French Drain Gravel Calculator and How to Use It

Calculate gravel for a French drain by multiplying the trench’s length, width, and gravel depth to find its volume, then add a modest allowance for settling and uneven excavation. Enter those three dimensions below in feet; the calculator returns the estimated gravel volume in cubic feet and cubic yards.

Formula

Volume in cubic feet = length × width × gravel depth. To convert cubic feet to cubic yards, divide by 27, because one cubic yard contains 27 cubic feet. The calculator’s first result is the geometric volume before any allowance for settling or excavation variation.

• Length (L): trench length in feet.
• Width (W): inside trench width in feet.
• Gravel depth (D): depth occupied by gravel in feet.
• Volume: cubic feet (ft³), or cubic yards (yd³) after dividing by 27.

For metric measurements, multiply length, width, and depth in metres to get cubic metres (m³). If you measure in centimetres, convert each dimension to metres first, or divide the product of the three centimetre measurements by 1,000,000.

Worked Example

Using the calculator’s default dimensions—20 ft long, 1 ft wide, and 1 ft deep:

1. Volume in cubic feet = 20 × 1 × 1 = 20 ft³.
2. Volume in cubic yards = 20 ÷ 27 = 0.7407… yd³.
3. Rounded to two decimal places, the results are 20.00 ft³ and 0.74 yd³.

These figures describe the rectangular space occupied by gravel. They do not include extra material for a bulging trench, an irregular bottom, compaction, or waste.

How to Measure the Trench

Measure the trench’s actual inside dimensions, not the overall drainage area. Length is the path the drain follows. Width is the distance between the trench sides. Gravel depth is only the portion of the trench that will contain stone; it is not necessarily the full trench depth if soil, a pipe, or another layer occupies part of it.

For a consistent trench, use representative dimensions. If width or depth changes noticeably along the route, divide the trench into sections, calculate each section separately, and add their volumes. For example, calculate two rectangular sections as L₁ × W₁ × D₁ and L₂ × W₂ × D₂, then sum the results. This is more reliable than using one average when the trench has a substantial change in size.

Check measurements in the same unit before multiplying. A length in feet multiplied by a width in inches and a depth in feet will not produce a correctly labelled cubic-foot result. Convert inches to feet by dividing by 12, or use one consistent metric unit throughout.

Choosing Gravel and Allowing for Extra

French drains commonly use clean, washed, coarse aggregate around a perforated drainpipe. The appropriate stone size and pipe arrangement depend on the project design, soil, and local conditions. Avoid assuming that any material called “gravel” will perform identically: fine particles or excess dust can affect the open spaces through which water moves. Follow the drain design’s material specifications and keep soil from mixing into the stone, often by using a suitable permeable geotextile where the design calls for it.

The calculated volume is a useful starting quantity, not an exact delivery guarantee. Trench walls may cave or bulge, the bottom may be uneven, and stone can settle as it is placed. A practical approach is to calculate the measured volume, then add a modest allowance—often around 5% to 10% for a reasonably uniform trench—and confirm the quantity with the aggregate supplier or project plan. The right allowance depends on how carefully the trench is excavated and how the material is sold; it is not a universal rule.

Bulk aggregate is often sold by volume, while some suppliers quote or deliver by weight. A cubic-yard estimate cannot be converted to a dependable tonnage without the material’s bulk density, which varies with stone type, grading, moisture, and packing. Ask the supplier for the product’s conversion information rather than treating a generic tons-per-yard figure as exact.

Pipe, Fabric, and Common Calculation Mistakes

A trench-volume calculation estimates the space available for the whole gravel layer. It does not subtract the volume of a pipe. For a typical drainpipe, the pipe occupies a relatively small part of the trench, and the stone surrounds it; many practical estimates therefore use the full trench volume. If a design requires a more exact net quantity, subtract the pipe’s outside cylindrical volume: π × (outside diameter ÷ 2)² × pipe length, using consistent units. Do not subtract the pipe’s inside capacity—the relevant space displaced is its outside volume.

• Confusing trench depth with gravel depth: account for any space taken by soil cover or other layers.
• Using mixed units: convert every dimension to feet or every dimension to metres before calculating.
• Forgetting cubic yards conversion: divide cubic feet by 27, not by 3.
• Ordering only the exact geometric volume: real excavations are rarely perfectly rectangular, and gravel may settle.
• Assuming a volume tells you weight: use the supplier’s density or conversion for the actual aggregate.

What the Estimate Does—and Does Not—Tell You

This calculation answers how much space the planned gravel layer occupies. It does not determine whether a French drain will solve a water problem, whether the slope and outlet are adequate, or which pipe and stone specification is suitable. Drain performance depends on the route, elevation, soil, water source, outlet conditions, and installation details. A drain with no effective place for collected water to go may not work as intended.

For long or irregular projects, keep a simple section-by-section measurement list and total the volumes before ordering. Check the excavated trench dimensions before placing material; if the dimensions differ from the plan, update the estimate. If runoff, groundwater, retaining structures, or building foundations are involved, have the drainage design assessed by an appropriately qualified local professional.

Frequently Asked Questions

How many cubic yards of gravel do I need for a French drain?
Multiply trench length, width, and gravel depth in feet to get cubic feet, then divide by 27. Add an allowance if the excavation is uneven or the stone may settle.

How do I calculate gravel for a French drain in metres?
Multiply length × width × gravel depth, with all dimensions in metres. The result is cubic metres. For centimetre measurements, convert to metres first or divide their product by 1,000,000.

Should I subtract the drainpipe from the gravel volume?
Usually, a practical estimate uses the full trench volume because the pipe is surrounded by gravel and occupies a relatively small share of it. For a precise net estimate, subtract the pipe’s outside cylindrical volume.

How much extra gravel should I order?
For a uniform, carefully dug trench, a modest 5%–10% allowance can account for variation and settling, but conditions differ. Use the project specification or ask the supplier if the excavation is irregular.

How many tons are in a cubic yard of French drain gravel?
There is no single exact conversion. Weight per cubic yard depends on aggregate type, grading, moisture, and packing, so request the supplier’s density or volume-to-weight conversion for the specific material.

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in l | Trench length | ft | 20 | 0
in w | Trench width | ft | 1 | 0
in d | Gravel depth | ft | 1 | 0
out cf | Gravel volume | ft³ | l * w * d | 2
out cy | Gravel volume | yd³ | l * w * d / 27 | 2