French Drain Calculator
Estimate drainage gravel, perforated pipe, filter fabric, trench excavation, slope drop, material weight, bags, and project cost.
Plan a French drain before excavation
A French drain calculator estimates trench volume, drainage gravel, perforated pipe, filter fabric, and elevation drop from the drain length, width, depth, pipe diameter, and slope.
The gravel estimate subtracts the pipe volume from the trench, then adds the selected allowance. Optional purchasing settings convert the result into weight, bags, bulk volume, and material cost.
A working drain also needs a suitable outlet, positive grade, compatible washed stone, nonwoven geotextile where appropriate, utility locating, and a layout that does not threaten foundations or neighboring property.
Drainage gravel required
----
Gravel weight
--
--
Perforated pipe
--
--
Filter fabric
--
Wraps the trench cross-section with overlap.
Estimated material cost
--
Gravel, pipe, fabric, fittings, and delivery.
Excavation
--
Pipe displacement
--
Gravel bags
--
Elevation drop
--
Spoil volume
--
French drain material breakdown
Shows calculated quantities, formulas, and purchasing assumptions.
| Item | Quantity | Calculation | Estimated cost |
|---|---|---|---|
| Run the calculator to see the material breakdown. | |||
Planning note: Confirm the outlet elevation, pipe orientation, aggregate specification, fabric type, utility locations, frost conditions, permits, and local drainage rules before digging.
How to use the French drain calculator
- Measure the route: Follow the planned drain path, including turns and the run to a suitable outlet.
- Enter trench width and depth: Use the actual excavated dimensions, not only the pipe size.
- Select pipe diameter and slope: Enter the outside diameter and planned fall along the drain.
- Add an allowance: Include extra material for uneven excavation, connections, trimming, and site changes.
- Review optional purchasing settings: Adjust gravel density, bag size, fabric overlap, local prices, fittings, and delivery.
- Verify the outlet: Make sure the calculated elevation drop can discharge water without creating erosion or a nuisance.
French drain formulas
The calculator treats the trench as a rectangular prism and the pipe as a cylinder. It subtracts pipe displacement because the pipe occupies space that otherwise would be filled with gravel.
Trench volume = length x width x depth
Pipe volume = pi x radius squared x length
Gravel = (trench volume - pipe volume) x allowance
Elevation drop = length x slope / 100
A 50-foot trench that is 12 inches wide and 18 inches deep has 75 cubic feet of excavation. After subtracting a 4-inch pipe and adding 10% allowance, the estimate is about 2.88 cubic yards of drainage gravel.
Slope, outlet, and route planning
Start with the outlet
Work backward from a legal, stable discharge point. A drain cannot perform reliably when the outlet is blocked, submerged, or higher than the collection area.
Check total fall
A percentage that looks small can create a substantial drop over a long run. Compare the calculated fall with the available site elevations before excavation.
Avoid low spots
Use a laser level, builder's level, or taut string line to maintain continuous grade and prevent sections that hold sediment or standing water.
Choosing gravel, pipe, and filter fabric
Material selection affects both drainage capacity and long-term clogging resistance. Confirm the complete assembly for the soil, groundwater, loading, and outlet conditions rather than choosing materials by volume alone.
Drainage aggregate
Use clean, washed, angular stone sized for the specified drainage system. Fine material and stone dust can reduce void space and slow water movement.
Perforated pipe
Rigid and flexible products have different installation requirements. Confirm diameter, perforation orientation, bedding, fittings, cleanouts, and load rating.
Filter fabric
A compatible nonwoven geotextile can separate soil from stone while allowing water through. Fabric selection should match soil gradation and the drainage design.
French drain planning checklist
Before digging
- Locate utilities: Request the applicable utility-marking service before excavation.
- Check permissions: Review permits, easements, right-of-way limits, and discharge rules.
- Record elevations: Confirm the inlet, route, and outlet can maintain positive fall.
- Protect structures: Review foundation depth, footings, retaining walls, trees, and nearby paving.
Before backfilling
- Inspect grade: Check the trench and pipe for low points and reverse slope.
- Test the outlet: Confirm water can leave freely without erosion or backflow.
- Photograph the route: Record pipe, fittings, cleanouts, fabric, and crossings for future work.
- Protect the assembly: Keep soil out of washed stone and avoid crushing or displacing the pipe.
Excavation safety and utility-marking reference: 811 Before You Dig - Safe Digging Process.
Is a French drain the right drainage solution?
A French drain is designed to intercept water moving through saturated soil or collecting below the surface. It is not automatically the best fix for every wet yard, overflowing gutter, foundation leak, or low spot. Identify where the water comes from and where it can legally go before sizing materials.
| Water problem | Likely first option | When a French drain helps | What to verify |
|---|---|---|---|
| Subsurface seepage across a slope | Interceptor or curtain drain placed across the water path. | It can collect shallow groundwater before it reaches the lower yard or structure. | Seepage depth, seasonal groundwater, slope stability, outlet elevation, and soil type. |
| Water flowing over the surface | Regrading, swale, catch basin, or channel drain. | It may supplement surface controls when soil also stays saturated below grade. | Whether runoff can enter the stone trench fast enough during heavy rain. |
| Roof or downspout discharge | Solid, nonperforated conveyance pipe to an approved outlet. | A separate French drain may address wet soil, but it should not be treated as a substitute for direct roof-water conveyance. | Downspout capacity, cleanouts, freeze protection, discharge rules, and overflow path. |
| Water beside a foundation | Correct grading, gutters, waterproofing, footing drainage, or sump evaluation. | Only as part of a design that accounts for footing depth and avoids undermining the structure. | Foundation type, basement leakage source, soil pressure, waterproofing, and professional design needs. |
| A low area with no gravity outlet | Regrading, approved dry well, sump pump, or another engineered discharge method. | Only if collected water has enough storage and a dependable way to leave the system. | Soil infiltration, groundwater level, overflow location, pump power, and local restrictions. |
Roof-runoff management reference: U.S. Environmental Protection Agency - Disconnect / Redirect Downspouts.
How site conditions change the drain design
Two drains with the same length can need different trench dimensions, stone envelopes, pipe products, and access points. Use the calculator for quantity after these site conditions have been considered in the layout.
Fine clay or silty soil
Fine particles can migrate into the aggregate and fabric. Soil-compatible geotextile selection, adequate stone volume, and protection from sediment-laden surface runoff become especially important.
High groundwater or constant flow
Continuous water can exceed the assumptions of a simple landscape trench. Pipe capacity, downstream discharge, backup protection, and the effect on nearby structures may require professional evaluation.
Driveway or vehicle loading
Traffic changes pipe stiffness, cover, compaction, aggregate, and trench restoration requirements. Do not use a lightly buried landscape assembly beneath a driveway without a load-rated design.
Freezing conditions
Standing water at the outlet, shallow solid-pipe runs, and poorly graded sections may freeze. Local frost depth, snow storage, discharge exposure, and seasonal maintenance should influence the route.
Trees and aggressive roots
Excavation can damage structural roots, while roots may later enter joints or outlets where moisture persists. Review tree-protection zones and provide maintainable connections where blockage is possible.
Long routes and multiple turns
Turns add fittings, friction, layout risk, and cleaning difficulty. Add pipe and fabric allowance, minimize sharp bends, and plan accessible cleanouts at practical locations.
Before relying on the material total
Confirm the trench cross-section at several points along the route. A drain that widens around roots, utilities, fittings, catch basins, or unstable soil can use substantially more stone and fabric than one uniform calculation suggests.
Site-specific soil information: USDA Natural Resources Conservation Service - Web Soil Survey.
Cleanouts, inspection, and troubleshooting
A buried drain is easier to maintain when access is planned before backfilling. Provide a way to inspect important junctions and the outlet, then keep a record of the route, depth, fittings, and nearby utilities.
Cleanout locations
Consider access at the upstream end, major direction changes, long runs, and transitions to solid outlet pipe. Keep caps protected but reachable.
Outlet protection
Use a visible, stable termination that resists animals, debris, erosion, mower damage, and backflow without restricting discharge.
Inspection record
Photograph each stage and save measurements. Mark cleanouts and the outlet on a site plan before landscaping hides the system.
| Symptom | Possible cause | First check | Likely response |
|---|---|---|---|
| Wet area remains after normal rain | The trench misses the water path, is too shallow, or has insufficient outlet fall. | Compare wet-ground observations with the recorded drain route and elevations. | Correct grading, extend interception, or obtain a site-specific drainage assessment. |
| Little or no outlet flow | Blocked outlet, crushed pipe, reverse grade, sediment, roots, or no water reaching the trench. | Inspect the outlet and accessible cleanouts during wet conditions. | Clear the termination, inspect the pipe, and correct damaged or poorly graded sections. |
| Stone surface fills with soil | Surface sediment is entering, fabric is damaged, or soil separation is unsuitable. | Check uphill erosion, exposed aggregate, fabric seams, and landscaping runoff. | Control sediment at the source and repair contaminated or unprotected sections. |
| Erosion develops at discharge | Outlet velocity or concentration is too high for the receiving surface. | Inspect after a storm for scour, soil loss, or water crossing property boundaries. | Stabilize or redesign the outlet and confirm that the discharge location is permitted. |
| Drain works briefly, then slows | Sediment loading, outlet obstruction, fabric blinding, root intrusion, or undersized capacity. | Compare performance over several storms and inspect all accessible points. | Clean maintainable components and investigate the failed section before replacing materials blindly. |
Frequently Asked Questions
How much gravel do I need for a French drain?
Multiply trench length, width, and depth to find excavation volume, then subtract the pipe volume and add an allowance for uneven digging and settlement. This calculator reports cubic yards, cubic meters, weight, and bag count so you can compare bulk and bagged purchasing options.
What slope should a French drain have?
The correct slope depends on the pipe, soil, site elevations, flow, and outlet. A commonly discussed planning value is about 1%, or roughly 1/8 inch per foot, but the final grade should follow the drainage design and local requirements. Continuous positive fall matters more than hitting a nominal number while leaving low spots.
How wide and deep should the trench be?
Trench dimensions depend on the drainage problem, pipe diameter, stone envelope, soil, frost, groundwater, surface loading, and outlet elevation. A shallow landscape drain and a foundation drainage system are not interchangeable. Use a site-specific design when water could affect a building or slope.
Does the pipe volume make a noticeable difference?
Yes, especially in a narrow trench or when a larger pipe is used. The pipe displaces gravel along the entire route. Subtracting its outside cylindrical volume gives a more realistic stone estimate than treating the trench as completely filled with aggregate.
How much filter fabric is needed?
The calculator adds the trench bottom width, both side depths, and the selected overlap, then multiplies that wrap width by the drain length and allowance. Actual roll width, seams, end closures, turns, and manufacturer installation instructions can increase the purchase quantity.
Can a French drain discharge anywhere downhill?
No. Discharge must comply with local rules and should not cause erosion, icing, flooding, foundation damage, or a nuisance on another property. Some sites need daylight discharge, a dry well, storm connection, sump system, or another approved outlet designed by a qualified professional.
Other useful calculators
Disclaimer: This French drain calculator is for general educational and preliminary material-planning use only. It provides mathematical estimates from user-entered trench dimensions, pipe diameter, slope, material allowance, gravel density, fabric overlap, and prices. It does not determine whether a French drain is suitable for a property, diagnose the source of water, design foundation drainage, establish required pipe capacity, approve an outlet, locate utilities, or replace site investigation, engineering, surveying, geotechnical advice, plumbing advice, waterproofing design, permits, manufacturer instructions, or contractor judgment. Actual quantities and performance can vary because of soil type, groundwater, rainfall intensity, contributing drainage area, trench shape, overexcavation, settlement, aggregate gradation, fabric selection, pipe stiffness, perforation orientation, fittings, cleanouts, frost, roots, traffic loads, buried services, outlet restrictions, erosion, and local codes. Excavation can expose workers and property to cave-ins, utility strikes, unstable soil, water intrusion, falls, and structural damage. Contact the applicable utility-marking service before digging, keep excavations safe, verify property and easement boundaries, protect nearby foundations and slopes, and confirm that discharge will not harm neighboring property or public infrastructure. Use a qualified drainage contractor, civil engineer, geotechnical professional, plumber, waterproofing specialist, or local authority when conditions are uncertain or when drainage affects a building, retaining wall, roadway, septic system, steep slope, or regulated stormwater system.
Last updated: June 10, 2026