The slope face is the whole calculation
A bank six feet high laid back at two horizontal to one vertical does not have six feet of face. It has the hypotenuse: six times the square root of one plus four, which is six times 2.236, or 13.42 feet. Over a hundred feet of shoreline that is 1,342 square feet of slope rather than the 600 square feet the vertical height suggests. Order off the vertical and you buy 45 percent of the stone.
Carrying the armour a couple of feet over the crest adds another 200 square feet, so the cover area on the defaults is about 1,542. Eighteen inches of layer over that is 2,313 cubic feet, and the toe key trench at three feet by two feet along a hundred feet of bank adds 600 more, for 2,913 cubic feet of space to fill — a shade over 108 cubic yards.
Voids, which are most of the difference between space and tons
That 2,913 cubic feet is not 2,913 cubic feet of rock. Random stone packs with air between it, commonly around 60 to 70 percent stone by volume for placed riprap, and the air weighs nothing. At 65 percent the actual rock is 1,893 cubic feet, which at 165 pounds per cubic foot is 312,500 pounds, or 156 tons. Add ten percent for waste and loss and the order is 172 tons.
Put another way, that is 1.72 tons per foot of shoreline on the defaults, and per-foot is the number worth carrying around, because the bank length is the thing that changes when somebody walks the site again.
The solid share is also the field most worth asking about rather than accepting. Moving it from 65 to 60 percent takes thirteen tons off the same job, and moving it to 70 adds thirteen. Nobody can supply that number for your material except the quarry selling it and the person placing it.
The filter layer and the fabric
Armour stone stops wave energy. It does not stop the bank washing out through the gaps between the rocks, which is why a graded filter layer and a geotextile normally sit underneath, and why a riprap job that failed usually failed from below rather than from the front. Six inches of bedding over 1,542 square feet is 771 cubic feet, and crushed stone as delivered runs around 100 pounds per cubic foot including its own voids, so that is 38.6 tons before waste, 42.4 after.
Fabric is area plus laps. Fifteen percent added to 1,542 square feet is 1,773, and a 12.5 by 300 foot roll covers 3,750, so one roll does it with a good deal left over — which is normal, because rolls come in the sizes they come in.
What is deliberately not here
No stone size, no gradation, no layer thickness, no slope, no toe depth, no fabric specification. All of those are design outputs for a specific bank with a specific exposure, water level range, boat wake and whatever is undermining it, and getting them from a calculator is how banks end up with expensive rock sitting in the water below where it was placed.
Anything built in, on or over water is permitted work in most places, and the permission usually comes from more than one desk. The state environmental or natural resources agency, the Army Corps of Engineers district for the waterway, the local building department, and on many lakes the utility or authority that holds the shoreline licence all have a say, and what each one wants is different from one state and one waterbody to the next. None of it is on this page. Start with the state agency and the Corps district for the water you are on, and get a marine contractor who works that lake or that stretch of river involved before the arithmetic turns into an order.
Questions people ask
How many tons of riprap do I need per foot of shoreline?
Work the face first, because that is where the error lives. On a six foot bank at two to one the sloped face is 13.42 feet long, so a foot of shoreline is 13.42 square feet of face plus whatever crest you carry over the top. Eighteen inches of armour over that is about 23 cubic feet, and at 65 percent stone and 165 pounds per cubic foot that is roughly 1.24 tons a foot before the toe key and before waste. With a three by two foot key trench and ten percent waste the defaults on this page land at about 1.72 tons a foot. Change any of the thickness, the slope or the solid share and that figure moves substantially.
Why is my riprap order so much bigger than length times height?
Because length times height is the vertical projection of the bank, not the surface the rock sits on. A slope is a hypotenuse: at two to one it is 2.236 times the vertical height, at three to one it is 3.162 times. Over a hundred feet of six foot bank, that is 1,342 square feet of face against the 600 you get from the vertical, which is more than twice as much. The toe key trench, the crest, the void fraction and the waste allowance all pile on top of that, but the slope length is the term that dominates and it is the one most often left out.
What percentage of a riprap layer is actually stone?
Commonly somewhere around 60 to 70 percent by volume for placed riprap, with the rest voids between the rocks, but it depends on the gradation and on how the stone is placed. It matters because you buy tons and you fill a volume: at 65 percent, a cubic yard of finished layer contains about 17.6 cubic feet of actual rock rather than 27. Moving that assumption by five points changes a hundred and fifty ton order by about thirteen tons. Ask the quarry what their material runs at, and ask the person placing it whether it is being dumped or set, because the two do not pack the same.
Do I need filter stone and fabric under riprap?
Whether a particular bank needs them, and what grading and specification they should be, is a design question for an engineer or a marine contractor rather than something a calculator decides. What is worth understanding is why the question exists: armour stone dissipates wave energy but does nothing to stop the soil behind it migrating out through the gaps, and a riprap job that has failed has usually failed from underneath rather than from the face. The bedding layer and the geotextile are what address that. This page will quantify them if your design calls for them and leaves them at zero if it does not.
Do I need a permit to put riprap on my shoreline?
Almost certainly, and from more than one body. Work in, on or over water is permitted work in most places: the state environmental or natural resources agency, the Army Corps of Engineers district for that waterway, the local building department and often the utility or authority holding the shoreline licence each have requirements, and they differ by state and by waterbody. Some of them care about the slope, the stone size, where the toe sits and how far below the water line the work goes, which means the permit can change the arithmetic on this page rather than just approving it. Start there before you order stone.