Drain Rock Calculator: How Much Gravel You Need

A drain rock calculator works out the volume and weight of gravel needed to fill a trench, French drain, or drainage bed. You enter the length, width, and depth of the area, and the calculator returns the volume in cubic feet or cubic yards, plus the estimated tons of rock required. This article explains the math behind the calculator, how to use it correctly, and what to watch out for when ordering drainage aggregate.

Formula

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

Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft)

Since depth is entered in inches, it is converted to feet by dividing by 12. To get cubic yards, divide the cubic feet by 27 (because 1 cubic yard = 27 cubic feet). To estimate weight in tons, multiply the cubic feet by the density of the rock in tons per cubic foot. A typical value for drainage gravel is about 0.05 tons per cubic foot (roughly 100 lb per cubic foot), which corresponds to a density of about 1.6 metric tons per cubic meter.

Worked Example

Using the default values: Length = 50 ft, Width = 1 ft, Depth = 12 in.

1. Convert depth to feet: 12 in ÷ 12 = 1 ft
2. Volume in cubic feet = 50 × 1 × 1 = 50 cu ft
3. Volume in cubic yards = 50 ÷ 27 ≈ 1.85 cu yd
4. Estimated tons = 50 × 0.05 = 2.5 tons

So for a 50-foot-long, 1-foot-wide, 1-foot-deep trench, you need about 50 cubic feet (1.85 cubic yards) of rock, which weighs roughly 2.5 tons. Always confirm the density with your supplier, as it varies with rock type and moisture.

What Is Drain Rock and Why Use It?

Drain rock—also called drainage gravel, crushed stone, or aggregate—is a coarse, angular material used to create pathways for water to flow through soil. Unlike sand or fine gravel, drain rock has large voids between stones that allow water to move quickly. It is commonly used in:

• French drains (trenches filled with gravel and a perforated pipe)
• Foundation footing drains
• Behind retaining walls for drainage
• Underneath downspout outlets
• As a base layer for patios or walkways

The angular shape of crushed rock helps it lock together, preventing shifting while maintaining open spaces for water flow. Rounded river rock can also be used but may compact more and reduce drainage capacity.

How to Measure Your Drainage Area

Accurate measurements are the key to a correct estimate. Follow these steps:

1. Length: Measure the full extent of the trench or bed along its centerline. Use a tape measure or measuring wheel for long runs.
2. Width: For a trench, this is the bottom width. If the trench sides slope, measure the average width (top width + bottom width ÷ 2).
3. Depth: Measure from the bottom of the trench to the level where the gravel will reach. If the trench will be covered with soil or fabric, subtract that thickness.
4. Compaction factor: If the gravel will be compacted (e.g., under a patio), add 10–15% to the volume to account for settling.

Common Mistakes and How to Avoid Them

Even with a good calculator, people often make these errors:

• Mixing units: Entering depth in inches but length in feet without converting. Always convert everything to the same unit (feet is easiest).
• Forgetting the pipe: In a French drain, the perforated pipe takes up space. For a 4-inch pipe, subtract about 0.09 cubic feet per linear foot (the pipe’s volume). This reduces the rock needed slightly.
• Ignoring the fabric: If you wrap the trench in landscape fabric, the fabric takes up negligible space, but it affects how much rock you need at the top and bottom. Usually you fill to the top of the fabric, so measure accordingly.
• Ordering by weight without checking density: Different rocks weigh differently. A cubic foot of crushed granite weighs more than a cubic foot of lightweight lava rock. Always ask your supplier for the specific weight per cubic yard.

Drain Rock vs. Other Aggregates

Not all gravel is the same. Here’s how drain rock compares to other common materials:

• Pea gravel: Small, rounded stones (about 3/8 inch). It drains well but can wash away easily and doesn’t lock together. Better for decorative areas than for underground drainage.
• Crushed stone #57: Angular stones about 1 inch in size. This is a popular choice for drain fields because it has large voids and doesn’t compact too tightly.
• Crushed stone #3: Larger stones (1–2 inches) with even bigger voids. Good for very wet areas, but may be too large for narrow trenches.
• Sand: Drains poorly and should never be used in a French drain—it fills the voids and stops water flow.

For most residential drainage projects, #57 crushed stone is the standard recommendation because it balances void space and stability.

Cost and Ordering Considerations

Drain rock is sold by the cubic yard (loose) or by the ton (weighed). The calculator gives you both volume and weight, so you can compare prices. As a rough guide, a cubic yard of dry crushed stone weighs about 1.4 to 1.7 tons (2,800–3,400 lb). When ordering, you usually need to round up to the nearest quarter or half yard. It’s wise to order 10% extra to account for settling and spillage.

If you’re having rock delivered, consider the delivery fee and whether you can accept a bulk truckload. For small projects, buying bagged rock from a home center is more expensive per cubic foot but avoids delivery costs.

Safety and Installation Tips

Working with drain rock involves heavy lifting and sharp edges. Follow these safety practices:

• Wear gloves and eye protection when handling or shoveling rock.
• Use a wheelbarrow to move rock—avoid lifting heavy buckets repeatedly.
• If the trench is deep (over 3 feet), slope the sides or use shoring to prevent collapse.
• Call 811 before digging to locate underground utilities (in the U.S.).
• When installing a French drain, wrap the gravel in filter fabric to prevent soil from clogging the voids.

Also, ensure the drain has a proper outlet (e.g., a daylight pipe or a dry well) so water doesn’t pool at the end of the trench.

Frequently Asked Questions

How deep should a French drain be?

Most French drains are 12 to 24 inches deep. The depth depends on the water table and the frost line in your area. The bottom of the drain should be below the level where water is entering (e.g., below a foundation footer).

What size gravel is best for drainage?

Crushed stone in the 1-inch to 1.5-inch range (like #57) is ideal for most drainage systems. It has enough void space (about 30–40%) to carry water quickly while remaining stable.

How much area does a ton of drain rock cover?

At a depth of 1 foot, a ton of typical crushed stone covers about 20 square feet (since 1 ton ≈ 20 cubic feet). At 6 inches deep, it covers about 40 square feet. Use the calculator to get an exact figure for your dimensions.

Can I use river rock instead of crushed stone?

Yes, but river rock is round and will settle and lock together less effectively. It may also shift under load. For underground drainage, crushed angular stone is preferred because it maintains voids better.

Should I put landscape fabric over the drain rock?

Yes, wrap the trench with filter fabric (geotextile) before adding rock, and fold it over the top after filling. This prevents soil and silt from washing into the gravel and clogging it over time.

How do I calculate for a sloped trench?

If the trench bottom slopes, measure the average depth (at the midpoint) and use that in the calculator. For a uniform slope, the volume is the same as a level trench with the same average depth.

Related

• Building Square Calculator: How to Find Area
• Buco Concrete Calculator: How to Use It
• Buco Brick Calculator: How to Estimate Bricks

in l | Length | ft | 50 | 0
in w | Width | ft | 1 | 0
in d | Depth | in | 12 | 0
out vol_ft | Volume | cu ft | l * w * (d / 12) | 2
out vol_yd | Volume | cu yd | (l * w * (d / 12)) / 27 | 2
out tons | Estimated tons | tons | vol_ft * 0.05 | 2