How to calculate rip rap
Example: a 50 ft shoreline, 8 ft up the slope, with an 18-inch layer is 400 × 18 ÷ 12 = 600 cubic feet = 22.2 cubic yards — about 29.5 tons, plus 10% extra.
How thick should the layer be?
A common rule of thumb is a layer at least 1.5–2 times as thick as the typical stone size, so stones interlock without gaps. For 6–12 inch stone that usually means 12–24 inches.
| Use | Typical stone | Typical thickness |
|---|---|---|
| Ditch lining, small drainage | 4–8 in | 12 in |
| Culvert outlets, slopes | 6–12 in | 18 in |
| Pond and lake shorelines | 12–24 in | 24–36 in |
Don't skip the fabric
Place geotextile fabric (and sometimes a bedding layer of smaller stone) under rip rap so soil isn't washed out from beneath the rocks.
Frequently asked questions
How much does a cubic yard of rip rap weigh?
About 1.3–1.4 tons (2,600–2,800 lb) because of the gaps between stones.
How many square feet does a ton of rip rap cover?
About 14 sq ft at 18 inches thick, or 20 sq ft at 12 inches.
Do I need a permit to place rip rap?
Often yes along lakes, rivers and wetlands — check with your local or state environmental agency first.
What size rip rap do I need?
It depends on water flow and wave action. Small ditches use 4–8 inch stone; shorelines often need 12 inches or larger.
Related calculators
These figures are estimates for planning only. Material weights and coverage vary by supplier, moisture and compaction. Always confirm quantities and prices with your supplier before ordering. See our terms & disclaimer.