Rip Rap Calculator: Measuring the Face, Not the Map
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A Maryland bank is losing ground in one of two ways. On the Bay and the tidal rivers it is wave action and storm tides taking the edge of a lawn or the foot of a bluff a little at a time. Inland it is a creek cutting the outside of a bend after a summer thunderstorm, or the ground at a culvert outlet washing out from under a drive. Either way the fix is stone heavy enough that moving water cannot lift it, and the rip rap calculator turns a measured bank face into the tonnage that takes.
A 5-foot bank has an 11-foot face: where the tape goes
This is the measurement that goes wrong, and it goes wrong in the same direction every time. Armor stone covers the sloping face, so the area is the slope distance times the length of shoreline, not the width you would read off a plan.
Take the tape down the face itself, from the toe to the crest. Where the bank is too soft or too wet to walk, work it out instead: a bank standing 5 feet high on a 2:1 slope runs 10 feet out horizontally, and the face is the diagonal of those two, which is 11.2 feet. Multiply that by the run of the shoreline.
Odd outlines break up rather than average out. A shore that turns a corner into a cove is two runs with two different face heights, and the multiple-areas option holds up to five. A culvert outlet is its own rectangle at its own thickness. Do not fold them together; the thicknesses are rarely the same.
Grade the bank to 2:1 or flatter before any stone goes on. A steeper face needs engineering rather than more rock on top of it.
Entering a face, and the ceiling on the depth box
Shape first, then the dimensions in feet, then thickness in inches. The form accepts 0.25 to 24 inches, and armor stone is the one material on this site that runs into the top of that range rather than the bottom.
A 24-inch course fits exactly. Anything thicker goes in as two runs on the same footprint and gets added by hand. A 30-inch course over 300 square feet of face, for instance, is 300 square feet at 24 inches plus 300 at 6, which is 600 plus 150 cubic feet, or 750 in total. That comes to 27.8 cubic yards and roughly 43.1 tons.
Pick the material so the weight is right, since the select is what sets the conversion, and read the cubic yards, tons and cubic feet that come back together. The result can be emailed across as a written quote.
Layer thickness from a farm pond to a wake-hit shore
Thickness follows stone size: one and a half to two times the largest rock, never less than 12 inches on anything doing real work.
| Where the stone is going | Stone size class | Layer thickness |
|---|---|---|
| Farm pond bank, downspout outlet, slow stream | 4 to 8 inch | 12 inches |
| Tidal creek and river bank, moderate exposure | 4 to 8 inch | 12 to 18 inches |
| Chesapeake shoreline taking boat wake, faster channels | 8 to 12 inch | 18 to 24 inches |
| Anything on an engineered plan | as specified | as specified |
Where two sizes would both do, take the larger stone and the thicker course. Weight is what resists the water.
What a ton actually covers once it is a foot thick
The coverage figures on this material surprise people, and they are correct.
| Layer thickness | One ton covers | Tons per 100 sq ft of face |
|---|---|---|
| 12 inches | about 17 sq ft | 5.7 |
| 18 inches | about 12 sq ft | 8.6 |
| 24 inches | about 9 sq ft | 11.5 |
At roughly 3,100 lb per cubic yard, which is 1.55 tons and the heaviest figure this site publishes, a ton of armor stone is only about two thirds of a cubic yard. That is why a bank job that looks small on paper arrives as a serious load.
Cubic yards is the volume of the course, tons is the ordering unit, and cubic feet is what you want when adding a short apron to an order already placed.
A 65-foot tidal creek bank and a culvert apron under the drive
Both are invented for this page.
The creek bank. A face 5 feet high on a 2:1 slope measures 11.2 feet down the slope, and the run is 65 feet, so the area is 728 square feet. At 18 inches, that is 728 times 1.5, or 1,092 cubic feet. Divided by 27 it comes to 40.4 cubic yards, which at 1.55 is about 62.7 tons.
That figure is the armor only. Under it goes non-woven geotextile from a landscape supplier, since fabric is not stocked here, or 3 to 4 inches of #4 Crushed Stone as a bedding layer. Four inches over the same 728 square feet is 242.7 cubic feet, or 8.99 cubic yards, which at roughly 1.45 tons per cubic yard is about 13 tons. Two materials, two lines on the order.
The culvert apron. A 15 inch pipe under a gravel drive wants an apron carried 4 to 6 pipe diameters downstream, so 5 to 7.5 feet. Take 7 feet long by 7 feet wide, which is 49 square feet, at 12 inches. That is 49 cubic feet, 1.81 cubic yards, or about 2.81 tons. Small enough to hand-set, and the place scour starts if it is skipped.
Fabric, the buried toe, and getting a loaded truck close
Three things decide whether the stone stays where you put it, and none of them is tonnage.
The bedding layer comes first, because rock set straight onto bare soil settles as soon as water pulls the ground out from beneath it. The toe row goes in below the waterline or below the expected scour depth, buried, since failure starts at the bottom and works up. And the top layer is placed rather than tipped, set by machine or by hand so the pieces lock against each other.
Before that toe row goes in below the ordinary high water line of a tidal shoreline, creek or pond, check what approval your state and local authorities require. Work in and along water is commonly permitted work.
Access is worth checking before the order rather than after. The last stretch down to a pond or a shoreline is soft ground, and a loaded truck that cannot reach the work means the stone gets handled twice. Take the tonnage to the RipRap listing, set the quantity and pick a date there. Delivery is arranged at checkout by ZIP code and the delivery page covers what the driver needs at the tipping spot.
Two jobs get sent here that do not want armor stone at all. A dry creek bed that only runs after a storm is decorative work for Large River Rock, sized on the river rock calculator. Water sitting in the ground behind a wall or under a lawn is a drainage problem for clean washed stone, sized on the stone calculator.
After the first serious storm, walk the bank. Look at the toe for displaced pieces, at the gaps for exposed soil, and at the apron for scour, then put the shifted stones back and fill the low patches from what you held in reserve. Ten minutes, twice a year, is most of the upkeep this material asks for.
Common questions
The pieces in one load are not all the same size. Does that change my tonnage?
It changes how far the load goes, which comes to the same thing. Stone running to the large end of a size class leaves bigger voids between pieces and covers less face per ton than the coverage figures suggest. The tick box adding 5 percent for spillage covers the pieces that roll past the toe into the water on the way down, not the size mix, so treat it as a starting point and round the tonnage up on top of it. Whatever is left over is the stock for the first repair after a storm.
Do I measure the slope or the horizontal distance across the bank?
The slope, every time. On a 2:1 bank the footprint reading is about a tenth short, and the gap widens as the bank steepens. Run the tape from the toe to the crest along the face. If the bank is too soft to walk, take the vertical height and the horizontal run and use the diagonal: 5 feet up and 10 feet out gives a face of 11.2 feet, not 10.
The shoreline turns a corner into a cove. Can I do it in one go?
Use the multiple-areas option and give each straight run its own entry, up to five. The two legs rarely share a face height, and the sheltered leg inside a cove often takes a smaller stone and a thinner course than the exposed one outside it. Entering them separately keeps those differences, and the results add up to one tonnage at the end.
Should I include the flat strip above the crest?
Include the part the stone actually covers. Armor usually carries a foot or two past the crest so a storm tide cannot get in behind it, and that strip is at the same thickness as the face, so add it to the area. What does not go in the same entry is the toe trench, which is deeper than the course and worth calculating on its own before you add it to the total.
What goes underneath the big stone, and when does that arrive?
As two products on two lines. The bedding is either fabric, which is not a tonnage at all, or 3 to 4 inches of #4 stone at about 1.45 tons per cubic yard. The armor is the thick course at 1.55. On the creek bank above that is roughly 13 tons under about 63, and they have to arrive in that order, since the bedding is worthless once the armor is on top of the wrong thing.
Related calculators
Banks don't armor themselves.
RipRap by the ton with free delivery, plus the #4 bedding stone to go under it.
