French Drain Rock Calculator: How Much Gravel

This French drain rock calculator works out the volume and weight of gravel or crushed stone needed to fill a trench for a French drain. You enter the trench length, width, and depth (in feet or meters), plus the gravel density if you want a weight estimate, and it returns the required volume in cubic feet, cubic yards, and cubic meters, along with the approximate weight in tons and kilograms. The calculator uses the standard formula for the volume of a rectangular prism, then applies a typical gravel density to convert volume to weight.

Formula

The calculator uses the basic geometric formula for the volume of a rectangular prism:

Volume = Length × Width × Depth

Where:

• Length (l) – the total length of the trench, in feet (or meters).
• Width (w) – the average width of the trench, in feet (or meters).
• Depth (d) – the average depth of the trench, in feet (or meters).
• Density (dens) – the bulk density of the gravel, typically 90–120 lb/ft³ (1,440–1,920 kg/m³).

To convert volume to weight: Weight = Volume × Density. For US tons, divide pounds by 2,000; for metric tonnes, divide kilograms by 1,000.

Worked Example

Using the default values from the calculator: a trench 30 ft long, 1 ft wide, and 1.5 ft deep, with gravel density 105 lb/ft³.

1. Volume in cubic feet: 30 × 1 × 1.5 = 45 ft³
2. Volume in cubic yards: 45 ÷ 27 = 1.67 yd³
3. Volume in cubic meters: 45 × 0.0283168 = 1.27 m³
4. Weight in pounds: 45 × 105 = 4,725 lb
5. Weight in tons: 4,725 ÷ 2,000 = 2.36 tons

So for a typical 30-foot French drain, you would need about 1.67 cubic yards (1.27 m³) of gravel, weighing roughly 2.36 tons (2.14 metric tonnes).

What Is a French Drain and Why Does Rock Size Matter?

A French drain is a trench filled with gravel or crushed stone that redirects surface water and groundwater away from a structure. The gravel creates a high-permeability path for water to flow toward a perforated pipe (if used) or simply to a lower area. The size of the rock matters because it affects both drainage speed and the risk of clogging.

Common gravel sizes for French drains:

• 1/2-inch (12 mm) – fine gravel, good for areas with very fine soil, but may clog more easily.
• 3/4-inch (19 mm) – the most common choice; balances drainage and stability.
• 1 to 1.5-inch (25–38 mm) – coarse gravel, used when high flow is expected or when wrapping with filter fabric.

Larger rock leaves bigger voids, allowing water to move faster, but if the rock is too large, soil can wash in and clog the drain. Most professionals recommend 3/4-inch washed gravel for a standard residential French drain.

How to Measure Your Trench Correctly

Accurate measurements are the key to a correct estimate. Here’s what to do:

1. Length: Run a tape measure along the centerline of the trench from start to end. For curved drains, measure along the curve, not straight line.
2. Width: Measure the width at several points and average them. Trenches dug by hand are often wider at the top than the bottom.
3. Depth: Measure from the ground surface to the bottom of the trench. Again, take multiple readings and average.
4. Add extra: Add 10–15% to the volume to account for compaction, spillage, and irregular sides.

If you plan to wrap the gravel in filter fabric, the fabric takes up negligible volume, but the gravel will settle over time, so a little extra is wise.

Common Mistakes and Misconceptions

One frequent mistake is forgetting that the trench is not completely filled with gravel if you’re also installing a pipe. The pipe displaces some volume, but for a typical 4-inch (10 cm) pipe, that’s only about 0.1 ft³ per foot of pipe (0.008 m³ per meter) – usually less than 1% of the total, so it’s often ignored. However, if you’re using a large pipe (e.g., 6-inch or more) or a drain box, you may want to subtract that volume.

Another misconception is that “gravel� has a single density. In reality, density varies with rock type and compaction:

• Loose gravel: 90–110 lb/ft³ (1,440–1,760 kg/m³)
• Compacted gravel: 110–130 lb/ft³ (1,760–2,080 kg/m³)
• Crushed stone (e.g., limestone): about 100–120 lb/ft³ (1,600–1,920 kg/m³)

If you order by weight, use the density from your supplier if possible; otherwise, 105 lb/ft³ is a reasonable average.

How to Order Gravel: Volume vs. Weight

Gravel is sold either by the cubic yard (or cubic meter) or by the ton (or metric tonne). To convert between them, you need the density. For example, if you need 1.67 yd³ and your supplier sells by the ton, multiply 1.67 by the density in lb/yd³ (roughly 2,835 lb for 105 lb/ft³) to get about 4,725 lb, or 2.36 tons.

Always confirm with your supplier whether the price is per ton or per cubic yard, and whether delivery is included. It’s also a good idea to order about 10% extra to avoid running short, especially if the trench is irregular.

Alternatives to Traditional Gravel

While washed gravel is the standard, some situations call for alternatives:

• Pea gravel – smaller, rounded stones; drains well but can shift and clog more easily.
• Crushed stone – angular pieces that lock together, providing more stability; often preferred for steep slopes.
• Recycled concrete – cheaper but may have fines that reduce drainage; use only if washed.
• Plastic drainage cells – prefabricated panels that create a void; easier to install but more expensive.

For most residential French drains, washed 3/4-inch crushed stone is the best balance of cost, drainage, and longevity.

Safety and Installation Tips

Working with gravel and trenches involves physical labor and some hazards:

• Wear gloves and safety glasses when handling rock.
• Check for underground utilities before digging – call your local utility locator service.
• Keep trench walls stable; if deeper than 3 feet (1 m), consider shoring or a trench box.
• Use a wheelbarrow or chute to move gravel, and avoid lifting heavy loads repeatedly.
• If using a perforated pipe, lay it with the holes facing down (or as per manufacturer instructions) and slope it 1–2% toward the outlet.

Frequently Asked Questions

How much gravel do I need for a 100-foot French drain?
For a 100 ft × 1 ft × 1.5 ft trench, the volume is 150 ft³ (5.56 yd³). At 105 lb/ft³, that’s 15,750 lb, or about 7.9 tons. Add 10% extra, so roughly 8.7 tons.

Can I use sand instead of gravel in a French drain?
Sand has much lower permeability than gravel and will clog quickly. It is not recommended. If you need a finer material, use a geotextile fabric around the gravel to keep soil out while allowing water in.

What is the best gravel size for a French drain?
Washed 3/4-inch (19 mm) crushed stone is the most common and reliable choice. It provides good drainage without excessive voids that allow soil to infiltrate.

Should I wrap the gravel in fabric?
Yes, wrapping the gravel (and pipe) in a non-woven geotextile fabric prevents fine soil from entering the drain and clogging it. The fabric adds minimal cost and significantly extends the drain’s life.

How deep should a French drain be?
Typically 18–24 inches (45–60 cm) deep, with the bottom below the problem area’s water level. The exact depth depends on the soil and the water source; deeper is better for intercepting groundwater.

Do I need to subtract the pipe volume from the gravel calculation?
For a standard 4-inch pipe, the difference is negligible (less than 1% of the total). For larger pipes or drains with multiple pipes, you can subtract the pipe’s volume using the formula πr²l, but it’s often easier to just order 10% extra gravel.

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in l | Trench length | ft | 30 | 0
in w | Trench width | ft | 1 | 0
in d | Trench depth | ft | 1.5 | 0
in dens | Gravel density | lb/ft³ | 105 | 0
out vol_ft | Volume | ft³ | l * w * d | 2
out vol_yd | Volume | yd³ | (l * w * d) / 27 | 2
out vol_m | Volume | m³ | (l * w * d) * 0.0283168 | 2
out weight_lb | Weight | lb | (l * w * d) * dens | 0
out weight_t | Weight | tons | ((l * w * d) * dens) / 2000 | 2