French Drain Calculator — Gravel Volume & Pipe Capacity
Enter trench dimensions and pipe size to get the gravel quantity, estimated weight, and the drainage area the pipe can handle at a design rainfall intensity.
ft
in
in
in
%
Manning's equation (pipe flowing full, n = 0.013)
- 1
Pipe cross-section area
π × (0.1667 ft)² = 0.087266Full bore area of the pipe in square feet. - 2
Hydraulic radius
0.3333 ÷ 4 = 0.083333For a full circular pipe, hydraulic radius = diameter ÷ 4. - 3
Slope fraction
1% ÷ 100 = 0.01 - 4
Manning flow (ft³/s)
(1 ÷ 0.013) × 0.0873 × 0.0833^(2/3) × 0.01^(1/2) = 0.1281 - 5
Flow capacity (GPM)
0.1281 × 449.83 = 58
How does this calculator work?
Gravel needed = L × W × D (ft) − pipe volume. Pipe capacity (GPM) = Manning's: (1/0.013) × A × (D/4)^(2/3) × slope^(1/2). Typical materials: ¾-in clean stone at ~2,700 lbs/yd³. Maintain ≥ 0.5% slope; check that your drainage area fits the capacity shown at 1 in/hr. Wrap trench in geotextile fabric to prevent clogging.
Formula
How this is calculated
A French drain is a gravel-filled trench containing a perforated pipe that collects and redirects groundwater or surface runoff away from a structure. The trench is excavated to the required depth, lined with non-woven geotextile fabric, packed with clean crushed stone around and above the pipe, and sloped continuously toward a discharge point.
The gravel quantity equals the trench volume (length × width × depth, all converted to feet) minus the cylindrical volume of the pipe itself. The weight estimate uses a typical bulk density of about 2,700 lbs/yd³ for ¾-inch clean crushed stone — confirm with your supplier as density varies by material. A conversion of 27 ft³ per cubic yard applies throughout.
Pipe flow capacity is calculated with Manning's open-channel-flow equation Q = (1/n) × A × R^(2/3) × S^(1/2), where n = 0.013 is the Manning roughness coefficient for smooth drainage pipe, A is the full cross-sectional area (conservative — assumes the pipe flows completely full), R is the hydraulic radius (D/4 for a circle), and S is the slope as a decimal fraction. The result in GPM is also converted to a catchment area the drain can handle at 1 inch per hour of rainfall intensity — a typical residential design-storm benchmark. For heavier storms, clay soils, or larger catchment areas, increase the pipe diameter or add a second parallel run.
Frequently asked questions
Use ¾-inch clean crushed stone or washed pea gravel — coarse enough that it won't clog (no fines or sand), yet small enough to pack around the pipe. Wrap the entire trench in non-woven geotextile fabric to prevent soil migration into the gravel over time and keep the system flowing freely.
A slope of at least 0.5% (about ½ inch drop per 8 feet run) is recommended to ensure gravity drainage — 1% is more reliable and gives roughly 40% more flow capacity. The steeper the slope, the higher the pipe capacity shown in this calculator.
The 'drain area at 1 in/hr' result shows the catchment area the pipe can handle at a common design storm intensity. Measure your actual drainage area in square feet; if it exceeds that value, either increase the pipe diameter, steepen the slope, or add a second parallel drain. For heavy clay soils that pond water, allow a safety factor of at least 1.5–2×.
Also known as
TG we-Calculate Editorial Team. (2026). French Drain Calculator — Gravel Volume & Pipe Capacity [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/french-drain-calculator
TG we-Calculate Editorial Team. "French Drain Calculator — Gravel Volume & Pipe Capacity." TG we-Calculate. 2026. https://we-calculate.com/calculator/french-drain-calculator.
TG we-Calculate Editorial Team, "French Drain Calculator — Gravel Volume & Pipe Capacity," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/french-drain-calculator
@misc{wecalculate_french_drain_calculator, title = {French Drain Calculator — Gravel Volume & Pipe Capacity}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/french-drain-calculator}}, year = {2026}, note = {TG we-Calculate} }
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