Rip Rap Calculator for Lakeshore Banks, Stream Scour and Culvert Outlets

Bank work gets done at low water in late summer, and that is a short season. Ice-out and spring runoff are what tear a bank apart, and they are also what stops you standing in it with a tape. So the measuring happens when the water drops and the stone goes in before the fall rain.

Four ways a bank order comes up short

The flat footprint gets measured instead of the slope face. Forty feet of shoreline with a six foot face is 240 square feet of stone, not forty feet of anything. Run the tape down the bank, from the toe to the top, and multiply that by the length.

A landscape depth goes into the box. Three or four inches is a mulch layer. Armor stone is installed at 12 inches at the very least, and often 18 or 24, so a ton of it covers a fraction of what a ton of landscape stone does.

The toe is left sitting on the surface. Failure starts at the bottom. The lowest course goes below the waterline or below the expected scour depth, and that buried stone has to be in the order. Before it goes in below the ordinary high water line of a lake, pond or brook, find out what approval your state and local authorities require. Work in and along water is commonly permitted work, and what is required follows the waterbody rather than the size of the job.

Bedding is forgotten. Rock straight onto soil looks solid for a season, then the bank washes out from underneath it. Non-woven geotextile, which a landscape supplier carries and this site does not, or 3 to 4 inches of #4 crushed stone, goes down first.

Measuring the bank from the toe up, not from the lawn out

Measure the slope face, not the plan view. Length along the shoreline or the channel, then the face distance from the toe up to where the armoring stops. Multiply the two.

Layer depth is set by the stone: one and a half to twice the size of the largest pieces, and 12 inches is the floor for erosion work. Four to eight inch stone usually takes 12 to 18 inches of layer. Bigger rock, or faster water, takes 24.

Cut the slope back to 2:1 or flatter before any stone lands on it. Steeper than that is an engineering question, not a stone-quantity question. Highway and spillway work has its own specified classes and depths, so follow the plan rather than this page.

What goes in the boxes

Shape first. Rectangle covers a bank run or an apron. Multiple areas, up to five of them, handles a shoreline that changes height along its length, which most of them do: split it where the face gets taller and enter each stretch separately.

Length and width in feet, using the face distance as the width. Depth in inches, anywhere from 0.25 to 24, and for armor stone you will be at the top of that range. Then the material select, because the weight follows it.

The 5% compaction and spillage box is optional. Rip rap is placed rather than compacted, so the more useful allowance is the round-up described further down.

Cubic yards, tons and cubic feet come back together, and Email my quote puts the figures in writing.

Heaviest thing on the site, thinnest coverage

Rip rap runs about 3,100 lb per cubic yard, which is 1.55 tons. It is the heaviest material published here, and combined with a foot-deep layer it gives coverage figures that look wrong until you think about them.

Cubic yards = face area sq ft x layer depth in, divided by 12, divided by 27. Tons = yards x 1.55.

Choosing between the two size ranges

Four to eight inch stone suits pond banks, slow streams and the outfall below a downspout or a small culvert. Eight to twelve inch stone is for lake edges taking boat wake and for channels that run fast after a thaw. The steep streams coming off the hills move stone that a pond would never touch, as the 2011 tropical storm demonstrated across the mountain towns.

Between two sizes, take the larger and the thicker layer. Weight is the only thing holding it there.

Hand setting works up to about six or eight inch stone if you have gloves and a strong back. Above that it is machine work.

Forty-five feet of lakeshore bank

Hypothetical. A camp lot where wind and boat wake have cut the bank back at high water. Forty-five feet long, a four and a half foot face, armored at 18 inches with six to twelve inch RipRap.

• 45 x 4.5 = 202.5 sq ft of face
• 202.5 x 1.5 ft = 303.75 cu ft
• 303.75 / 27 = 11.25 cu yd
• 11.25 x 1.55 = 17.44 tons

Bedding, if you use stone rather than fabric, at 4 inches over the same face: 202.5 x 0.333 = 67.5 cubic feet, 2.50 yards, about 3.63 tons of #4 stone.

Round the armor up to 18 tons. Walk the access before ordering; the last stretch down to the water is usually the softest ground on the lot, and shifting a pile twice is the expensive part of the job.

A culvert outlet apron on a camp road

Hypothetical. A 24 inch culvert under a camp road is scouring a hole below it. The apron runs five and a half pipe diameters downstream, so 11 feet, and 8 feet wide to take the spread, at a 12 inch layer.

• 11 x 8 = 88 sq ft
• 88 x 1 ft = 88 cu ft
• 88 / 27 = 3.26 cu yd
• 3.26 x 1.55 = 5.05 tons

Bedding at 4 inches over the same area: 29.3 cubic feet, 1.09 yards, about 1.58 tons of #4 stone.

Set the apron stones so they interlock rather than leaving them where the bucket drops them, and bury the downstream lip. Then leave it alone. Walk the bank after the first hard storm and again after ice-out, push back anything that has shifted, and fill any thin patch from the reserve. Ten minutes twice a year is the whole maintenance program.

Where the bank is visible from the house and appearance counts as much as armoring, large river rock is the rounder alternative, and a purely decorative channel is sized on the dry creek beds tool instead of this one. Water arriving at the top of the bank from a downspout or a foundation line is a separate problem, solved with drain rock before any armor goes in. If the access track itself is too soft to carry a loaded truck, firm it first with base stone for the access.

Once the tonnage is settled, set a quantity and a delivery date on the product page. Delivery is arranged at checkout by ZIP code, and access and tipping covers what the truck needs, which near water means firm ground and clear room well back from the edge. The full calculator list has the rest of the materials.

Common questions

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