French Drain Calculator, Gravel, Pipe, Fabric, Slope and Cost Estimator
Calculate the gravel, perforated pipe, filter fabric, excavation volume, slope drop, and estimated cost for a French drain installation. This tool is built for U.S. contractors, drainage installers, landscapers, and homeowners planning yard drainage, foundation drainage, and trench drain material orders.
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Rebar Weight Chart
US Standard Rebar Sizes (#2–#18) with weight per foot, diameter, and cross-sectional area.
View Chart →How to Use This French Drain Calculator
Enter trench size
Start with drain length, trench width, and trench depth. These three values control excavation volume and the amount of stone required.
Select pipe and stone assumptions
Choose the pipe diameter and stone density. The calculator subtracts pipe displacement so the stone estimate is closer to a real order quantity.
Set slope and waste
Input the target slope and ordering allowance. This helps you check total drop from start to outlet and avoid under-ordering stone or fabric.
Review material and cost totals
Results show gravel in cubic feet, cubic yards, and tons, plus pipe, fabric, labor guidance, and estimated project cost.
French Drain Reference Values
| Item | Typical Planning Value | Why It Matters |
|---|---|---|
| Residential trench width | 8 to 12 inches | Common width range used for many yard drainage runs. |
| Pipe size | 4 inch perforated pipe | Common residential drainage diameter. |
| Planning slope | 1% | Produces about 1 foot of drop over 100 feet. |
| Stone bedding under pipe | 2 inches minimum | IRC R405 footing drains require at least 2 inches of washed gravel or crushed rock under perforated pipe. |
| Stone cover above pipe | 6 inches minimum | IRC R405 footing drains require at least 6 inches of the same material above the pipe. |
| Fabric overlap allowance | 12 inches | Common planning assumption for wrapped nonwoven geotextile. |
What This French Drain Calculator Measures
A French drain is a gravel-filled trench that routes water into a perforated pipe and carries it to an approved discharge point. For yard drainage, the key material questions are trench volume, stone quantity, pipe length, filter fabric coverage, and the amount of slope available from inlet to outlet.
This calculator focuses on material planning first. It estimates trench excavation, gravel after pipe displacement, total stone tonnage using supplier density assumptions, filter fabric area, and rough labor hours based on installation method.
If you only need stone volume, use the french drain gravel calculator. If you need broader aggregate takeoffs, the gravel calculator, drain rock calculator, and drainage rock calculator can help cross-check your order.
Code and standard points that affect planning
For foundation drainage, IRC Section R405 requires drainage tiles, gravel or crushed stone drains, perforated pipe, or other approved systems to be installed at or below the top of footing or below the bottom of the slab and discharged into an approved drainage system. The same section requires perforated pipe to be placed on at least 2 inches of washed gravel or crushed rock and covered with at least 6 inches of the same material.
Where gravel or crushed stone drains are used for foundation drainage, IRC R405 also states that the aggregate should extend at least 1 foot beyond the outside edge of the footing and 6 inches above the top of footing, then be covered with an approved filter membrane material. For flexible corrugated drainage tubing, ASTM F405 historically covered nominal 3 to 6 inch pipe dimensions used in subsurface drainage work.
For trench safety, OSHA 29 CFR 1926 Subpart P requires a protective system for trenches 5 feet deep or greater unless the excavation is made entirely in stable rock. That matters for long or deep drainage trenches, especially on foundation work or sites with poor soil stability.
Material assumptions behind the calculator
The stone estimate is based on trench volume minus the outside volume displaced by perforated pipe. Fabric quantity is based on trench wrap width, two trench sidewalls, trench bottom, and an overlap allowance. Slope drop is calculated from run length and slope percentage so you can check whether the outlet elevation is realistic before excavation starts.
Stone tonnage depends on the product your supplier delivers. Clean washed stone can run around 1.30 to 1.40 tons per cubic yard, while denser crushed material can be higher. Use the supplier’s certified conversion when available because freight and order minimums are often based on tons, not loose cubic yards.
Sample Calculation Scenarios
🌧️ Backyard low spot drain
Length: 60 ft
Width: 12 in
Depth: 18 in
Pipe: 4 in perforated pipe
With a 1% slope, the drain needs about 0.6 feet of total drop from start to outlet. For aggregate ordering and spoil removal, it also helps to compare the excavation with the excavation calculator and the backfill calculator.
🏠 Foundation perimeter drain planning
Length: 140 ft
Width: 16 in
Depth: 24 in
Pipe: 4 in perforated pipe
Foundation drainage work should follow IRC R405, especially pipe bedding, stone cover, and approved discharge requirements. If site grading also needs correction, compare your material quantities with the base material calculator or subbase calculator for adjacent hardscape repairs.
🚜 Long side-yard machine install
Length: 220 ft
Width: 12 in
Depth: 20 in
Pipe: 4 in perforated pipe
At 1% slope, a 220-foot run needs about 2.2 feet of total fall. If the site cannot provide that elevation change, a different discharge strategy or sump connection may be needed.
Frequent French Drain Planning Mistakes
Using trench dimensions without checking actual outlet elevation. A drain can have enough stone and pipe on paper but still fail if there is no positive discharge path.
Ordering stone from gross trench volume only. Pipe displacement reduces the stone envelope, and waste should be added after the base estimate, not before.
Skipping filter fabric or choosing the wrong fabric type. Drainage work typically uses nonwoven geotextile, not decorative weed barrier fabric.
Assuming all stone weighs the same. Washed stone, drain rock, and crushed stone can produce different tons-per-yard conversions.
Ignoring trench safety. Once trenches reach 5 feet deep, OSHA protective system rules may apply unless the excavation is entirely in stable rock.
Planning Notes for Drainage Installations
For yard drainage, a French drain usually works best when it intercepts water before it spreads across low areas or reaches the foundation. Many contractors combine the drain with grading corrections, downspout extensions, or discharge into a dry well or approved storm system.
For foundation drainage, code requirements are stricter because hydrostatic pressure and subsurface water can affect the building envelope. IRC R405 is the main reference point for footing drain location, bedding, aggregate cover, and discharge, while AASHTO M288 is commonly used as a reference for drainage geotextile selection.
If your project also includes imported fill, base repair, or hardscape replacement after trenching, compare related quantities with the crushed stone calculator, road base calculator, or driveway base calculator.
⚠️ Field check before excavation
Verify buried utilities, outlet elevation, property line setbacks, and local stormwater discharge rules before trenching. Material math is only one part of a drainage installation.
✅ Ordering tip
Drainage stone is often sold by the ton, while trench takeoffs are easier to compute in cubic feet or cubic yards. Check both values before calling a supplier so truck quantity and delivery charges match the actual order.
French Drain Questions
Many residential French drain trenches are about 8 to 12 inches wide, though wider trenches may be used for higher inflow, foundation drainage, or difficult soil conditions. BuildersCalculator notes that 8 to 12 inches works for many common installations, but site conditions and outlet constraints control the final layout.
For foundation drains, IRC R405 requires perforated pipe to be placed on at least 2 inches of washed gravel or crushed rock and covered with at least 6 inches of the same material. Yard drains often follow similar good practice even when not installed as footing drains.
Clean washed drainage aggregate is commonly used because it provides void space for water movement and does not compact like dense graded base. Suppliers often recommend washed drain rock or similar open-graded stone for perforated pipe installations.
At 1% slope, a 100-foot run needs 1.0 foot of total fall from start to outlet. This calculator converts slope percent directly into total drop so you can test if the discharge point is realistic before excavation begins.
No. Excavation volume is based on full trench dimensions, but stone volume is usually lower because the perforated pipe displaces some of that space and some installations also leave soil cover above the stone. This calculator reports both values separately.
OSHA requires a protective system for trenches 5 feet deep or greater unless the excavation is made entirely in stable rock. A competent person also needs to inspect trenches daily and after conditions change, especially after rain or water intrusion.
Sources and Methodology
- IRC 2024 / IRC R405 Foundation drainage: footing drain location, gravel or crushed stone drainage, 2-inch bedding under perforated pipe, 6-inch cover above pipe, approved filter membrane and approved discharge system. Public summary: 2024 IRC Chapter 4, Section R405 summary
- ASTM F405: historical standard specification for corrugated polyethylene drainage tubing in nominal 3 to 6 inch sizes for soil drainage applications. Reference summary: ASTM F405 summary
- AASHTO M288: commonly referenced geotextile standard for subsurface drainage and separation applications. Public technical references: AASHTO M288 technical sheet
- OSHA 29 CFR 1926 Subpart P: trench safety requirements, including protective systems for trenches 5 feet or deeper and inspection duties of the competent person. OSHA reference: OSHA trenching and excavation safety fact sheet
- Competitor functionality review: input patterns and common trench defaults reviewed from Omni Calculator French Drain Calculator and BuildersCalculator French Drain Gravel Calculator
Built by Muhammad Ramzan Babar, physics researcher (PhD candidate).
Last reviewed: June 25, 2026.
Disclaimer
This calculator provides estimates for planning purposes. For permitted structural work, foundations, multi-story construction, retaining walls over 4 feet, and commercial projects, calculations must be verified by a licensed structural engineer per IBC 2024 §1604. ConcreteCalculate.com is not liable for structural decisions made from these estimates.
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