How this rip rap calculator works
Rip-rap is armor stone sold by the ton, and the estimate is the same volume × density as any aggregate — the difference is the layer is thick and the stone is big:
tons = yd³ × density × (1 + waste%) → rounded up to ¼ ton
At the defaults — a 30 × 6 ft bank strip, a 12 in layer of R-3, 10% allowance — the order is 10 tons, about $550; the full chain is in the load math section below.
Rip-rap size classes and where they go
Rip-rap is sorted crushed or quarried stone in size classes, and the class is chosen by how hard the water hits — bigger stone for faster flow and steeper banks:
- R-3 machine rip-rap (3–6 in) — 1.35 t/yd³: gentle slopes, slow flow, splash aprons. A 12 in layer. The default.
- R-4/R-5 (6–12 in) — 1.3: moderate current, pond and lake edges, culvert outfalls. An 18–24 in layer.
- Large rip-rap (12–24 in) — 1.25: spillways, fast channels and wave zones. A 24 in+ layer; big stone stacks with more void, so it weighs less per yard.
- Bedding stone under it: a graded filter layer or geotextile between soil and armor — the layer that stops the whole thing sinking.
How thick should a rip-rap layer be?
About twice the median stone size — enough for stones to interlock and cover the bank with no gaps. Thinner and the slope shows through and undermines:
- R-3 (3–6 in): 12 in layer.
- R-4/R-5 (6–12 in): 18–24 in layer.
- Large (12–24 in): 24 in and up.
- Toe deeper: thicken the layer at the bottom of the slope, where scour is worst.
- Always over a filter: geotextile or a graded bedding layer — never armor straight on soil.
How much area does a ton of rip rap cover?
Table 1 is coverage per ton of R-3 by layer thickness; Table 2 maps size class to the layer it wants.
| Layer | 1 ton covers |
|---|---|
| 9″ | 27 ft² |
| 12″ | 20 ft² |
| 18″ | 13 ft² |
| Class | Stone | Layer |
|---|---|---|
| R-3 | 3–6″ | 12″ |
| R-4/R-5 | 6–12″ | 18–24″ |
| Large | 12–24″ | 24″+ |
How to measure a bank for rip-rap
- Measure the slope area — the length of bank times the slope face, not the horizontal, since armor covers the incline.
- Set the layer thickness to about twice the stone size — 12 in for R-3, more for bigger classes.
- Pick the size class by the water speed and slope; the density follows.
- Leave the allowance at 10% for the irregular stacking of big stone, more on a rough bank.
- Enter the per-ton price, delivered — and remember armor ships and places by machine, so the haul and the excavator are real lines.
Worked example: rip rap for a 30 × 6 bank strip
The default, worked line by line — a 30 × 6 ft slope face, 12 in layer of R-3 at 1.35 t/yd³, 10% allowance, $55 a ton:
volume = 180 × (12 ÷ 12) = 180 ft³ = 6.67 yd³
weight = 6.67 × 1.35 = 9 tons exact
allowance = 9 × 1.10 = 9.9 → order 10 tons
cost = 10 × $55 = $550 · 1 ton covers ≈ 20 ft² at a 12″ layer
Armor stone disappears fast: 10 tons — half a tandem load — for a bank strip barely bigger than a parking space, because a foot-thick layer of dense stone adds up quick. Which is why the fabric underneath matters so much. Skip it and that 10 tons works its way into the bank and vanishes in a few storm seasons, and you buy it twice.
What this rip-rap estimate doesn’t include
On a rip-rap job the stone is often the smaller half of the bill — the excavator that places it and the fabric under it decide whether the armor lasts. Everything below sits outside the tonnage and belongs on a full estimate:
- Geotextile / filter fabric. Non-woven fabric or a graded bedding layer under the armor — non-negotiable.
- Bedding stone. A graded filter course where fabric alone won’t do.
- Toe and key trenches. The excavated anchor at the base and edges of the armor.
- Machine placement. Rip-rap is placed by excavator — the other half of the job cost.
- Delivery. Truckload haul of heavy, sorted stone.
- Engineering and permits. Channels, outfalls and waterway work are designed and permitted — this is a planning estimate, not a spec.
Rip rap FAQ
How much rip rap do I need?
Slope area × layer thickness ÷ 12 ÷ 27 × the stone’s density: the default 30 × 6 ft bank strip at a 12 in layer is 6.67 yd³ — 9 tons of R-3 rip-rap, ordered as 10 tons with 10% allowance, about $550 at $55 a ton.
How thick should a rip rap layer be?
Twice the median stone size is the working rule: a 12 in layer for 3–6 in machine rip-rap, 18–24 in for the big 12 in class. Thinner layers leave bank showing between stones and wash out from underneath.
What size rip rap do I need?
Fast water and steep banks take bigger stone. R-3 (3–6 in) armors gentle slopes and slow flow; R-4/R-5 (6–12 in) handles moderate current and pond edges; the 12–24 in class is for spillways, fast channels and wave zones. On an engineered outfall the size is specified by the designer — this is a planning tool, not a substitute for the spec.
How many tons of rip rap are in a cubic yard?
About 1.35 for 3–6 in machine rip-rap, sliding to 1.25 for the largest classes — big stone stacks with more void space, so a yard of it weighs less. The size picker carries the right figure.
How much area does a ton of rip rap cover?
About 20 ft² at a 12 in layer in R-3 — 27 ft² at 9 in, 13 ft² at 18 in. Armor stone goes fast: a modest bank eats tons in a hurry, which is why it ships by the truckload.
Do I need fabric under rip rap?
Yes — non-woven geotextile between soil and stone. It stops the bank from pumping up through the voids and the stone from sinking in, which is the usual reason a rip-rapped slope fails early.
How much does rip rap cost?
Commonly $40–80 per ton in 2026 depending on size class and haul — large, sorted stone costs more to make and move. The default bank strip runs about $550 in stone; placement by machine is the other half of the job.
Sources and standards
Rip-rap sizing sits between landscaping and engineering, so the figures here lean on the agencies that publish armor-stone specs rather than any single product standard:
- State DOT rip-rap classes (R-3, R-4/R-5): the graded armor-stone size classes and their layer thicknesses.
- NRCS / USACE streambank guidance: the twice-the-stone-size layer rule and the filter-fabric requirement.
- Standard estimating densities: 1.25–1.35 t/yd³ by class — larger stone stacks lighter.
- The engineer of record: channels and outfalls are designed, not guessed — this is a planning tool, and the EOR’s spec governs.
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