---
title: "French Drain Calculator: Gravel Volume, Slope Drop, and the Burrito Method Explained"
canonical: "https://theyieldgrid.com/french-drain-calculator/"
model_id: "tyg-794"
model_version: "1.0.0"
last_reviewed: "2026-08-24T08:27:17"
reviewed_by: "Umer Hayiat"
---

# French Drain Calculator: Gravel Volume, Slope Drop, and the Burrito Method Explained

> Canonical calculator: [https://theyieldgrid.com/french-drain-calculator/](https://theyieldgrid.com/french-drain-calculator/)

## What this calculator does

Home - Free Gardening Calculators & Tools - French Drain Calculator: Gravel Volume, Slope Drop, and the Burrito Method Explained A french drain fails for one of two reasons: the wrong slope, or the wrong fabric strategy. Most homeowners dig the trench, drop in a pre-socked corrugated pipe, backfill with gravel, and declare the job done. Within two years, the system is fully blocked. Silt migrates through the gravel column and plugs the fine pores of the factory-wrapped sock from the outside in. No inspection reveals it until the water is back in the basement. The failure is structural, not accidental.

## Inputs

| Input | ID | Type | Unit | Range or choices | Required |
|---|---|---|---|---|---|
| Trench Length | `frdrain_length` | number | feet | 1 to 5000 | No |
| Trench Width | `frdrain_width` | number | inches | 4 to 72 | No |
| Trench Depth | `frdrain_depth` | number | inches | 4 to 120 | No |
| Desired Slope (%) | `frdrain_slope` | number | % | 0.5 to 20 | No |
| Pipe Diameter | `frdrain_pipe` | number | most common: 4″ | 2 to 24 | No |

## Outputs

| Output ID | Default state |
|---|---|
| `frdrain_length_err` |  |
| `frdrain_width_err` |  |
| `frdrain_depth_err` |  |
| `frdrain_slope_err` |  |
| `frdrain_pipe_err` |  |
| `frdrain_results` | 0.00 yd³ gravel needed Your french drain calculator result: see the breakdown below. Gravel fill 0% pipe / 100% gravel Gravel Pipe Displacement Measurement Value Total Trench Volume Pipe Displacement Gravel Required Slope Drop (total) Geotextile Fabric Needed Reference: Common French Drain Scenarios (1% Slope, 4″ Pipe) Length (ft) Width × Depth Gravel (yd³) Slope Drop (in) Fabric (sq ft) Recommended Supplies for This Project 4-oz Non-Woven Geotextile Fabric Rolls NDS Catch Basins Washed River Ro |
| `frdrain_out_primary` | 0.00 yd³ gravel needed |
| `frdrain_out_trench` |  |
| `frdrain_out_pipe` |  |
| `frdrain_out_gravel` |  |
| `frdrain_out_slope` |  |
| `frdrain_out_fabric` |  |

## Formula and method

Our calculator accurately subtracts the volume of the pipe from the total trench to ensure you don't over-order gravel. Show the calculation steps Step 1: Slope Drop Converts your percentage grade into a concrete elevation change in inches across the full run. SlopeDrop (in) = Length (ft) x 12 x (Slope% / 100) A 100-ft run at 1% grade drops exactly 12 inches. At 2%, it drops 24 inches. This number tells you how much lower your outlet needs to be compared to your inlet before a single shovel of dirt is moved. Step 2: Trench Volume Calculates the total excavated space in cubic yards, which is the unit your gravel supplier quotes in. TrenchVol (yd³) = [Length (ft) x (Width (in) / 12) x (Depth (in) / 12)] / 27 Dividing width and depth by 12 converts inches to feet. Dividing the result by 27 converts cubic feet to cubic yards. No compaction factor is applied at this stage. Step 3: Pipe Displacement Applies the cylinder volume formula to the pipe cross-section over the run length. PipeVol (yd³) = pi x (PipeDiameter (in) / 24)² x Length (ft) / 27 Dividing diameter by 24 converts to radius in feet (diameter divided by 2 gives radius in inches, divided by 12 converts to feet). The result is divided by 27 to reach cubic yards. For a standard 4-inch pipe at 100 feet, displacement is approximately 0.32 yd³, which is meaningful at scale. Step 4: Gravel Required GravelRequired (yd³) = TrenchVol - PipeVol This is the net fill volume. Add 10% when placing your order to account for uneven compaction, minor spillage, and gravel settlement over the first season. Step 5: Geotextile Fabric Area Calculates the fabric needed to line the trench walls and bottom using the Burrito Method, with a 20% area addition for laps and top fold. FabricSqFt = [(2 x Depth (ft)) + Width (ft)] x Length (ft) x 1.20 The factor 1.20 adds 20% for edge overlaps where the fabric sheets meet and for the top fold that closes the gravel bundle. Purchase fabric in 6-ft wide rolls for trenches up to 18 inches wide; 8-ft wide rolls for deeper or wider excavations. Assumptions and Limits Fabric method: All fabric area calculations assume the Burrito Method, meaning a single continuous wrap around the outside of the entire gravel column. Pre-socked pipe is excluded from this model. Gravel type: Assumes clean, washed drainage gravel or river rock with negligible fines. Crushed stone with fine material reduces effective void space and will cause the drain to underperform relative to this calculation. Single pipe run: The pipe displacement formula applies to one continuous pipe. Branching configurations require separate calculations for each segment. Uniform cross-section: The calculator assumes a rectangular trench cross-section of constant width and depth. Trapezoidal trench walls (typical of backhoe excavation) will produce slightly more spoil volume than shown. No compaction factor in primary result: Order at least 10% more gravel than the calculated figure. Pea gravel and river rock settle differently; larger stone settles less. Slope check is advisory: The 1% minimum threshold reflects widely cited drainage practice and is consistent with IRC surface drainage guidance. Sub-surface perforated pipe drainage on very flat sites may require additional engineering review beyond what this tool provides. Fabric overlap is estimated: The 20% area buffer is a reasonable field estimate. Sites with tight corners, multiple direction changes, or wide trenches may require additional fabric. Input limits: Length 1 to 5,000 ft; width 4 to 72 inches; depth 4 to 120 inches; slope 0.5 to 20%; pipe diameter 2 to 24 inches.

## Verified worked examples

### Example 1: Standard Backyard Swale (50-ft Run)

Trench Length: 50 ft Trench Width: 12 in Trench Depth: 18 in Desired Slope: 1% Pipe Diameter: 4 in Result: Trench Volume = 2.78 yd³, Pipe Displacement = 0.16 yd³, Gravel Required = 2.62 yd³, Slope Drop = 6.0 inches, Geotextile Fabric = 240 sq ft. A single cubic yard delivery plus a partial second yard covers the gravel. At 1%, the outlet sits exactly 6 inches lower than the inlet, which is a manageable excavation challenge on most residential lots. Two standard 6-ft wide by 50-ft fabric rolls provide more than enough material with overlap.

### Example 2: Long Driveway Perimeter Run (150-ft at 1.5%)

Trench Length: 150 ft Trench Width: 18 in Trench Depth: 24 in Desired Slope: 1.5% Pipe Diameter: 4 in Result: Trench Volume = 16.67 yd³, Pipe Displacement = 0.49 yd³, Gravel Required = 16.18 yd³, Slope Drop = 27.0 inches (2.25 ft), Geotextile Fabric = 990 sq ft. The slope drop of 27 inches over 150 feet is significant. Confirm that the outlet end of the trench does not daylight too shallow to be covered or, alternatively, too deep to connect to an existing storm inlet. This is precisely the scenario where running a laser transit level across the full run before excavation prevents a costly elevation mismatch.

### Example 3: Foundation Perimeter Drain (100-ft, 6-inch Pipe)

Trench Length: 100 ft Trench Width: 12 in Trench Depth: 30 in Desired Slope: 1% Pipe Diameter: 6 in Result: Trench Volume = 9.26 yd³, Pipe Displacement = 0.73 yd³, Gravel Required = 8.53 yd³, Slope Drop = 12.0 inches (1.0 ft), Geotextile Fabric = 720 sq ft. A 6-inch pipe on a 12-inch-wide trench is a tight fit; the tool will flag this because there is less than 3 inches of gravel clearance on either side of the pipe. In practice, widening to an 18-inch trench for a 6-inch pipe is strongly recommended by most drainage contractors. This example illustrates why pipe-to-trench ratio matters, not just gravel volume.

## Assumptions

Trench Length: 50 ft Trench Width: 12 in Trench Depth: 18 in Desired Slope: 1% Pipe Diameter: 4 in Result: Trench Volume = 2.78 yd³, Pipe Displacement = 0.16 yd³, Gravel Required = 2.62 yd³, Slope Drop = 6.0 inches, Geotextile Fabric = 240 sq ft. A single cubic yard delivery plus a partial second yard covers the gravel. At 1%, the outlet sits exactly 6 inches lower than the inlet, which is a manageable excavation challenge on most residential lots. Two standard 6-ft wide by 50-ft fabric rolls provide more than enough material with overlap. Our calculator accurately subtracts the volume of the pipe from the total trench to ensure you don't over-order gravel. Show the calculation steps Step 1: Slope Drop Converts your percentage grade into a concrete elevation change in inches across the full run. SlopeDrop (in) = Length (ft) x 12 x (Slope% / 100) A 100-ft run at 1% grade drops exactly 12 inches. At 2%, it drops 24 inches. This number tells you how much lower your outlet needs to be compared to your inlet before a single shovel of dirt is moved. Step 2: Trench Volume Calculates the total excavated space in cubic yards, which is the unit your gravel supplier quotes in. TrenchVol (yd³) = [Length (ft) x (Width (in) / 12) x (Depth (in) / 12)] / 27 Dividing width and depth by 12 converts inches to feet. Dividing the result by 27 converts cubic feet to cubic yards. No compaction factor is applied at this stage. Step 3: Pipe Displacement Applies the cylinder volume formula to the pipe cross-section over the run length. PipeVol (yd³) = pi x (PipeDiameter (in) / 24)² x Length (ft) / 27 Dividing diameter by 24 converts to radius in feet (diameter divided by 2 gives radius in inches, divided by 12 converts to feet). The result is divided by 27 to reach cubic yards. For a standard 4-inch pipe at 100 feet, displacement is approximately 0.32 yd³, which is meaningful at scale. Step 4: Gravel Required GravelRequired (yd³) = TrenchVol - PipeVol This is the net fill volume. Add 10% when placing your order to account for uneven compaction, minor spillage, and gravel settlement over the first season. Step 5: Geotextile Fabric Area Calculates the fabric needed to line the trench walls and bottom using the Burrito Method, with a 20% area addition for laps and top fold. FabricSqFt = [(2 x Depth (ft)) + Width (ft)] x Length (ft) x 1.20 The factor 1.20 adds 20% for edge overlaps where the fabric sheets meet and for the top fold that closes the gravel bundle. Purchase fabric in 6-ft wide rolls for trenches up to 18 inches wide; 8-ft wide rolls for deeper or wider excavations. Assumptions and Limits Fabric method: All fabric area calculations assume the Burrito Method, meaning a single continuous wrap around the outside of the entire gravel column. Pre-socked pipe is excluded from this model. Gravel type: Assumes clean, washed drainage gravel or river rock with negligible fines. Crushed stone with fine material reduces effective void space and will cause the drain to underperform relative to this calculation. Single pipe run: The pipe displacement formula applies to one continuous pipe. Branching configurations require separate calculations for each segment. Uniform cross-section: The calculator assumes a rectangular trench cross-section of constant width and depth. Trapezoidal trench walls (typical of backhoe excavation) will produce slightly more spoil volume than shown. No compaction factor in primary result: Order at least 10% more gravel than the calculated figure. Pea gravel and river rock settle differently; larger stone settles less. Slope check is advisory: The 1% minimum threshold reflects widely cited drainage practice and is consistent with IRC surface drainage guidance. Sub-surface perforated pipe drainage on very flat sites may require additional engineering review beyond what this tool provides. Fabric overlap is estimated: The 20% area buffer is a reasonable field estimate. Sites with tight corners, multiple direction changes, or wide trenches may require additional fabric. Input limits: Length 1 to 5,000 ft; width 4 to 72 inches; depth 4 to 120 inches; slope 0.5 to 20%; pipe diameter 2 to 24 inches. Fabric method: All fabric area calculations assume the Burrito Method, meaning a single continuous wrap around the outside of the entire gravel column. Pre-socked pipe is excluded from this model. Gravel type: Assumes clean, washed drainage gravel or river rock with negligible fines. Crushed stone with fine material reduces effective void space and will cause the drain to underperform relative to this calculation. Single pipe run: The pipe displacement formula applies to one continuous pipe. Branching configurations require separate calculations for each segment. Uniform cross-section: The calculator assumes a rectangular trench cross-section of constant width and depth. Trapezoidal trench walls (typical of backhoe excavation) will produce slightly more spoil volume than shown. No compaction factor in primary result: Order at least 10% more gravel than the calculated figure. Pea gravel and river rock settle differently; larger stone settles less. Slope check is advisory: The 1% minimum threshold reflects widely cited drainage practice and is consistent with IRC surface drainage guidance. Sub-surface perforated pipe drainage on very flat sites may require additional engineering review beyond what this tool provides. Fabric overlap is estimated: The 20% area buffer is a reasonable field estimate. Sites with tight corners, multiple direction changes, or wide trenches may require additional fabric. Input limits: Length 1 to 5,000 ft; width 4 to 72 inches; depth 4 to 120 inches; slope 0.5 to 20%; pipe diameter 2 to 24 inches. Critical Warnings The Fabric Sock is a Structural Failure, Not a Feature: Pre-wrapped corrugated pipe (the kind sold with a black fabric sock already attached) is designed for situations where the pipe is buried directly in native soil, with no gravel envelope. When you bury a socked pipe inside a gravel-filled trench, silt from the surrounding soil washes through the coarse gravel, reaches the sock, and clogs its fine pores from the outside. The gravel does not stop silt. The sock cannot self-clean. The pipe becomes inoperable, typically within one to three seasons. Sub-1% Slope Creates Sedimentation Inside the Pipe: Corrugated pipe has internal ridges that trap silt when water velocity is insufficient to carry it through. A slope below 1% produces slow-moving water that drops its sediment load inside the pipe rather than flushing it to the outlet. The tool flags this as a hard failure. Verify grade with a laser transit level, not a visual estimate or a standard bubble level. Outlet Elevation Must Be Verified Before Excavation: The slope drop output gives you the elevation difference between inlet and outlet. If your lot does not have enough vertical relief to achieve that drop at the outlet location you have in mind, no amount of careful installation will make the system work. Check outlet feasibility before committing to a trench route. Minimum Standards Maintain a minimum 1% grade along the full pipe run. Verify with a laser transit level, not estimation. Use 4-oz non-woven geotextile fabric wrapping the entire outside of the gravel column. Do not use woven geotextile; woven fabric has larger pores and passes more fine sediment. Leave a minimum 3 inches of gravel clearance on all sides of the pipe. A 4-inch pipe requires at least a 10-inch-wide trench; 12 inches is the practical standard. Use only clean, washed drainage gravel or river rock. Avoid limestone screenings, concrete gravel, or any crushed stone with dust or fines content. Competitor Trap: Nearly every other french drain calculator online asks you for trench dimensions and outputs a single gravel volume number. None of them address the fabric question at all. That omission is expensive: a homeowner who reads “order 6 cubic yards of gravel” and then goes to the hardware store and buys socked pipe because it is sold right next to the drainage gravel has just set up a system that will fail within two years. The burrito method, the pipe selection guidance, and the exact fabric square footage are not optional commentary. They are the part of the calculation that determines whether the project works. Understanding how surface water reaches your french drain helps right-size the entire system. The yard drainage catch basin calculator handles the inlet sizing problem, which is the upstream companion to the trench volume calculation here. For sites where pipe flow capacity is the design constraint rather than volume, Manning’s equation quantifies how pipe diameter, slope, and roughness combine to set maximum flow rate.

## Limitations and safety

Fabric method: All fabric area calculations assume the Burrito Method, meaning a single continuous wrap around the outside of the entire gravel column. Pre-socked pipe is excluded from this model. Gravel type: Assumes clean, washed drainage gravel or river rock with negligible fines. Crushed stone with fine material reduces effective void space and will cause the drain to underperform relative to this calculation. Single pipe run: The pipe displacement formula applies to one continuous pipe. Branching configurations require separate calculations for each segment. Uniform cross-section: The calculator assumes a rectangular trench cross-section of constant width and depth. Trapezoidal trench walls (typical of backhoe excavation) will produce slightly more spoil volume than shown. No compaction factor in primary result: Order at least 10% more gravel than the calculated figure. Pea gravel and river rock settle differently; larger stone settles less. Slope check is advisory: The 1% minimum threshold reflects widely cited drainage practice and is consistent with IRC surface drainage guidance. Sub-surface perforated pipe drainage on very flat sites may require additional engineering review beyond what this tool provides. Fabric overlap is estimated: The 20% area buffer is a reasonable field estimate. Sites with tight corners, multiple direction changes, or wide trenches may require additional fabric. Input limits: Length 1 to 5,000 ft; width 4 to 72 inches; depth 4 to 120 inches; slope 0.5 to 20%; pipe diameter 2 to 24 inches. Critical Warnings The Fabric Sock is a Structural Failure, Not a Feature: Pre-wrapped corrugated pipe (the kind sold with a black fabric sock already attached) is designed for situations where the pipe is buried directly in native soil, with no gravel envelope. When you bury a socked pipe inside a gravel-filled trench, silt from the surrounding soil washes through the coarse gravel, reaches the sock, and clogs its fine pores from the outside. The gravel does not stop silt. The sock cannot self-clean. The pipe becomes inoperable, typically within one to three seasons. Sub-1% Slope Creates Sedimentation Inside the Pipe: Corrugated pipe has internal ridges that trap silt when water velocity is insufficient to carry it through. A slope below 1% produces slow-moving water that drops its sediment load inside the pipe rather than flushing it to the outlet. The tool flags this as a hard failure. Verify grade with a laser transit level, not a visual estimate or a standard bubble level. Outlet Elevation Must Be Verified Before Excavation: The slope drop output gives you the elevation difference between inlet and outlet. If your lot does not have enough vertical relief to achieve that drop at the outlet location you have in mind, no amount of careful installation will make the system work. Check outlet feasibility before committing to a trench route. Minimum Standards Maintain a minimum 1% grade along the full pipe run. Verify with a laser transit level, not estimation. Use 4-oz non-woven geotextile fabric wrapping the entire outside of the gravel column. Do not use woven geotextile; woven fabric has larger pores and passes more fine sediment. Leave a minimum 3 inches of gravel clearance on all sides of the pipe. A 4-inch pipe requires at least a 10-inch-wide trench; 12 inches is the practical standard. Use only clean, washed drainage gravel or river rock. Avoid limestone screenings, concrete gravel, or any crushed stone with dust or fines content. Competitor Trap: Nearly every other french drain calculator online asks you for trench dimensions and outputs a single gravel volume number. None of them address the fabric question at all. That omission is expensive: a homeowner who reads “order 6 cubic yards of gravel” and then goes to the hardware store and buys socked pipe because it is sold right next to the drainage gravel has just set up a system that will fail within two years. The burrito method, the pipe selection guidance, and the exact fabric square footage are not optional commentary. They are the part of the calculation that determines whether the project works. Understanding how surface water reaches your french drain helps right-size the entire system. The yard drainage catch basin calculator handles the inlet sizing problem, which is the upstream companion to the trench volume calculation here. For sites where pipe flow capacity is the design constraint rather than volume, Manning’s equation quantifies how pipe diameter, slope, and roughness combine to set maximum flow rate.

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## Provenance

- Model ID: `tyg-794`
- Model version: `1.0.0`
- Reviewed by: Umer Hayiat
- Page modified: 2026-08-24T08:27:17
- Runtime SHA-256: `1b353c9586a98c3144ef9ff954c4eb92c1cf37d3a92c3594c0bdcdc192ca52b8`

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