French Drain Calculator

Calculate French drain capacity, pipe sizing, and storage volume. Design subsurface drainage systems using Darcy's Law and Manning's equation. Free professional-grade drainage calculator.

Calculate French Drain Capacity

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Design Requirements

Design Flow

cfs

Peak flow rate the French drain must handle

Trench Dimensions

Trench Length

Total length of the French drain

Trench Width

Width of the excavated trench

Trench Depth

Depth of the excavated trench

Pipe Properties

Pipe Diameter

Internal diameter of the perforated pipe

Don't know this value? Look it up

Pipe Material

Material affects Manning's n roughness coefficient

Soil & Aggregate Properties

Slope

Longitudinal slope of the trench (ft/ft or m/m)

Soil Type

Surrounding soil affects infiltration capacity

Don't know this value? Look it up

Aggregate Type

Aggregate in trench affects storage capacity

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