Rip Rap Calculator

Rip rap is the layer of loose, heavy stone you see armoring a shoreline, a bridge abutment, a culvert outlet, or the face of a steep slope. Its whole job is to take the energy out of moving water before that energy can carve away the soil underneath. Sizing an order comes down to two things: how much area you are covering and how thick the stone layer needs to be.

The volume is straightforward. Multiply the length by the width to get the area in square feet, multiply by the layer thickness in feet, and you have cubic feet. Divide by 27 for cubic yards, the unit quarries sell by. Turning cubic yards into tons is where rock type matters, because riprap is full of voids between the stones and the void fraction depends on how the stone was placed. Large stone tipped straight off a truck runs 35 to 40% voids and lands around 1.3 to 1.45 tons per cubic yard. Machine-placed or well-graded material, where smaller stone fills the gaps, packs closer to 1.55 to 1.65. Quarries quote anywhere from about 1.25 to 1.65 depending on gradation, so pick the option that matches how yours will be placed and confirm the figure on the ticket before you order.

Two field rules keep a riprap job from failing. First, the layer should be at least 1.5 times as thick as the largest stone, and 12 to 18 inches is typical for moderate flows; thin layers shift and expose the bank. Second, put a filter fabric or a granular bedding layer underneath. Without it, water works the fine soil up through the gaps and the whole armor slowly sinks and unravels, a failure that looks fine for a season and then lets go.

Order about 10 percent extra. Stone settles into the voids, slopes are never as flat as the tape says, and coming up short on a half-finished bank is a bad day.