Drainage Gravel Calculator: Volume & Weight

This drainage gravel calculator works out the volume of gravel you need for a trench, French drain, or bed, and converts that volume to weight in tons and pounds. You enter the length, width, and depth of the area, plus the gravel density, and you get back the cubic yards, cubic feet, and estimated weight. It uses the standard formula for a rectangular prism, which fits most drainage projects.

Formula

The calculator uses the volume of a rectangular prism: Volume = Length × Width × Depth. All measurements must be in the same unit before multiplying. Since depth is entered in inches, it is first converted to feet by dividing by 12.

• L = length of the trench or bed (feet)
• W = width of the trench or bed (feet)
• D = depth of the gravel layer (inches, converted to feet by D ÷ 12)
• Ï� = density of the gravel (pounds per cubic foot, lb/ft³)

Volume (ft³) = L × W × (D ÷ 12)
Weight (lb) = Volume × �
Tons = Weight ÷ 2000 (US short ton)

Worked Example

Using the default values: length = 20 ft, width = 1 ft, depth = 12 in, density = 105 lb/ft³.

1. Convert depth to feet: 12 in ÷ 12 = 1 ft
2. Volume = 20 × 1 × 1 = 20 ft³
3. Convert to cubic yards: 20 ÷ 27 = 0.74 yd³ (rounded to 2 decimals)
4. Weight = 20 ft³ × 105 lb/ft³ = 2100 lb
5. Tons = 2100 ÷ 2000 = 1.05 tons

The calculator returns 20.0 ft³, 0.74 yd³, 2100 lb, and 1.05 tons for the defaults.

Why Gravel for Drainage?

Gravel is the standard fill for French drains, septic leach fields, and foundation footing drains because it creates large void spaces between stones. Those voids allow water to flow freely while preventing soil from clogging the pipe. The gravel also provides structural support, keeping the trench open and protecting the perforated pipe from crushing.

Not all gravel works equally well. Clean, angular crushed stone (typically ¾-inch to 1½-inch) is preferred because the sharp edges lock together and leave bigger gaps. Rounded pea gravel packs too tightly and can shift under load. The density you enter should match the specific gravel type—common values range from 95 lb/ft³ for loose pea gravel to 125 lb/ft³ for dense crushed limestone.

How to Measure Your Drainage Area

Accurate measurements matter more than precision in the formula. For a trench or French drain, measure the length along the centerline of the trench, not the top edge. The width is the average width of the trench—if it tapers, measure at the middle. Depth is the total depth from the surface to the bottom of the trench, but you only need the depth of the gravel layer, not the soil above it.

• For a French drain with pipe: the gravel depth is from the bottom of the trench to the top of the gravel cover (usually 1–2 inches above the pipe).
• For a gravel bed under a slab: measure the full thickness of the gravel layer.
• For a dry well or soakaway pit: treat it as a rectangular or cylindrical volume—if cylindrical, the formula changes (Ï€ × r² × depth).

If your trench is not a perfect rectangle (e.g., sloped sides), add 5–10% to the volume to account for the extra material.

Choosing the Right Gravel Density

Density is the variable most people get wrong. The calculator defaults to 105 lb/ft³, which is a reasonable average for clean crushed stone. But actual densities vary:

• Pea gravel (rounded, ⅜–½ inch): 95–100 lb/ft³
• Crushed stone (angular, ¾ inch): 100–110 lb/ft³
• Crushed limestone (dense): 110–125 lb/ft³
• Bank run gravel (with sand and fines): 115–130 lb/ft³

If you are ordering by the ton from a quarry, ask the supplier for the exact density they use. If you are buying by the cubic yard, the density only matters for estimating truck weight—you can skip it and just use the volume.

Common Mistakes and How to Avoid Them

The most frequent error is mixing units—entering depth in inches while length is in feet, or vice versa. This calculator converts depth automatically, but if you are doing it by hand, always convert everything to feet first. Another common mistake is forgetting that a cubic yard is 27 cubic feet, not 9.

People also underestimate the volume of a sloped trench. If your trench is deeper at one end (which is typical for a French drain to maintain slope), the depth you enter should be the average depth, not the deepest point. Using the deepest point overestimates by 10–20%.

Finally, remember that gravel compacts. The volume you calculate is the loose volume you need to order. If you are using a plate compactor, the final compacted volume will be about 5–10% less, so order slightly more than the calculated amount.

Limitations of This Calculator

This calculator assumes a straight rectangular trench with uniform width and depth. Real-world drainage projects often have curves, slopes, or irregular cross-sections. For a curved trench, measure the length along the curve and use that as the length—the formula still works because the cross-section is constant. For a sloped trench, use the average depth.

The density value is an estimate. Actual weight depends on moisture content and the specific rock type. Wet gravel can weigh 5–10% more than dry. If you are hauling it yourself, check your vehicle’s payload rating—a full cubic yard of gravel weighs about 1.4 tons (2800 lb), which exceeds the capacity of most pickup trucks.

Alternatives to Gravel for Drainage

Gravel is not the only option. For some applications, you might consider:

• Plastic drainage cells (geocomposite) – lightweight panels that create void space under patios or green roofs, but they are more expensive and not suitable for deep trenches.
• Perforated pipe with no gravel – in very sandy soil, you can wrap the pipe in filter fabric and skip the gravel, but this is uncommon and requires careful soil analysis.
• Recycled concrete aggregate – cheaper in some areas, but it can have higher fines and may not drain as well as clean crushed stone.
• Expanded clay pellets – lightweight and clean, but costly for large projects.

For most residential French drains, clean crushed stone remains the best balance of cost, performance, and availability.

Frequently Asked Questions

How much gravel do I need for a 50-foot French drain?

Assuming a standard 12-inch-wide trench and 12-inch-deep gravel layer, the volume is 50 × 1 × 1 = 50 cubic feet, or about 1.85 cubic yards. At 105 lb/ft³, that weighs about 5250 lb (2.6 tons). Adjust the width and depth to your actual trench dimensions.

What size gravel is best for drainage?

For most French drains and drainage trenches, ¾-inch to 1½-inch clean crushed stone is recommended. The angular shape creates larger voids than rounded gravel, and the size is large enough to resist being washed away but small enough to be handled easily. Avoid pea gravel for drainage—it packs too tightly.

Should I add 10% extra gravel for compaction?

Yes, it is a good practice to add 5–10% to your calculated volume. Gravel compacts when tamped or when water flows through it, especially if it contains fines. Ordering extra also covers measurement errors and irregular trench shapes. For a typical residential project, an extra 0.5 cubic yard is usually sufficient.

How do I convert cubic yards to tons?

Multiply the volume in cubic yards by the density in pounds per cubic foot, then divide by 2000. For example, 1 cubic yard = 27 ft³. At 105 lb/ft³, that is 27 × 105 = 2835 lb, or about 1.42 tons. Use the density from your supplier for a more accurate conversion.

Can I use this calculator for a circular dry well?

No, this calculator is for rectangular trenches only. For a circular dry well, use the formula for a cylinder: π × (radius)² × depth. For example, a 4-foot diameter well (radius 2 ft) that is 5 feet deep has volume π × 4 × 5 ≈ 62.8 ft³, or about 2.3 cubic yards.

What happens if I use too little gravel?

Insufficient gravel means the trench can’t hold the water that flows into it, leading to flooding, soggy soil, or a failed drain. The gravel also protects the pipe from soil pressure; too little gravel can cause the pipe to crush or clog. Always err on the side of slightly more gravel than less.

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in len | Length | ft | 20 | 0
in wid | Width | ft | 1 | 0
in dep | Depth | in | 12 | 0
in den | Gravel density | lb/ft³ | 105 | 0
out vol_ft | Volume | ft³ | len * wid * (dep / 12) | 1
out vol_yd | Volume | yd³ | (len * wid * (dep / 12)) / 27 | 2
out weight_lb | Weight | lb | (len * wid * (dep / 12)) * den | 0
out weight_t | Weight | tons | ((len * wid * (dep / 12)) * den) / 2000 | 2