How this rip rap calculator works
Rip-rap is volume × density like any stone — the differences are that the layer is thick, the density slides with stone size, and everything arrives by the truckload:
tons = yd³ × density × (1 + allowance%) → rounded up to ¼ ton
Worked example: rip rap for a 30 × 6 bank strip
180 ft² armored 12 in thick is 180 ft³ — 6.67 yd³. In 3–6 in machine rip-rap at 1.35 t/yd³ that's 9 tons; with 10% allowance the order is 10 tons, about $550. One ton covers ≈ 20 ft² at this layer.
How much does a ton of rip rap cover?
| 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″+ |
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.
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.
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