Tons of rip rap for a bank, shoreline or culvert — from the area to armour and the layer thickness.
Rip Rap Calculator
Inputs
Measure DOWN the slope face, not the horizontal distance.
Rule of thumb: twice the median stone size, and never less than the largest stone.
Results
Rip rap needed
20.00 tons
tons = area × thickness × 100 lb/ft³ placed ÷ 2,000 (rock at ~60% packing)
Calculated with BuildCalculators.com
Calculated with BuildCalculators.com · buildcalculators.com/rip-rap-calculator
Estimates are for planning only — confirm quantities with your supplier or contractor before ordering.
Rip rap is erosion armour: large angular stone placed over a bank, shoreline or outfall so that moving water spends its energy on rock instead of soil. The estimate is area × layer thickness at PLACED density — about 100 lb per cubic foot, which is solid rock at roughly 60% packing, since a rip rap layer is deliberately full of voids. Two rules carry the sizing: the layer should be about twice the median stone size, and the width you measure is down the slope face, not the horizontal distance a plan view shows.
A = length along bank × slope widthV = A × (thickness ÷ 12)tons = ft³ × 100 lb ÷ 2,0001 ton ≈ 20 ft² at a 12 in layerGranite and limestone run about 165 lb per cubic foot solid, but a placed rip rap layer is one-third air — the voids between stones are where the water’s energy dies. 100 lb per cubic foot placed is the working figure suppliers and DOT tables use, and it is why a ton of rip rap covers far less area than a ton of gravel.
The stone has to be heavy enough that the design flow cannot roll it. Ditches and gentle swales manage with 3–6 inch stone; ordinary creek banks and pond shores want 6–12 inch; strong current, wave fetch or outfall turbulence pushes into 12–24 inch class rock. Engineers size it from velocity and shear formulas — for anything protecting a structure or upstream of a neighbour, that is a calculation worth paying for.
Twice the median stone size, and never thinner than the largest stone — a 6–12 inch stone mix wants a 12–18 inch blanket. Thinner than the biggest rock means single-stone coverage with soil showing through the gaps, which is exactly where the bank starts unravelling.
Water does not stop at the rock — it moves through the voids and works on the soil below. Geotextile filter fabric (or a 4–6 inch gravel bedding layer) lets water pass while holding soil in place. Rip rap on bare earth looks identical on day one and fails from underneath by year two.
Armouring an eroding bank with 6–12 in stone at a 12 in layer.
20 tons of rip rap
Just over one truckload. Stone goes in from the top of the bank with an excavator where access allows — 20 tons is a brutal hand-placement job.
Stopping the scour hole where a drive culvert discharges.
About 5 tons
Aprons want the stone keyed slightly below grade at the downstream edge, or the scour simply relocates to the end of your rock.
Wave protection with larger 8–15 in stone, thicker layer.
45 tons — three truckloads
Run the stone from a foot above the high-water line to two or three feet below the low line. Waves attack the zone the water level crosses, not one fixed height.
At 100 lb/ft³ placed density.
| Layer thickness | Coverage per ton |
|---|---|
| 6 in | ≈ 40 ft² |
| 9 in | ≈ 27 ft² |
| 12 in | ≈ 20 ft² |
| 18 in | ≈ 13 ft² |
| 24 in | ≈ 10 ft² |
Common retail classes; engineered jobs size from flow data.
| Class | Stone size | Typical use | Layer |
|---|---|---|---|
| Small | 3–6 in | Ditches, swales, downspout outfalls | 6–9 in |
| Medium | 6–12 in | Creek banks, pond shores | 12–18 in |
| Large | 12–24 in | Fast current, wave fetch, outfalls | 24 in + |
Rip rap runs $40–$90 per ton delivered in 2026, rising with stone size — big rock is harder to quarry, load and haul. Machine placement typically adds $30–$60 a ton on top of material, and on most banks placement is not optional.
Small 3–6 inch rip rap sits at the bottom of the range; hand-pickable and cheap to handle. Class rock above 12 inches costs more per ton and needs an excavator with a thumb to place properly. The material line in a quote is often smaller than the machine line — normal for this work.
Work at the waterline is regulated: many US states require a permit for armouring a natural creek or lake shore, and the Corps of Engineers takes an interest in navigable waters. A phone call before ordering beats undoing 45 tons of stone after a notice.
For gentle slopes and low flows, vegetation with erosion-control matting costs a fraction and looks like a bank rather than a quarry. Gabion baskets — wire cages of smaller stone — suit tight or vertical spots and use cheaper rock — priced on the crushed stone calculator. Rip rap wins on durability and zero maintenance where the water genuinely works.
A bank 6 ft high at a 2:1 slope has a face over 13 ft wide — nearly double the horizontal distance an aerial view shows. Measure down the face with a tape, or the order comes up almost half short.
The classic rip rap failure: stone on bare soil, water flowing through the voids, bank washing out from underneath while the rock armour sits proudly on a growing hollow. Fabric or gravel bedding first, always.
If the design flow can move the stone, the armour migrates downstream one storm at a time. When in doubt between two classes, take the bigger — the cost difference is small against doing the job twice.
Stretching tonnage by spreading thin leaves soil visible between stones — each gap a starter hole for erosion. Hold the thickness rule and shrink the area covered this season instead; a properly armoured half is worth more than a token whole.
At a 12 inch layer, about 20 square feet; at 18 inches, roughly 13. Smaller stone laid thinner stretches further — 3–6 inch rock at a 6 inch layer covers about 40 ft² per ton.
Match the water: 3–6 inch stone for ditches and gentle swales, 6–12 inch for most creek banks and pond shores, 12–24 inch where current or wave action is strong. Bigger, faster water needs bigger stone — sized properly, from local flow data, for anything structural.
The working rule is twice the median stone size, and never thinner than the largest stone in the mix. For common 6–12 inch bank stone that means a 12–18 inch layer, laid over filter fabric.
Yes — geotextile filter fabric or a 4–6 inch gravel bedding layer. Without it, water pulls the soil out through the gaps between stones and the bank fails underneath while the rock stays put.
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Embed the Rip Rap Calculator anywhere — it is free, stays in sync with this page, and sizes itself to fit. The only condition is the credit line in the snippet, which links your readers to the full version with the formulas and worked examples.
This calculator uses published constants from named industry sources, each asserted in an automated test that runs on every build. The formula is printed with the result above, and any correction we make is logged on the changelog.
It is a planning estimate. It does not replace an engineer, a site visit or a written quote — confirm quantities with your supplier before ordering.