A Complete Guide from Kirk Allen Landscape Supply, NJ
If you’re planning to install a French drain on your property, you’ve probably realized it’s not as simple as digging a trench and tossing in some rocks. The success of a French drain depends heavily on choosing the right gravel—and the right amount of it.
At Kirk Allen Landscape Supply, we’ve helped thousands of New Jersey homeowners and contractors source high-quality drainage materials, and we know firsthand how a little math upfront can save a lot of headaches later. Let’s break it down so you can order confidently and avoid costly miscalculations.
What Is a French Drain?
A French drain is a simple yet highly effective drainage system designed to redirect surface water or groundwater away from a specific area, usually to prevent flooding or erosion. It consists of:
- A sloped trench
- Perforated drain pipe (optional but recommended)
- Gravel or crushed stone
- Landscape fabric
- Topsoil or decorative stone as a finishing layer
The gravel in a French drain is critical—it acts as a filter and provides structure to the drainage path.
Why Is Calculating Gravel So Important?
Using too little gravel can result in a clogged or ineffective drain. Too much gravel? You’re wasting money and time. The key is to calculate based on the dimensions of your trench and the drainage goals of your property.
Plus, if you’re ordering from a landscape supplier like us, knowing how many cubic yards you need ensures you’re not over- or under-ordering—both of which can delay your project.
Step-by-Step Guide to Calculating Gravel for Your French Drain
Step 1: Measure Your Trench
Grab a measuring tape and determine the length, width, and depth of your planned trench.
Let’s say you’re digging a trench that is:
- 50 feet long
- 12 inches wide (1 foot)
- 18 inches deep (1.5 feet)
You’ll use these numbers to calculate the total volume of your trench.
Step 2: Use the Volume Formula
The formula for volume is:
Length × Width × Depth = Cubic Feet
For the trench above:
50 ft (L) × 1 ft (W) × 1.5 ft (D) = 75 cubic feet
Step 3: Convert Cubic Feet to Cubic Yards
Most gravel is sold by the cubic yard, so you’ll need to convert:
Cubic Feet ÷ 27 = Cubic Yards
75 ÷ 27 ≈ 2.78 cubic yards
Step 4: Adjust for Pipe and Fabric
If you’re installing a 4-inch perforated pipe, it will displace some of that volume. Subtract about 0.2 to 0.3 cubic yards for every 50 feet of pipe.
So, your adjusted gravel estimate might be around:
2.78 – 0.3 = 2.48 cubic yards
To be safe, round up and order 2.5 to 3 cubic yards.
💡 Pro Tip: Always order 10–15% extra to account for settling and compaction. So, for our example, ordering 3.0 to 3.25 cubic yards of gravel would be ideal.
What Type of Gravel Should You Use?
Not all gravel is equal. Here are the best options for French drains:
1. ¾” Crushed Stone
The most common and effective. It allows water to pass through easily without compacting.
2. Washed Gravel
Free of dust and debris, which helps prevent clogs in your drainage system.
3. River Rock (for decorative topping)
Can be used on top for a finished look, though not always necessary.
At Kirk Allen Landscape Supply, we stock all these options and can help you decide what’s best based on soil type and budget.
Landscape Fabric: Don’t Skip It
While not part of the gravel calculation, geotextile landscape fabric is essential. It lines the trench to prevent soil from clogging your drain over time. Wrap the fabric around the gravel and pipe like a burrito—open on top if you’re topping with more gravel or soil.
We carry professional-grade fabric that resists tearing and helps your system last for decades.
Example Calculation Recap
| Trench Dimension | Value |
|---|---|
| Length | 50 ft |
| Width | 1 ft |
| Depth | 1.5 ft |
| Volume (cu. ft) | 75 |
| Volume (cu. yd) | 2.78 |
| Adjusted for pipe & rounding | 3.0 cu. yd approx. |
Frequently Asked Questions
How deep should the gravel be in a French drain?
A typical French drain trench is about 18 to 24 inches deep, with 12-18 inches of gravel below the pipe and 4-6 inches above it, depending on soil conditions and drainage goals.
Can I use pea gravel instead of crushed stone?
Pea gravel is smooth and can shift easily. While it might work in light-duty residential drains, it’s not recommended for French drains due to its tendency to move and compact over time. Stick to angular crushed stone for best results.
How much does gravel cost?
Gravel prices in NJ vary depending on type and supplier, but you can expect to pay between $40 to $60 per cubic yard for quality drainage gravel. Delivery is often available at an additional charge.
Call us at 973-347-3117 for a quote based on your zip code.
Need Help? Let’s Make It Easy.
We know calculating materials can be intimidating, especially if you’re installing your first French drain. If you’re located anywhere in New Jersey, Kirk Allen Landscape Supply is ready to help. We offer:
- Bulk gravel in all sizes
- Delivery directly to your site
- Friendly, expert advice
- Landscape fabric, drain pipe, and more
Just give us a call at 973-347-3117 or stop by the yard to speak with a pro.
Final Thoughts
Installing a French drain is a smart investment in your property’s long-term health, especially if you’re dealing with water pooling, basement leaks, or soggy lawns. By taking the time to accurately calculate the amount of gravel you need, you’re setting your system up for success—and saving money and effort in the long run.
Whether you’re a DIYer or a professional landscaper, Kirk Allen Landscape Supply has the gravel, materials, and guidance to get the job done right. Reach out today—we’re happy to help you drain smarter, not harder.
