Work out how much rip rap you need to armor a slope, ditch, shoreline or culvert outlet. Enter the area and layer thickness and this rip rap calculator gives you tons, cubic yards and the cost. This free online rip rap calculator works on any phone or computer, with no sign-up or download.
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Rip rap needed
— tons
Quick answer: tons of rip rap = area (sq ft) × thickness (in) ÷ 324 × 1.76. Make the layer at least 1.5 × the median stone size, and at least 6 in.
Tons = L (ft) × W (ft) × thickness (in) ÷ 324 × 1.76
Michigan’s erosion-control guidance (Riprap BMP) sets the minimum layer thickness at 1.5 times the D50 (the median stone size) or 6 in, whichever is greater. It also says the largest stones should be no more than 1.5 times the D50.
| Median stone size (D50) | Minimum thickness | 1 ton covers about |
|---|---|---|
| 4 in | 6 in | 31 sq ft |
| 6 in | 9 in | 20 sq ft |
| 8 in | 12 in | 15 sq ft |
| 12 in | 18 in | 10 sq ft |
| 18 in | 27 in | 7 sq ft |
The same Michigan guidance reproduces a USDA Soil Conservation Service table of typical stone sizes, assuming solid rock at 165 lb per cu ft:
| Stone weight | Mean spherical diameter |
|---|---|
| 50 lb | 10 in |
| 100 lb | 13 in |
| 300 lb | 18 in |
| 500 lb | 22 in |
| 1,000 lb | 27 in |
| 2,000 lb | 34 in |
The calculator uses 1.7636 tons per cubic yard (130.6 lb per cu ft), the factor the Tennessee DOT Roadway Design Guidelines use to estimate graded rock. Placed rip rap has large gaps between the stones, so the weight per yard depends on the rock and how it is placed. If your quarry quotes a different factor, enter it as a custom density.
Also: rectangular prism, bed, trench, room
V = L × W × H
L = length, W = width, H = height or depth.
Example: A shoreline 50 ft long, 10 ft measured up the slope, armored 18 in thick: 50 × 10 × 1.5 = 750 cu ft = 27.8 cu yd × 1.76 = 49 tons.
Also: square feet to cubic feet
V = A × d
A = area, d = depth or thickness.
Example: A ditch lined over 300 sq ft (length × the width around the ditch surface), 12 in thick: 300 cu ft = 11.1 cu yd, about 19.6 tons.
Also: round hole, post hole, tank, planter
V = π × r² × h
r = radius (half the diameter), h = height.
Example: A round apron 10 ft across at a culvert outlet, 12 in thick: π × 5² × 1 = 78.5 cu ft = 2.91 cu yd, about 5.1 tons.
A bank 80 ft long, 6 ft along the slope, with 6 in median stone (9 in thick): 80 × 6 × 9 ÷ 324 = 13.3 cu yd × 1.76 = 23.5 tons, or about 26 tons with 10% extra.
Multiply the area to cover (length × width along the slope, in feet) by the layer thickness in feet to get cubic feet, divide by 27 for cubic yards, and multiply by about 1.76 tons per cubic yard. A 50 × 10 ft slope armored 18 in thick needs 27.8 cu yd, or about 49 tons.
A common rule, used for example in Michigan’s erosion-control guidance, is at least 1.5 times the median stone size (D50) or 6 in, whichever is greater. For 12 in median stone that is 18 in thick. The size itself depends on water speed and slope and should come from a design for anything beyond small residential work.
The calculator uses 1.76 tons per cubic yard, the Tennessee DOT’s estimating factor for graded rock. Real loads vary with the rock and how tightly it is placed, so ask your quarry for their figure and enter it as a custom density.
Usually, yes: a geotextile or a granular filter layer keeps soil from washing out between the stones. Michigan’s guidance calls for granular filter layers at least 6 in thick, or 3 times the filter’s D50, and for burying the top and toe of any fabric.
It depends on the water velocity, the slope and wave action. Larger flows need bigger stone. Many suppliers sell rip rap by class or by a size range such as 6–12 in or 12–24 in. For streambanks, culvert outlets and shorelines, get the size from your local permit office or an engineer.
Often, yes. Work along streams, lakes, wetlands and shorelines usually needs a state or federal permit in the US, and sometimes a local one. Check before you order.
Yes. The CalcWall Rip Rap Calculator is completely free and works online in any browser, on a phone, tablet or computer. There is nothing to download and no account to create, and the math runs right in your browser, so results appear as you type.