properties. The system relies on gravity and the high permeability of gravel to collect water and channel it to a safe discharge point.

French drains are commonly installed around building foundations to prevent basement flooding, across soggy lawns to improve drainage, behind retaining walls to relieve hydrostatic pressure, and along driveways to manage runoff. The key components are a trench, washed angular gravel, a perforated drain pipe, and landscape fabric to prevent soil migration into the gravel. Our French drain calculator helps you determine the optimal pipe size, gravel volume, and trench dimensions based on Darcy's Law.

How to Use the French Drain Calculator

Using this French drain calculator is straightforward. Enter the drainage area in square feet (the surface area contributing water to the drain), the local rainfall intensity in inches per hour (use local storm data: a 1-in-10-year 1-hour storm is typically 1.5-3 in/hr), the total drain length in feet, the trench width in inches, the trench depth in feet, and the drain slope as a percentage (minimum 1%). Select the gravel type that best matches your available fill material.

Our calculator uses Darcy's Law (Q = K × A × S) to compute the flow capacity of the gravel-filled trench and Manning's equation to size the perforated pipe. The results show estimated water inflow, trench capacity, recommended pipe diameter, and the gravel volume needed. If the trench appears undersized, the calculator suggests adjustments such as increasing width, depth, or slope.

French Drain Formula (Darcy's Law)

The drainage capacity of a French drain is governed by Darcy's Law, the fundamental equation for fluid flow through porous media:

Q = K × A × S

Where:

• Q = Flow rate through the gravel trench (ft³/s or gpm)
• K = Hydraulic conductivity of the gravel fill (m/s or ft/s) — depends on gravel size and angularity
• A = Cross-sectional area of the trench perpendicular to flow (ft²)
• S = Hydraulic gradient (slope of the drain), dimensionless

Clean angular crushed gravel has a hydraulic conductivity (K) of 0.01-0.1 m/s, which is 1,000 to 10,000 times higher than native clay soil. This is why the gravel fill is the primary flow path — native soil contributes negligibly to drainage. The perforated pipe at the bottom of the trench adds a low-resistance channel that can carry water much faster than percolation through gravel alone, especially during peak flow events.

Applications of French Drains

Foundation Drainage: French drains are installed around building perimeters at the footing level to intercept groundwater before it reaches the basement or crawlspace. The drain should extend at least to the bottom of the footing and slope away at a minimum 1% grade.

Yard Drainage: For soggy lawns and low-lying areas that collect water after rain, a French drain can redirect surface and subsurface water to a storm drain, dry well, or daylight outlet. A typical yard drain uses a 4-6 inch perforated pipe in a 12-inch wide trench filled with 3/4-inch crushed gravel. A rainwater collection calculator can help estimate runoff volumes.

Retaining Wall Drainage: Hydrostatic pressure behind retaining walls can cause structural failure. A French drain placed at the base of the wall relieves this pressure by collecting and redirecting groundwater. These drains typically require higher capacity due to the large volume of water that can accumulate behind a wall.

Agricultural and Road Drainage: In agriculture, French drains (also called tile drains) lower the water table to improve crop yields. Along roads and highways, they prevent frost heave and pavement degradation by keeping the subgrade dry.