French Drain Gravel Calculator
French Drain Gravel Calculator: Complete Guide to Materials, Sizing & Cost
A French drain is one of the most effective solutions for waterlogged yards, wet basements, and poor surface drainage. But before you dig a single inch of trench, you need to know exactly how much gravel to order, what pipe size to use, and how the entire system is sized. A French drain gravel calculator removes all the guesswork — giving you gravel volume in cubic feet, cubic yards, and tons along with filter fabric area and drain pipe length, all from just a few measurements.
This guide walks through everything: how to calculate gravel for a French drain, which gravel type to use, proper trench dimensions, slope requirements, and what the full installation will cost.
What Is a French Drain and How Does It Work?
A French drain is an underground drainage system built inside a gravel-filled trench, typically with a perforated drain pipe at the bottom. Water — whether from surface runoff or rising groundwater — seeps down through the gravel, enters the perforated pipe through its holes, and flows by gravity to a catch basin or outlet at a lower elevation.
The gravel serves two purposes: it provides structural support around the perforated pipe, and it creates void space for water to move freely down to the pipe. Without adequate gravel surrounding the pipe, soil and sand clog the pipe perforations and the system fails.
A French drain system is the right solution when you have standing water in your yard after rain, water pooling against a foundation, or wet basement walls caused by hydrostatic pressure.
French Drain Size: Trench Dimensions That Matter
Before you can calculate gravel for a French drain, you need to establish the trench dimensions. These three measurements drive every calculation:
Trench Width
For most residential applications, a trench width of 8 to 12 inches is standard. A wider trench holds more gravel, which increases the system’s water storage capacity while it drains — useful in areas with heavy rainfall or poor soil permeability.
Trench Depth
Standard French drain depth is 18 to 24 inches. This allows approximately:
- 3 inches of gravel beneath the pipe
- 4-inch perforated drain pipe
- 8–12 inches of gravel above the pipe
- 3 inches of topsoil or sod cap at the surface
For foundation drainage or areas with significant slope variation, deeper trenches (24–36 inches) ensure the pipe maintains a consistent slope throughout its length.
Trench Length
Simply the total run of the French drain system from start to outlet. Residential systems range from 20 to 100+ feet. Longer systems need careful attention to slope to ensure gravity moves water all the way to the outlet.
How to Calculate Gravel for a French Drain
The French drain gravel calculator uses trench volume minus pipe volume to determine the total gravel volume needed.
Step 1: Calculate total trench volume
Trench Volume (cu ft) = Width (ft) × Depth (ft) × Length (ft)
Step 2: Subtract pipe volume
Pipe Volume (cu ft) = π × (Pipe Diameter ÷ 2)² × Length
For a standard 4-inch pipe (actual outside diameter = 4.215 inches = 0.3513 ft):
- Pipe volume per foot = π × (0.3513 ÷ 2)² × 1 = 0.097 cu ft per linear foot
Step 3: Account for soil cap
The top 3 inches of the trench is typically backfilled with the original topsoil or sod, not gravel. Subtract that volume: Soil cap volume = Width (ft) × 0.25 ft × Length (ft)
Step 4: Result is your gravel volume
Gravel Volume = Trench Volume − Pipe Volume − Soil Cap Volume
Sample Calculation: 50-Foot French Drain
- Trench: 1 ft wide × 1.75 ft deep × 50 ft long
- Total trench volume: 1 × 1.75 × 50 = 87.5 cu ft
- Pipe volume: 0.097 × 50 = 4.85 cu ft
- Soil cap (3 in): 1 × 0.25 × 50 = 12.5 cu ft
- Gravel needed: 87.5 − 4.85 − 12.5 = 70.15 cu ft ≈ 2.6 cubic yards
To convert cubic yards to tons, multiply by the gravel density (~1.4 tons/yd³ for clean angular gravel): 2.6 × 1.4 = ~3.6 tons
Always add 10% for compaction and spillage when ordering.
How Much Rock for 100 Feet of French Drain?
Using the same standard dimensions (12-inch wide trench, 18-inch deep, 4-inch pipe):
| Drain Length | Gravel (cu ft) | Gravel (cu yd) | Gravel (tons) |
|---|---|---|---|
| 20 ft | 23 | 0.85 | 1.2 |
| 50 ft | 58 | 2.15 | 3.0 |
| 100 ft | 116 | 4.3 | 6.0 |
| 150 ft | 174 | 6.4 | 9.0 |
| 200 ft | 232 | 8.6 | 12.0 |
These estimates are based on standard assumptions. A trench gravel calculator or drain stone calculator lets you input your actual dimensions for a precise result.
French Drain Gravel Type: What to Use
Choosing the right gravel is not a minor detail — the wrong material causes premature failure through clogging or poor drainage.
Best Gravel for French Drains
¾-inch washed angular gravel (crushed stone) is the industry standard for French drain systems that include a perforated drain pipe. Angular stone packs around the pipe without shifting, while the irregular edges create large void spaces that allow water to percolate freely. Washed aggregate has no fine particles or dust that would otherwise migrate into the pipe.
Is ¾ Crushed Rock Good for a French Drain?
Yes — ¾-inch crushed stone (also called #57 stone or clean crushed limestone) is the most recommended French drain gravel type. It drains well, stays in place, and doesn’t clog easily. It’s what most drainage contractors specify by default.
Can I Use #57 Gravel for a French Drain?
Yes. #57 gravel is ¾-inch crushed stone — it’s the same material. It’s widely available, inexpensive, and performs well in both residential and commercial French drain systems. It’s the preferred choice for how much gravel around a drainage pipe is specified in most drainage design guides.
What NOT to Use
- Pea gravel: Too round and smooth. It migrates easily and can shift into pipe perforations, causing clogs.
- Sand: Immediately clogs pipe perforations.
- Limestone screenings or crusher dust: Fine particles fill void spaces and block drainage.
- River rock (large smooth stones): Works for pipeless French drains but provides poor contact around perforated pipe.
Pipeless French Drain
For surface drainage applications without a perforated pipe, larger river rock (1.5–3 inch) can be used to fill the trench. This is sometimes called a dry creek bed or channel drain and works well for directing surface runoff. The French drain aggregate calculator still works the same way — just remove the pipe volume step.
French Drain Fabric: Filter Fabric Requirements
Filter fabric (geotextile) lines the trench walls and pipe before gravel is added. Its job is to prevent surrounding soil from migrating into the gravel bed over time, which would eventually clog the system. Using quality French drain fabric extends the life of the system from 5–10 years to 20+ years.
How much filter fabric do you need?
Fabric Area = (2 × Trench Depth + Trench Width) × Trench Length + 1 ft overlap on each side
For a 12-inch wide, 18-inch deep, 50-foot trench:
- (2 × 1.5 + 1) × 50 = 4 × 50 = 200 sq ft of filter fabric
Fabric typically comes in rolls of 3, 4, or 6-foot widths. Choose a width that wraps the trench in one piece to minimize seams.
French Drain Pipe: Size and Slope Requirements
Pipe Size
For most residential French drain installations, a 4-inch perforated PVC pipe (SDR35 or schedule 40) is standard. The 4-inch size handles typical residential runoff volumes and is widely available at home improvement stores.
For larger drainage areas, high water tables, or commercial properties, 6-inch pipe is the appropriate step up.
Minimum Slope Requirements
Water moves through the French drain pipe by gravity. Insufficient slope causes water to sit in the pipe, reducing drainage capacity and creating conditions for sediment buildup.
| Pipe Size | Minimum Slope (inches per foot) |
|---|---|
| Up to 2.5 inches | ¼ inch per foot |
| 3 to 6 inches | ⅛ inch per foot |
| 8 inches or larger | 1/16 inch per foot |
For a 4-inch pipe on a 50-foot run at ⅛ inch per foot slope, the outlet end of the pipe will be 6.25 inches lower than the inlet. This must be factored into your trench depth when there are changes in grade across the installation area.
How Much Water Can a French Drain Handle?
A properly sized 4-inch French drain pipe on an adequate slope can handle approximately 20–50 gallons per minute of water flow depending on slope and pipe condition. For most residential applications, this exceeds the peak runoff from a typical rain event.
The limiting factor is usually the gravel permeability and surrounding soil infiltration rate, not the pipe capacity. Clay soils drain slowly — even a well-built French drain won’t handle a sudden heavy downpour instantly, but it will manage the water over time and prevent long-term saturation.
For areas with very heavy clay soil or drainage areas larger than half an acre, consult a licensed civil engineer to size the system correctly.
Cost of French Drain Per Foot
French drain installation cost depends on depth, soil conditions, accessibility, and your region’s labor rates.
| Installation Type | Cost Per Linear Foot |
|---|---|
| DIY (materials only) | $3 – $8/ft |
| Professionally installed (simple) | $20 – $35/ft |
| Professionally installed (complex/deep) | $35 – $60/ft |
| Interior basement French drain | $50 – $100/ft |
Material cost breakdown per foot (DIY, 12-inch wide × 18-inch deep trench):
- Gravel: $1.50 – $3.00/ft
- 4-inch perforated pipe: $0.75 – $1.50/ft
- Filter fabric: $0.30 – $0.60/ft
- Total materials: ~$2.55 – $5.10/ft
A 50-foot residential French drain runs approximately $125 – $255 in materials for a DIY installation.
Frequently Asked Questions
How do you calculate gravel for a French drain?
Multiply your trench width (ft) × depth (ft) × length (ft) to get total trench volume. Subtract the volume of the drain pipe and the soil cap layer at the top (typically 3 inches). The remaining volume is how much gravel for the French drain you need. Convert cubic feet to cubic yards by dividing by 27, then multiply by the gravel density (~1.4 tons/yd³) to get tons. A French drain gravel calculator automates all of this instantly.
How many inches of gravel under a French drain?
Standard practice is a minimum of 3 inches of gravel beneath the perforated drain pipe before it’s placed in the trench. This elevates the pipe off the trench floor and allows water to enter the pipe from the bottom as well as the sides. Some contractors use up to 6 inches of base gravel in areas with high water table issues.
How much rock do I need for 100 feet of French drain?
For a standard 12-inch wide, 18-inch deep trench with a 4-inch perforated pipe, 100 feet of French drain requires approximately 4.3 cubic yards or 6 tons of gravel. This accounts for the pipe volume, a 3-inch soil cap, and assumes ¾-inch washed angular stone. A French drain rock calculator gives you the precise number when you enter your actual trench dimensions.
What gravel should be used for a French drain?
¾-inch washed angular crushed stone (also called #57 stone) is the recommended French drain gravel type for most installations. It drains effectively, packs well around the pipe, and won’t migrate into perforations. Always use washed aggregate — unwashed stone contains fines that compact into the gravel bed and reduce drainage over time.
Is ¾ crushed rock good for a French drain?
Yes — ¾-inch crushed rock is the go-to specification for French drain installations with perforated pipe. Its angular shape creates good interlocking contact with minimal movement, and the consistent sizing ensures adequate void space for water flow. It’s preferred over pea gravel, river rock, or any smooth-sided aggregate.
Can I use #57 gravel for a French drain?
Yes. #57 gravel is the ASTM designation for ¾-inch clean crushed aggregate — the same material most drainage guides refer to as “clean washed angular gravel.” It’s readily available at most stone yards and home centers and is appropriate for residential French drain systems of all sizes.
Quick Reference: French Drain Gravel by Trench Size
| Trench Width | Trench Depth | Gravel per 10 ft | Gravel per 50 ft | Gravel per 100 ft |
|---|---|---|---|---|
| 8 in (0.67 ft) | 12 in (1 ft) | 4.7 cu ft | 23.5 cu ft (0.87 yd³) | 47 cu ft (1.74 yd³) |
| 10 in (0.83 ft) | 18 in (1.5 ft) | 8.6 cu ft | 43 cu ft (1.59 yd³) | 86 cu ft (3.19 yd³) |
| 12 in (1 ft) | 18 in (1.5 ft) | 11.6 cu ft | 58 cu ft (2.15 yd³) | 116 cu ft (4.3 yd³) |
| 12 in (1 ft) | 24 in (2 ft) | 16 cu ft | 80 cu ft (2.96 yd³) | 160 cu ft (5.93 yd³) |
Estimates use a 4-inch pipe and a 3-inch soil cap. Add 10% when placing your order.
Final Thoughts
Getting the gravel quantity right before you start digging saves a trip back to the stone yard mid-project and ensures you’re not under-ordering on a drainage system that needs to perform for decades. The French drain gravel calculator simplifies the math — enter your trench length, width, depth, and pipe diameter, and it outputs total gravel volume in cubic feet, cubic yards, and tons along with filter fabric area and drain pipe length.
Use the right gravel type (¾-inch washed angular stone), maintain proper slope (⅛ inch per foot minimum for 4-inch pipe), wrap the trench in filter fabric, and your French drain system will handle water effectively for 20+ years with no maintenance.