What Is a Rip Rap Calculator?
Rip rap is a layer of loose, angular rock used to armor shorelines, stream banks, culvert outlets, and slopes against erosion. This calculator estimates how much rip rap you need — in both cubic yards and tons — based on the area you want to cover, the depth of the stone layer, and the density of the rock.
How to Use It
Enter the length and width of the area in feet, the desired depth of the rip rap layer in inches, and the density of the stone in tons per cubic yard. Most quarried rip rap weighs around 1.65 tons per cubic yard, but it can range from about 1.3 to 1.8 depending on rock type and gradation — check with your supplier for an exact figure.
The Formula Explained
First the calculator finds the coverage area (length × width) and converts the depth from inches to feet (depth ÷ 12). Multiplying area by depth gives the volume in cubic feet, which is divided by 27 to convert to cubic yards. Finally, cubic yards are multiplied by the stone density to get the weight in tons:
$$\text{Tons} = \left(\text{Area} \times \text{Depth}_{ft} \div 27\right) \times \text{Density}$$
Worked Example
Suppose you want to cover an area 20 ft long by 10 ft wide with a 12-inch (1 ft) layer of rip rap at 1.65 tons/yd³. Area = 200 ft². Volume = \(200 \times 1 \div 27 = 7.407\) yd³. Weight = \(7.407 \times 1.65 \approx 12.22\) tons. Order a little extra to account for settling and uneven subgrade.
FAQ
How deep should rip rap be? A common rule is at least twice the maximum stone diameter, often 12–24 inches for moderate flow conditions.
Why convert to tons? Stone is usually sold and delivered by weight, so tons is the practical ordering unit.
Should I add extra? Yes — adding 5–10% for waste, voids, and settling is recommended.
