Enter the size of the bank, ditch or shoreline and pick a stone class. The calculator sets the layer thickness from the stone size and gives the tons and cubic yards of rip rap to order, plus the filter fabric underneath.
How to use the rip rap calculator
- For a flat area or a ditch lining, enter the length and the width. On a slope, measure the width down the face.
- For a sloped bank, choose Sloped bank and enter its length, its height and how far it runs horizontally. The calculator works out the face for you.
- Pick the stone class from your plans, or enter your own layer thickness.
- Keep 10% extra for the toe trench and uneven ground, and enter the quarry’s price per ton for a cost.
Rip rap formulas
Slope face (ft) = √(rise² + run²) Area (ft²) = length × width, or bank length × slope face Layer thickness = 1.5 × largest stone, at least 6 in Cubic yards = area × thickness (ft) ÷ 27 Tons = cubic yards × 1.5
North Carolina’s erosion control standard sets the minimum rip rap thickness at 1.5 times the largest stone and never less than 6 inches, so the biggest rocks are fully bedded rather than sitting proud of the layer. Tons use 3,000 lb (1.5 tons) per cubic yard of placed stone, the same 1.5 multiplier Angi’s rip rap calculator uses. The voids between angular stones are part of that figure, which is why it is lower than solid rock.
Worked example: a 100 ft bank, 4 ft high, 2:1 slope, Class B
- Slope face: √(4² + 8²) = 8.94 ft.
- Area: 100 × 8.94 = 894 ft².
- Thickness: Class B stones are up to 12 in, so 1.5 × 12 = 18 in (1.5 ft).
- Volume: 894 × 1.5 = 1,342 ft³, or 1,476 ft³ with 10% extra: 54.66 yd³.
- Tons: 54.66 × 1.5 = 82 tons.
The filter fabric under it covers the same 99 yd², plus 12 in at every overlap and the part buried at the top. The calculator’s starting example, a 40 × 8 ft Class A ditch lining 9 in thick, needs 14.7 tons with 10% extra.
Rip rap classes and thickness
Stone sizes for the four NCDOT classes, the layer each one needs and the rip rap per 100 ft² of face, before any extra:
| Class | Stone size | Layer thickness | Tons per 100 ft² |
|---|---|---|---|
| Class A | 2–6 in | 9 in | 4.17 tons |
| Class B | 5–12 in | 18 in | 8.33 tons |
| Class 1 | 5–17 in | 25.5 in | 11.81 tons |
| Class 2 | 9–23 in | 34.5 in | 15.97 tons |
Class names differ from state to state. Some suppliers sell Class A to D by diameter instead; use the largest stone in the gradation you are buying and enter 1.5 times that as your own thickness.
Which size of rip rap do I need?
Faster water and bigger waves move bigger stones. Angi’s guide matches the smallest stone (2–6 in) to small drainage ditches, mid sizes to moderate and steeper slopes and small channels, and 12–18 in stone to heavy-duty shoreline protection. For anything near a stream, lake or road, the size is usually set by a permit or an engineer’s design, so use that.
Filter fabric and bedding
Rip rap needs a filter under it or the soil washes out between the stones. NC DEQ asks for geotextile edges to overlap by at least 12 in with anchors every 3 ft, the upstream end buried at least 12 in, and a 4 in layer of fine gravel or sand on the fabric where big stones are dropped on it. A gravel filter instead of fabric goes down in layers at least 6 in thick.
Tips for ordering rip rap
- Buy by the ton. Rip rap is weighed on the quarry scale. If they quote a different weight per yard, enter it in the calculator.
- Key in the toe. The bottom of a slope is where rip rap fails first. A trench at the toe filled with stone uses extra rock, so keep the 10%.
- Ask for angular stone. Broken, angular stone locks together; rounded river stone rolls downhill.
- Plan access. A loaded dump truck is heavy and the stone is placed by excavator on big jobs. Keep trucks back from the top of the bank.
Questions
How much does a cubic yard of rip rap weigh?
About 1.5 tons (3,000 lb) once placed, including the voids between stones. Solid stone weighs more, so a quarry’s figure may differ.
How many square feet does a ton of rip rap cover?
About 24 ft² for a 9 in Class A layer and about 12 ft² for an 18 in Class B layer.
How thick should rip rap be?
At least 1.5 times the largest stone and never less than 6 in. For Class B stone up to 12 in, that is 18 in.
Do I need fabric under rip rap?
Yes, a geotextile or a graded gravel filter. Without one, water pulls soil out from between the stones and the slope slumps.
Sources
- Practice Standards and Specifications 6.15: Riprap (2025 draft), North Carolina Department of Environmental Quality.
Used for: NCDOT stone classes, layer thickness of 1.5 × the largest stone (6 in minimum), geotextile overlaps and filter layers. - Rip Rap Calculator: How Much Rip Rap Do I Need?, Angi, updated Jul 8, 2026.
Used for: 1.5 tons per cubic yard for rip rap; stone sizes matched to ditches, slopes and shorelines. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch and foot definitions; cubic foot and cubic yard conversions.