How Much Riprap Do I Need?

The short answer

For homeowner-scale riprap — dry creek beds, downspout outlets, pond edges, and small slope toes — on a typical 2:1 slope at 12 inches thick, order about 0.19 tons per linear foot per foot of bank height. The signature example: 20 linear feet of 2-foot bank = 20 × 2 × 0.19 ≈ 7.6 tons — order 8 to 8.5 tons. Riprap is sold by the ton, not the yard, because the stone has huge voids between pieces — your quarry's scale figure is always the number to trust.

Scope and a shoreline warning

This guide covers dry-land homeowner projects only: dry creek beds, downspout outlet aprons, pond and pool edges, and the toes of small landscape slopes. If your project is a shoreline, lakefront, riverbank, or tidal edge, stop here: real shoreline armoring almost always requires permits (dredge-and-fill, wetlands, and shoreline-management rules apply at multiple levels of government) and a licensed contractor with engineered stone sizing, filter design, and heavy equipment. DIY shoreline work risks fines and a failed installation that takes the bank with it. Nothing below is sized for wave or current energy.

Riprap size classes

Riprap is graded by weight class, but quarries sell it by approximate stone diameter. The three classes a homeowner will ever see:

A quarry's “R-4” won't match another's exactly — these are nominal classes, not tolerances. Always ask the supplier for the actual gradation (the mix of sizes in the load) and confirm it's angular crushed stone, not rounded river rock: angular faces interlock and resist sliding; round stone rolls.

The math: why tons per linear foot

Riprap layers sit on a slope, so you measure the slope surface, not the horizontal run. On the standard 2:1 slope (2 feet horizontal for every 1 foot of height), the slope length per 1 foot of height is the hypotenuse: √(1² + 2²) = √5 ≈ 2.24 feet.

Volume per linear foot per foot of bank height at 12-inch thickness: 2.24 ft (slope length) × 1 ft (height) × 1 ft (thickness) = 2.24 cubic feet. Rock weighs 4,500 pounds per cubic yard — that's 4,500 ÷ 27 ≈ 167 pounds per cubic foot. So 2.24 × 167 ≈ 374 pounds ≈ 0.19 tons per linear foot per foot of height at 12 inches thick.

Two caveats before you use this: the number is approximate — riprap has enormous voids between stones, so a cubic-foot calc overstates the stone and understates the air. And your supplier's tonnage is the safer figure: quarries calibrate their loads by scale weight, and they've seen what a given slope eats. Our cubic yards to tons conversion guide covers the density math in full if you want the background.

Tons per linear foot (2:1 slope)

Read your bank height down the left, thickness across the top — multiply by your linear footage. The 18-inch column is the 12-inch column × 1.5, as it should be:

Worked example and common projects

The worked example: 20 linear feet of 2-foot bank at 12 inches thick = 20 × 2 × 0.19 = 7.6 tons. Order 8 to 8.5 tons — the half-ton up covers breakage, irregular bank lines, and the stones that roll where you don't want them. Applying the same per-foot figures to typical homeowner projects:

*A flat downspout apron isn't a slope — use area × thickness instead: 6 × 4 × (8 ÷ 12) ÷ 27 = 0.59 cubic yards of layer volume. Large angular stone sits with roughly 35% voids, so the solid stone you pay for is about 0.59 × 2.25 tons per yard (rock at 4,500 lb/yd³) × 0.65 ≈ 0.86 tons — order 1 ton. Size the apron to catch the discharge: 4–6 feet long and a foot wider than the downspout's spread on each side. For the full creek-bed layout this riprap pairs with, see our dry creek bed rock guide — and if you're armoring a drain outlet that ties into a buried line, our French drain gravel guide covers the pipe and stone behind it.

Non-woven geotextile under everything

Every riprap installation gets non-woven geotextile laid directly on the prepared soil before the first stone goes down. Riprap fails from the inside out: water flows through the big voids, picks up soil fines, and washes the bank out from under the stone — a failure mode called piping. The fabric is the filter that stops it. Use non-woven (felted) geotextile, not the woven stuff: non-woven passes water while holding soil, and it doesn't tear when heavy angular stone drops on it. Overlap seams by 12 inches, pin it, and run it under the full riprap footprint plus a foot beyond on every side. Skip this step and the installation can look perfect on day one and start settling by the second rainy season.

Grading and install notes

Grade to a 2:1 slope or flatter before any stone arrives — steeper than 2:1 and the stone won't stay put without engineering. Toe it in: dig the bottom of the slope down and bury the first row of stone below finished grade — an exposed toe is where scour starts, and buried toes don't slide. Key the top: finish the upper edge by burying the last course into the bank or a shallow trench so runoff can't get behind the layer.

Place, don't dump. Set the largest stones at the toe and work upward — the toe carries the load and the big pieces there resist the most force. Dumping a load from the top of the bank segregates the sizes (big stones roll to the bottom, fines stay on top) and gives you exactly the wrong gradation. With R-4 you can hand-place; R-6 and up realistically want a skid steer or excavator with a thumb. Keep the thickness honest: probe the layer as you go — minimum twice the largest stone's diameter, so 12 inches for R-4 and 18 inches where the water has real energy.

For a custom run, our landscape rock calculator handles the rectangular-area version of this math (aprons, pads, flat beds) — use the slope table above for anything on an incline.

Common mistakes

Rounded stone. River rock looks great and rolls downhill — riprap must be angular so the pieces interlock. No fabric. The #1 cause of slow failure: soil pipes out through the voids within a couple of seasons. One stone deep. A single-stone veneer leaves gaps water exploits — build to twice the largest stone's diameter. Dumping instead of placing. Segregates sizes and destroys the gradation the quarry sold you. Ordering by yards. Voids make yard math unreliable for riprap; order and pay by the ton, and round up half a ton on irregular banks. DIY-ing a shoreline. Permits, wave energy, and liability — real shoreline work is a contractor job.