How to use the riprap calculator calculator
- 1
Measure along the slope face
Measure the distance up the slope, not the horizontal distance across it. The slope face is the surface being covered, and it is always larger than the footprint beneath it.
- 2
Add a section for each area
A bank with a different pitch at one end, or a channel with sloped sides and a flat bottom, is several sections rather than one.
- 3
Enter the layer depth from your design
Layer depth comes from the erosion-control design. This calculator does not suggest one, because it depends on the stone size, the water velocity, and what the slope has to withstand.
- 4
Add an allowance
Placed stone settles into voids and shifts during placement, so most estimates carry an allowance. Choose a percentage that reflects your stone and the slope.
- 5
Add a density for tons
Riprap is normally sold by the ton. Enter the density your supplier gives you, since YardCalc publishes none: stone density varies by rock type and by how the material is graded.
Formula
Riprap cubic yards = (slope face area(sq ft) x layer depth(ft)) / 27
For a slope, the face area is the horizontal footprint multiplied by the slope multiplier, or measured directly up the face.
Worked example
A slope face measuring 40 feet along the bank by 10 feet up the slope, at a 12 inch layer depth
- 1Find the slope face area: 40 x 10 = 400 square feet.
- 2Convert the depth: 12 in = 1 ft.
- 3Find the volume: 400 x 1 = 400 cubic feet.
- 4Convert to cubic yards: 400 / 27 = 14.81 cubic yards.
The slope needs about 14.81 cubic yards before any allowance. The 12 inch layer depth is the figure entered for this example, not a recommendation.
Worked examples
Why the footprint is the wrong measurement
The same bank measured horizontally as 40 feet by 8 feet instead of up the 10 foot face
- 1Footprint area: 40 x 8 = 320 square feet.
- 2Volume from the footprint: 320 x 1 / 27 = 11.85 cubic yards.
- 3Volume from the slope face: 14.81 cubic yards.
- 4Shortfall: 2.96 cubic yards, about 20 percent.
Measuring the flat footprint under-orders by a fifth here, and the error grows with the steepness of the bank. Measure up the face.
Converting to tons with a supplier density
The 14.81 cubic yards above, with a supplier-provided density of 100 pounds per cubic foot
- 1Convert to cubic feet: 14.81 x 27 = 400 cubic feet.
- 2Weight in pounds: 400 x 100 = 40,000 pounds.
- 3Convert to short tons: 40,000 / 2,000 = 20 tons.
About 20 short tons at that density. The 100 pounds per cubic foot is a figure entered for this example; ask your supplier for the density of the stone they are selling.
Measuring a slope face rather than a footprint
This is the mistake that dominates riprap estimates. Standing at the top of a bank, the natural measurement is the horizontal distance across it, but the stone covers the sloping surface, which is longer.
The relationship is the same one that applies to roofs: the sloping face is the hypotenuse over the horizontal run, so it is always longer than the run. A modest bank adds a little; a steep one adds a great deal.
The most reliable approach is to measure directly up the face with a tape laid on the slope, which gives the real dimension without any calculation. If you only have the horizontal distance and the height, the face length is the square root of the sum of their squares.
Volume, tons, and why no density is published
Riprap is almost always sold by the ton, while the geometry gives you a volume, so a density is needed to bridge them.
Stone density varies by rock type, and the density that matters for ordering is not the density of solid rock but the bulk density of the graded material, including the voids between stones. That figure depends on the gradation and how the stone is placed, which is why it comes from the supplier rather than from a table.
The gap between solid rock density and placed bulk density is large, so using a solid-rock figure will overstate the tonnage significantly. Ask the supplier what density they use for the specific gradation you are buying.
Why placed riprap settles
Unlike aggregate, riprap is individual stones placed or dumped into a layer, and the layer that results is not a neat prism of the nominal depth.
Stones settle into the voids between the stones beneath them, the surface is irregular by nature, and the toe of the slope usually needs a thicker section than the face. All of that consumes stone beyond the geometric figure.
That is why the allowance field exists and why riprap estimates commonly carry one. The right percentage depends on the stone size relative to the layer depth and on the slope, both of which you can see and a calculator cannot.
Common mistakes
Where riprap estimates go wrong:
- Measuring the horizontal footprint instead of the slope face.
- Using solid rock density rather than the placed bulk density of the graded material.
- Assuming a density instead of asking the supplier.
- Ordering the exact geometric volume with no allowance for settlement into voids.
- Forgetting the thicker section usually needed at the toe of the slope.
- Treating a channel with sloped sides and a flat bottom as one uniform area.
- Starting work on a shoreline or channel without checking whether a permit is required.
Sources and methodology
Every figure on this page comes from geometry, not from a product or materials database. Areas use the standard area formulas, volumes are area multiplied by depth, cylinders use pi times the radius squared times the height, and unit conversions use exact factors: 12 inches to a foot, 27 cubic feet to a cubic yard, and 9 square feet to a square yard.
YardCalc publishes no material density, coverage rate, product dimension, soil swell factor, compaction factor, or price. Those vary by product, by supplier, by soil, and by region, and a figure that is right in one case is wrong in another, so the calculator asks you for the values that apply to your job. Weight estimates appear only when you supply a density.
Nothing here is engineering, safety, or code guidance. Results are planning estimates. Read how every value on the site is derived and checked in the YardCalc calculation methodology.
Riprap cubic yards by slope face area and layer depth
Volume for a given slope face area at a given layer depth, before any allowance. Depth columns are reference values, not design recommendations, and the area must be the slope face rather than the footprint.
| Area | 6 in | 9 in | 12 in | 18 in | 24 in | 36 in |
|---|---|---|---|---|---|---|
| 100 sq ft | 1.85 | 2.78 | 3.7 | 5.56 | 7.41 | 11.11 |
| 200 sq ft | 3.7 | 5.56 | 7.41 | 11.11 | 14.81 | 22.22 |
| 400 sq ft | 7.41 | 11.11 | 14.81 | 22.22 | 29.63 | 44.44 |
| 600 sq ft | 11.11 | 16.67 | 22.22 | 33.33 | 44.44 | 66.67 |
| 800 sq ft | 14.81 | 22.22 | 29.63 | 44.44 | 59.26 | 88.89 |
| 1,000 sq ft | 18.52 | 27.78 | 37.04 | 55.56 | 74.07 | 111.1 |
| 1,500 sq ft | 27.78 | 41.67 | 55.56 | 83.33 | 111.1 | 166.7 |
| 2,000 sq ft | 37.04 | 55.56 | 74.07 | 111.1 | 148.1 | 222.2 |
Tons per cubic yard by entered density
How a density converts one cubic yard into weight. These densities span a range to show the arithmetic and are not values for any particular stone. Use the figure your supplier gives you.
| Entered density (lb/ft³) | Pounds per cubic yard | US short tons per cubic yard |
|---|---|---|
| 80 | 2,160 | 1.08 |
| 90 | 2,430 | 1.22 |
| 100 | 2,700 | 1.35 |
| 110 | 2,970 | 1.49 |
| 120 | 3,240 | 1.62 |
| 130 | 3,510 | 1.76 |
Frequently asked questions
- How do I calculate riprap?
- Multiply the slope face area by the layer depth, then divide by 27 for cubic yards. A 40 by 10 foot slope face at a 12 inch depth is 400 cubic feet, or 14.81 cubic yards.
- How many tons of riprap do I need?
- Multiply the volume by a density. YardCalc publishes no stone density, because it varies by rock type and gradation, and the figure that matters is the placed bulk density including voids. Ask your supplier and enter it above.
- Do I measure the slope or the flat area?
- The slope face, always. The stone covers the sloping surface, which is longer than the horizontal footprint beneath it. Measuring the footprint under-orders, and the error grows with the steepness.
- What size riprap should I use?
- This calculator does not specify stone size. Size and gradation depend on water velocity, wave action, and what is being protected, and they belong to an erosion-control design from a qualified engineer.
- How thick should the riprap layer be?
- Not something this page determines. Layer depth is part of the design and relates to the stone size and the conditions. Take it from your design or a qualified professional.
- Do I need filter fabric underneath?
- That is a design decision, not a quantity question. Many riprap installations use a filter layer to stop soil washing out through the stone, but whether and what to use comes from the design. Fabric quantity can be estimated with the landscape fabric calculator once specified.
Riprap volume is your measured slope face area multiplied by the layer depth you enter, converted at exactly 27 cubic feet per cubic yard. No stone size, gradation, layer depth, density, or price is published, weight appears only from a density you supply, and nothing here constitutes erosion-control design. Quantities come from exact geometry using the dimensions you enter. Confirm product dimensions, installation requirements, local codes, site conditions, safety requirements, and final order quantities with the appropriate manufacturer, supplier, contractor, or professional.
Results are planning estimates. Last reviewed 2026-07-28.