About the Rip Rap Calculator
Rip rap is a layer of large, angular stone placed on a slope to protect it from erosion by waves, currents or fast runoff. You see it along lake shorelines, stream banks, ditch outlets, culvert ends and the faces of ponds and dams. The weight and interlocking shape of the stones resist the force of moving water, while gaps between them absorb wave energy rather than reflecting it.
This rip rap calculator estimates how much stone a bank needs. Enter the length of bank to protect, its height and slope, and choose the median stone size, called D50. It works out the length of the sloping face, the area to cover, the layer thickness, the volume and the weight in tons. It also gives the approximate weight of a median stone and the geotextile needed beneath.
How to Use the Rip Rap Calculator
Enter the length of bank along the water's edge in feet.
Enter the height of bank to cover, measured vertically from the toe to the top of the stone.
Enter the slope as horizontal distance per 1 foot of rise, for example 2 for a 2:1 slope.
Choose the median stone size (D50).
The Formulas
slope length = height × √(1 + slope²)
area = bank length × slope length
thickness = 2 × D50
cubic yards = area × thickness ÷ 27
tons = cubic yards × 1.5
median stone ≈ π ÷ 6 × D50³ × 165 lb/ft³Step-by-Step Example
50 ft of bank, 6 ft high at a 2:1 slope, with 12-inch rip rap.
slope length 6 × √(1 + 2²) = 6 × 2.236 = 13.42 ft
area 50 × 13.42 = 671 ft²
thickness 2 × 12 in = 24 in = 2 ft
volume 671 × 2 ÷ 27 = 49.69 yd³
weight 49.69 × 1.5 = 74.54 tons
median stone about 86 lb
geotextile 671 × 1.15 = 771 ft²Choosing the Stone Size
The stone must be heavy enough that moving water cannot roll it. Engineers size rip rap from the water velocity, wave height and bank slope, and state agencies publish classes with a median size and a range of sizes in each. As a broad guide, small ditches and culvert outlets often use 6 to 9 inch stone, stream banks 12 to 18 inch, and exposed lake shorelines with large waves 18 to 24 inch or bigger. A good rip rap mix is well graded, with plenty of smaller stones to fill the gaps between larger ones, and angular rather than rounded so the pieces lock together.
Layer Thickness
The layer must be thick enough for stones to overlap and protect each other. The usual rule is at least twice the median size, D50, and no thinner than the largest stone. A thin layer of large rocks leaves gaps through which waves can reach and wash out the soil. On high-energy shorelines the layer is often thicker, and the stone may be placed in two layers with a smaller filter stone beneath.
Filter Fabric and Bedding
Water moving in and out of the stone layer will carry soil with it unless something stops it. A non-woven geotextile laid on the prepared slope, or a 6-inch bedding layer of smaller stone, acts as a filter. Without it, the bank behind the stones slowly washes away and the rip rap slumps. The calculator adds 15% to the fabric area for overlaps and for wrapping into the toe trench.
The Toe and the Top
Most rip rap failures start at the bottom, where currents scour the bed and undermine the stones. Extend the layer into a trench at the toe, at least as deep as the layer is thick, or below the expected scour depth. At the top, carry the stone above the high-water line and wave run-up, and tie it into the bank so water cannot get behind it. Both of these add stone beyond the face area, so increase your order to allow for them.
Permits
Work along lakes, rivers and wetlands is regulated in most places, often by both state agencies and federal bodies such as the US Army Corps of Engineers. Permits may set the stone class, the extent of work and the time of year it can be done. Check before you start.
Gentler Slopes Need Less Stone per Foot
It can seem odd, but flattening a bank increases the area to cover. A 6 ft bank at 2:1 has a face 13.4 ft long, while at 3:1 the face is 19 ft long, so the stone per foot of shoreline rises by about 40%. In return, a gentler slope is much more stable: stones are less likely to roll, waves run up and lose energy more gradually, and a smaller stone class may be acceptable. Engineers often balance the extra stone against the reduced risk of failure, and sometimes against the cost of excavation needed to cut the bank back.
Placing the Stone
Rip rap is best placed with an excavator, setting individual stones rather than dumping them from a truck at the top of the bank. Dumped stone tends to separate, with large rocks rolling to the bottom and small ones staying near the top, and it can tear the filter fabric. Work from the toe upward, keying each stone against its neighbours so that the finished surface is tight and fairly even, with smaller stones packed into the larger gaps.
Understanding Your Result
The headline gives the tons of rip rap.
The volume line shows the area, thickness and cubic yards.
The slope line gives the length of the sloping face.
The stone and fabric line gives the median stone weight and the geotextile area.
When Should You Use This Calculator?
Use it when budgeting shoreline or stream bank protection.
Use it to compare stone needs for a steeper or gentler slope.
Use it to check a contractor's tonnage estimate.
Common Mistakes
Measuring the bank height as the slope length. The sloping face is always longer.
Using rounded stone. Angular rock interlocks and stays put.
Leaving out the filter fabric. Soil washes out behind the stones.
Forgetting the toe. Scour at the bottom causes most failures.