Rip rap calculator
Design Reference · Erosion Control
Sizing Riprap
the Right Way
A field guide to D50, flow velocity, and wave height — the three numbers that decide whether your armor stone holds a bank or ends up downstream.
Riprap looks simple from the road — a slope of loose rock. But every stone on that slope is doing a specific job against a specific force, and undersizing even one class does not fail gracefully. It fails all at once, at the worst possible flow.
Why Riprap Fails
Most riprap failures aren't material failures — they're sizing failures. A stone that looks "big enough" on site can be well under the diameter the hydraulics actually demand. Once flow lifts the first few stones on a slope, the layer loses its interlock, and the failure propagates fast.
The fix isn't more rock. It's the right rock — sized to the velocity or wave action the slope will actually see, not to what fits the budget or what the last job used.
Meet D50
D50 is the median stone diameter — the size at which 50% of the rock by weight is larger and 50% is smaller. It's the single number that gates everything downstream of it: gradation class, layer thickness, tonnage, and cost.
Method One: Water Velocity
For channels, streams, and swales, D50 scales with the square of the design flow velocity. The FHWA HEC-23 approach (a common simplified form) looks like this:
where V = design velocity (ft/s), C = stability coefficient
The takeaway: doubling velocity roughly quadruples the required stone size. Underestimate the design flow by "just a little," and the sizing error is not little at all.
Method Two: Wave Height
Shoreline and lake revetments aren't sized off velocity — they're sized off wave action. Here D50 tracks maximum expected wave height, following USACE coastal engineering guidance. A shoreline that sees occasional 3-ft wind waves needs meaningfully larger armor than one sheltered to 1-ft chop, even at the same slope.
Getting the sizing mode right — velocity vs. wave height — matters as much as getting the number right. They are not interchangeable inputs.
From D50 to a Truckload Count
Once D50 is set, the rest is geometry and density:
| Step | What it needs |
|---|---|
| Layer volume | Length × Width × Thickness (1.5× D50) |
| Total weight | Volume × material density (limestone, granite, sandstone differ) |
| Cost estimate | Weight × unit cost per ton |
| Truckloads | Weight ÷ dump truck payload capacity |
Small changes in density assumptions move tonnage more than most people expect — granite and sandstone can differ by roughly 20% at the same volume.
Bringing It to Site
On paper, this is five inputs and two lookup tables. In practice, it's the difference between a slope that's still there in five years and a call-back after the first storm. Whichever tool you use to run these numbers, the discipline is the same: know your sizing mode, size to the design condition — not the average one — and don't round down on D50 to save a truckload.
Run These Numbers in Seconds
Riprap Estimator handles both sizing methods, layer volume, tonnage, and truckload counts — built to FHWA HEC-23 and USACE guidelines.
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