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Drainage Rock Calculator: How Much Do You Need?

What Is Drainage Rock?

Drainage rock is washed, clean-crushed aggregate specifically prepared to allow water to move freely through it. The defining characteristic of true drainage rock is the absence of fines — no clay, silt, stone dust, or organic material. In the aggregate industry, “washed” means the stone went through a water-rinse screening process that strips away everything smaller than the minimum particle size, leaving only uniform chunks with large open voids between them. That void space (called porosity) is what gives drainage rock its hydraulic function: water enters at one point, flows downward and laterally through the gaps, and exits where the system directs it.

Common drainage rock products include #57 stone (3/4 inch nominal), #4 stone (1–1.5 inch), and pea gravel (3/8 inch). Each has the same fundamental property — clean, washed, no fines — but the right choice depends on your application. Smaller sizes fit snugly around small-diameter perforated pipe and are easier to place in confined trenches. Larger sizes work well in dry wells and retention pits where volume matters more than fine placement.

Drainage rock weighs approximately 103 pounds per cubic foot, or 1.39 tons per cubic yard. Because it is washed and uniform, the actual weight can vary slightly with moisture retained in surface pores after washing, but 1.39 tons per cubic yard is the industry standard for estimating.

How to Calculate How Much Drainage Rock You Need

For trench applications (French drains, leach fields, drain lines), calculate the volume of the trench itself rather than treating it as a surface coverage problem.

French drain formula: Length (ft) × Width (ft) × Depth (ft) = Cubic feet → ÷ 27 = Cubic yards → × 1.39 = Tons

Worked example — 80-ft French drain trench, 18 inches wide, 18 inches deep:

The actual rock needed is slightly less than the full trench volume because the pipe itself and the geotextile sock displace some space — but since you’re working with bulk material delivered by the ton, ordering the full trench volume is the right approach. Rock settles into any remaining gaps naturally.

Coverage Table

Depth Per cubic yard Per ton Typical application
2 inches 162 sq ft 117 sq ft Surface drainage layer
4 inches 81 sq ft 58 sq ft Window well, area drain
6 inches 54 sq ft 39 sq ft Shallow French drain
12 inches 27 sq ft 19 sq ft Deep trench / dry well
18 inches 18 sq ft 13 sq ft Deep drain or retention pit

For trench work, it’s often easier to think in terms of cubic feet per linear foot: a 12-inch wide × 12-inch deep trench uses exactly 1 cubic foot per linear foot. A 12-inch wide × 18-inch deep trench uses 1.5 cubic feet per linear foot.

How Much for Common Drainage Projects

French drain along a 100-ft fence line, 1 ft wide × 1 ft deep

This is a minimal residential French drain for a lawn that stays wet after rain:

Dry well, 4 ft diameter × 4 ft deep

A dry well collects overflow from a downspout or sump pump discharge:

Window well drainage bed, 3 ft × 2 ft × 2 ft

Window wells fill with water if they lack a drainage layer at the bottom:

Perimeter drain around a 30 × 40 ft foundation, 18 in wide × 18 in deep

Foundation perimeter drains protect basements from hydrostatic pressure:

Buying and Delivery Tips

The single most important specification when ordering drainage rock is the word “washed.” Ask explicitly for washed stone and confirm the product contains no fines. Some suppliers sell “crushed drainage stone” that is clean on the surface but still contains residual dust from cutting — this material clogs faster in buried systems. Request a spec sheet or ask to see a handful of the stone before ordering large quantities.

For French drain projects, you’ll also need perforated pipe (4-inch diameter is standard for residential work) and geotextile filter fabric to wrap around the rock and pipe. The fabric prevents surrounding soil from migrating into the drain over time. If you skip the fabric, even perfectly clean drainage rock will eventually clog with clay and silt that washes in from the walls of the trench.

Drainage rock is sold by the ton at quarries and landscape supply yards. Minimum bulk deliveries typically run 1–2 tons (smaller suppliers) or up to 5 tons at large quarries. For projects under 1.5 tons, buying bagged 3/4-inch washed stone in 50-lb bags often costs less than a truck delivery with a minimum tonnage requirement and separate delivery fee.

Timing your delivery matters: have the trenches fully excavated and the pipe laid before the stone arrives. A tandem truck delivering 7–10 tons of rock has no room to maneuver on a residential site, and re-loading excess stone that fell in the wrong place is hard work. Mark the trench edges clearly and talk to the driver about where you want the material dropped.

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Shape
1.72US tons
Cubic yards
1.23 yd³
Cubic feet
33.33 ft³
Weight
3,433 lb

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Frequently Asked Questions

What size rock is best for drainage?

3/4-inch washed crushed stone (#57) is the most widely recommended size for drainage applications including French drains, leach fields, and dry wells. Its uniform particle size creates large, interconnected void spaces that allow water to pass through quickly without clogging. For window wells and area drains, 1.5-inch clean stone also works well and is easier to shovel around pipes. Avoid any stone labeled 'dense-grade,' 'crusher run,' or '#610' for drainage — the fine particles in those blends block water movement.

How much does a ton of drainage rock cover?

At 4 inches deep, one ton of drainage rock (1.39 tons/yd³) covers approximately 58 square feet. At 6 inches — a typical French drain trench depth — one ton covers about 39 square feet. For trench applications where you're filling a long narrow channel, it's usually easier to calculate volume by trench dimensions than by area.

How much rock do I need for a French drain?

For a standard French drain trench 4 inches wide and 12 inches deep, you need about 1.23 cubic feet of rock per linear foot of trench. A 50-foot French drain at those dimensions requires roughly 61.5 cubic feet, which is 2.3 cubic yards or 3.2 tons. Wider or deeper trenches use proportionally more rock — a trench 12 inches wide and 18 inches deep needs about 1.5 cubic feet per linear foot.

How deep should drainage rock be in a trench?

French drain trenches are typically 12–24 inches deep, with drainage rock filling the trench from 2 inches above the pipe to within 2–3 inches of the surface. The minimum rock envelope around a perforated pipe is 2 inches on all sides. Deeper trenches (18–24 inches) are used in areas with high water tables or where the drain must pass under frost depth.

How many cubic yards of drainage rock do I need?

To calculate cubic yards needed, multiply trench length (ft) × width (ft) × depth (ft) and divide by 27. For a 100-ft trench that is 1.5 ft wide and 1.5 ft deep, that's 100 × 1.5 × 1.5 = 225 cubic feet ÷ 27 = 8.3 cubic yards. For area applications, multiply the square footage by the depth in feet and divide by 27.

What is the difference between drainage rock and river rock?

Drainage rock is crushed, angular stone that has been washed to remove all fines and clay particles. River rock is naturally rounded and smooth, formed by water erosion over thousands of years. Both are clean and allow water passage, but crushed drainage rock stays in place in trenches better than round river rock, which tends to shift. River rock is often chosen for decorative surface applications where appearance matters, while drainage rock is preferred inside buried systems where performance is the priority.

How much does drainage rock cost per ton?

Washed drainage rock typically costs $18–$45 per ton at the supplier, with washed products running slightly higher than unwashed crushed stone. Delivery adds $50–$150 per load for residential quantities. For small French drain jobs under 2 tons, buying bagged 3/4-inch stone at a home improvement store may be more economical than a minimum-load bulk delivery, despite the higher per-unit cost.

How many square feet does a cubic yard of drainage rock cover?

One cubic yard of drainage rock covers 324 square feet at 1 inch deep, 108 square feet at 3 inches deep, and 81 square feet at 4 inches deep. For trench applications, think in terms of linear feet: a trench 1 foot wide and 1 foot deep consumes exactly 1 cubic foot of rock per linear foot, or 27 linear feet of 1×1 trench per cubic yard.

Last updated: 2026-06-29