Rip Rap Calculator
Tons of rip rap for shorelines, ditches and slopes by area and layer thickness, with Class A, B and C densities.
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On this page
Rip rap (riprap, rock armour, shot rock) is large angular stone placed to protect shorelines, pond banks, culvert outlets, ditches and slopes from erosion. Because it is sold by the ton and placed in thick layers, small measuring mistakes become expensive. This rip rap calculator converts area × layer thickness into tons, using placed densities for each rock class, and shows truckloads.
How to use the rip rap calculator
- Measure the area – along the slope
For a bank or slope, measure the slope length (from toe to top along the surface), not the horizontal distance, and multiply by the length of shoreline. A 2:1 slope is about 12% longer than its horizontal run; a 1.5:1 slope about 20% longer.
- Enter the layer thickness
Rip rap thickness is normally 1.5 times the median stone size (D50) and at least as thick as the largest stone. Typical: 12″ for small Class A rock, 18″ for Class B (6–12″), 24–30″ for Class C (12–24″).
- Choose the rock class
Smaller rock packs tighter and weighs slightly more per cubic yard; large rock has bigger voids. Pick the class your engineer or supplier specifies. “Bedding stone” is for the filter layer underneath.
- Keep a 10% allowance
Rip rap is dumped and placed roughly, so edges and overlaps use more than the theoretical volume. For hand-placed or machine-keyed rock you can reduce it to 5%.
- Add bedding and filter fabric
Most specifications call for 4–6″ of bedding stone (or a geotextile filter) under the rip rap. Run the calculator again with “Bedding stone” and the bedding thickness and add it to your order.
Rip rap formula
Slope length (ft) = √(rise² + run²) · Area = slope length × shoreline lengthCubic yards = Area (sq ft) × thickness (in) ÷ 324Tons = cubic yards × 1.5 (placed rip rap, typical) · add 10%
Example: pond bank 80 ft long, 6 ft rise over 12 ft run (2:1), 18″ of Class B rip rap
- Slope length = √(6² + 12²) = 13.4 ft
- Area = 13.4 × 80 = 1,073 sq ft
- Volume = 1,073 × 18 ÷ 324 = 59.6 yd³
- Tons = 59.6 × 1.49 (Class B) = 88.8 → +10% = about 98 tons – roughly 4–5 tri-axle loads
Rip rap sizes, classes and layer thickness
Classifications vary by state DOT and by country, but the common pattern is below. Always use the spec your engineer gives – these are typical values for estimating.
| Class / name | Stone size | D50 (median) | Typical layer | Tons / yd³ (placed) | Typical use |
|---|---|---|---|---|---|
| Class A / Type 1 / “small” | 3″ – 6″ | ~4″ | 12″ | 1.55 | Ditch lining, small swales, culvert aprons |
| Class B / Type 2 / “medium” | 6″ – 12″ | ~9″ | 18″ | 1.49 | Pond banks, stream banks, moderate flow |
| Class C / Type 3 / “large” | 12″ – 24″ | ~15″ | 24–30″ | 1.42 | Lake shorelines, high-velocity channels, outfalls |
| Class D / heavy | 24″ – 36″+ | ~24″ | 36″+ | 1.40 | Coastal, large rivers, dam faces |
| Gabion stone | 4″ – 8″ | ~6″ | Basket depth | 1.42 | Filling gabion baskets and mattresses |
| Bedding / filter stone | 3/4″ – 3″ | – | 4–6″ | 1.42 | Filter layer under rip rap |
Rip rap coverage per ton
| Material | Density | Tons per yd³ | sq ft per ton at 12" | sq ft per ton at 18" | sq ft per ton at 24" | sq ft per ton at 30" |
|---|---|---|---|---|---|---|
| Rip rap, placed (typical) | 110 lb/ft³ | 1.49 | 18 | 12 | 9 | 7 |
| Small rip rap, 3"–6" (Class A/1) | 115 lb/ft³ | 1.55 | 17 | 12 | 9 | 7 |
| Medium rip rap, 6"–12" (Class B/2) | 110 lb/ft³ | 1.49 | 18 | 12 | 9 | 7 |
| Large rip rap, 12"–24" (Class C/3) | 105 lb/ft³ | 1.42 | 19 | 13 | 10 | 8 |
| Gabion stone, 4"–8" | 105 lb/ft³ | 1.42 | 19 | 13 | 10 | 8 |
| Bedding stone / filter layer | 105 lb/ft³ | 1.42 | 19 | 13 | 10 | 8 |
Coverage = 2,000 lb ÷ (density × depth in feet). Figures are for loose material; add 5–10% for waste and up to 20% for compacted base layers.
Slope length cheat sheet
Because rip rap goes on slopes, the surface area is always bigger than the footprint. Multiply the horizontal run by the factor below to get the slope length:
| Slope (H:V) | Angle | Multiply horizontal run by |
|---|---|---|
| 4:1 | 14° | 1.03 |
| 3:1 | 18° | 1.05 |
| 2:1 | 27° | 1.12 |
| 1.5:1 | 34° | 1.20 |
| 1:1 | 45° | 1.41 |
Slopes steeper than 1.5:1 usually need the rock keyed into a toe trench at the bottom – add the trench volume (typically 2 ft deep × 3 ft wide along the full length) to your estimate.
How much does rip rap cost?
Rip rap is priced per ton and varies widely with size and distance from the quarry. As a guide, expect $35–$100 per ton for Class A–C rock at the quarry, with large and heavy classes at the top of the range; delivery adds a per-load fee and big rock needs a dump truck with a reinforced bed. Installation by an excavator is often as much as the stone itself. Because a single project can run to 50–200 tons, getting the quantity right before you order matters more here than with any other material on this site.
Frequently asked questions
How many tons of rip rap are in a cubic yard?
Placed rip rap weighs about 1.4–1.55 tons per cubic yard depending on size – smaller rock packs tighter and weighs more per yard. Solid rock is about 2.2 tons per cubic yard, but placed rip rap is 30–40% voids.
How much rip rap do I need per linear foot of shoreline?
Multiply the slope length by the thickness: for a 10 ft slope with 18″ of rock, each linear foot needs 10 × 1.5 = 15 cubic feet = 0.56 yd³ ≈ 0.83 tons per foot. 100 ft of shoreline would need about 83 tons plus allowance.
How thick should rip rap be?
Use 1.5 × the median stone size (D50), and never less than the largest stone. In practice: 12″ for 3–6″ rock, 18″ for 6–12″ rock, 24–30″ for 12–24″ rock. Thickness is measured perpendicular to the slope.
Do I need fabric or bedding under rip rap?
Yes, almost always. Without a filter layer, soil washes out through the gaps between rocks and the bank fails behind the rip rap. Use a non-woven geotextile, 4–6″ of bedding stone, or both.
How many square feet does a ton of rip rap cover?
At 12″ thick, a ton of Class A rip rap covers about 17 sq ft; at 18″ about 12 sq ft; at 24″ about 9 sq ft. The coverage table above lists each class.
Can I use rip rap for a driveway or landscaping?
Small rip rap (3–6″) works well as a decorative border, around downspouts and in dry creek beds, and as a sub-base over very soft ground. It is too large to drive or walk on comfortably – use crushed stone or gravel for surfaces. See the landscape rock calculator for decorative uses.
