Rip rap looks like the simplest thing in civil works: put big rocks where the water is. It fails constantly, and almost always for the same two reasons — the stone was too small for the velocity, or there was no filter layer under it.
Velocity is the whole equation
The force water exerts on a stone rises roughly with the square of velocity. Double the flow speed and you don't need twice the rock — you need something closer to four times the weight to stay put.
This is why eyeballing it fails so reliably. A channel that looks gentle in summer can move at several feet per second in a storm, and the rock that sat there for three years vanishes in one afternoon.
Our rip rap calculator takes water velocity, flow turbulence, application type and rock specific gravity, and returns the size class you need.
Gradation matters as much as size
Rip rap isn't one size of rock. It's a graded mix, specified by D50 — the median diameter, where half the stone by weight is larger and half smaller.
| Class | Typical D50 | Typical use |
|---|---|---|
| Light / Class I | 6 in | Slow channels, shoreline toe |
| Class II | 9–12 in | Moderate flow, culvert outlets |
| Class III | 15–18 in | Fast flow, bridge abutments |
| Class IV+ | 24 in+ | High-energy spillways |
The range around D50 is deliberate. Smaller stones fill the voids between larger ones and interlock the mass. Uniformly sized rock has huge voids, doesn't lock together, and lets water accelerate through the gaps — which is exactly what you were trying to stop.
The filter layer is not optional
Here's the failure nobody sees coming, because it happens invisibly.
Water moves through rip rap, not just over it. If the rock sits directly on soil, that water washes the fine particles out from underneath. The rock doesn't move — the ground beneath it leaves. Then the stone settles into the void, the surface goes lumpy, and eventually the whole blanket slumps.
The fix is a filter layer between rock and soil, either:
Geotextile fabric — cheap, fast, and the usual modern choice. Overlap the seams generously and don't tear it dragging stone across.
Granular filter — a graded gravel layer. More work, more durable, better where the fabric might be punctured by heavy stone.
People skip this constantly because the rock looks fine on day one. It looks fine for a year or two. Then it doesn't.
Thickness and toe
Thickness: at least 2× D50, so 12-inch rock needs a 24-inch blanket minimum. A single layer of stone has nothing holding it — the blanket works because stones brace each other.
Toe: the bottom edge must be keyed in below the expected scour depth, usually in a trench. Rip rap fails from the toe upward far more often than from the top down. Undercut the bottom and the whole slope unravels like a pulled thread.
How much to order
Area × thickness gives volume, then convert with the rock's density — commonly around 1.5 to 1.7 tons per cubic yard, depending on stone type and how well it packs.
Add 10% for the toe trench and settling. Our rip rap calculator handles sizing, and the crushed stone calculator covers tonnage conversion for the filter gravel.
Building a wall instead? Our retaining wall calculator covers block counts and base.
Try the free calculator
Skip the manual math — get instant numbers for your own project:
Frequently Asked Questions
How do you size rip rap?
Primarily by water velocity, since force rises roughly with the square of speed. Turbulence at culvert outlets and bends requires larger stone than straight-channel flow at the same velocity.
What is D50 in rip rap?
The median stone diameter — half the rock by weight is larger and half smaller. Rip rap is a graded mix, not a single size, so the smaller stones lock the larger ones together.
Do you need fabric under rip rap?
Yes. Without a filter layer of geotextile or graded gravel, water washes fine soil out from beneath the rock and the whole blanket eventually slumps into the void.
How thick should a rip rap layer be?
At least twice the median stone diameter, so 12-inch rock needs a 24-inch blanket. The toe must also be keyed in below the expected scour depth.