Riprap Calculator
Measure width along the slope face, not the flat plan distance.
Landscaping
Riprap Calculator
Quick answer: riprap tons = area (ft²) × layer thickness (ft) ÷ 27 × (1 + overfill) × about 1.5 tons/yd³. A 50 ft × 8 ft bank with 9 in median stone placed 18 in thick is 22.2 yd³; with 10% overfill that is 24.4 yd³, or about 36.7 tons, over 460 ft² of fabric.
Estimate rock for shoreline and stream-bank protection, drainage ditches, and culvert outlets. Pick a stone size and the tool sets a layer about twice the median stone diameter, then returns tons, cubic yards, truck loads, and the filter fabric to go underneath.
Riprap on a slope
How to calculate riprap
Riprap is sized by its median stone diameter, called D50: half the stones by weight are larger than D50 and half are smaller. The layer has to be thick enough for stones to interlock, so design guides commonly set thickness at 1.5 to 2 times D50, and never less than the largest stone (D100). Once you have a thickness, the quantity is a simple area × thickness volume converted to tons.
L × W × T (ft) ÷ 27 × (1 + overfill) × density
= tons of riprap; filter fabric = L × W × (1 + overlap allowance)
- Measure along the slope. Use a tape laid on the bank from toe to top of protection. On a 2:1 slope that rises 4 ft, the face is about 8.9 ft, not 8 ft.
- Choose the stone size from your plan, permit, or engineer. Flow speed and wave exposure govern this, not the calculator.
- Set thickness at about 2 × D50 (minimum 1.5 × D50 or D100, whichever is larger).
- Multiply area × thickness for cubic feet; divide by 27 for cubic yards.
- Add 10% or so for an uneven subgrade and the toe key, then multiply by 1.4–1.6 tons per placed yd³.
- Add fabric equal to the slope area plus 10–20% for overlaps and anchoring at the top and toe.
Worked example: pond shoreline
A 30 ft stretch of pond bank, 6 ft measured down the slope, protected with 4–8 in stone (D50 ≈ 6 in) placed 12 in thick:
- Area: 30 × 6 = 180 ft²; net volume 180 × 1 = 180 ft³ ÷ 27 = 6.67 yd³
- With 10% overfill: 6.67 × 1.1 = 7.33 yd³
- Weight at 1.5 tons per yd³: 7.33 × 1.5 = 11.0 tons — one tandem load
- Filter fabric: 180 × 1.15 = 207 ft² (23 yd²)
Riprap per 100 ft² of slope
Net quantities with no overfill. Tons shown at 1.4, 1.5, and 1.6 tons per placed yd³ to bracket typical rock densities.
| Layer thickness | Fits D50 of about | yd³ per 100 ft² | Tons at 1.4 | Tons at 1.5 | Tons at 1.6 |
|---|---|---|---|---|---|
| 12 in | 6 in | 3.70 | 5.19 | 5.56 | 5.93 |
| 18 in | 9 in | 5.56 | 7.78 | 8.33 | 8.89 |
| 24 in | 12 in | 7.41 | 10.37 | 11.11 | 11.85 |
| 30 in | 15 in | 9.26 | 12.96 | 13.89 | 14.81 |
| 36 in | 18 in | 11.11 | 15.56 | 16.67 | 17.78 |
State highway departments publish their own riprap classes (Class I, II, Type A, B, and so on) with different gradations. When a plan names a class, use that agency's D50 and thickness.
Riprap placement tips
- Always use a filter layer. Without nonwoven or woven geotextile (or a graded gravel filter) under the rock, water pulls soil out through the gaps and the riprap sinks. Overlap fabric seams 12–18 in, shingled downstream and downslope.
- Key in the toe. Dig a trench at the bottom of the slope and fill it with rock so the blanket cannot slide or be undermined. Include the toe trench in your area.
- Do not use rounded rock alone. Angular quarry stone interlocks; smooth river cobbles roll out under flow.
- Place, don't dump from height. Dropping large stone onto fabric tears it. Place rock with an excavator bucket, starting at the toe and working up.
- Check permits first. Work along streams, lakes, and wetlands often needs a local, state, or U.S. Army Corps of Engineers permit that may specify stone class and thickness.
- Watch the slope. Slopes steeper than about 2:1 (horizontal:vertical) make riprap prone to sliding; flatten the bank or get an engineered design.
Riprap FAQ
How do you calculate how much riprap you need?
Multiply the slope area (length × width measured along the face) by the layer thickness in feet, divide by 27 for cubic yards, and multiply by about 1.5 tons per yard. A 50 × 8 ft bank with an 18 inch layer is 22.2 cubic yards, about 36.7 tons with 10% overfill.
How thick should riprap be?
Riprap is usually placed at 1.5 to 2 times the median stone size (D50), and at least as thick as the largest stones. For 6–12 inch stone with a D50 near 9 inches, that means a layer about 14 to 18 inches thick. Follow your permit or engineer when one specifies thickness.
How much does a cubic yard of riprap weigh?
Placed riprap typically weighs about 1.4 to 1.6 tons per cubic yard. Solid rock is much heavier, but riprap has 30 to 40 percent voids between stones. Granite and dense limestone sit at the high end; ask your quarry for its figure.
Do you need filter fabric under riprap?
Yes, in almost all cases. A geotextile or gravel filter layer stops water from washing soil out through the gaps between stones. Plan fabric equal to the slope area plus 10 to 20 percent for seam overlaps and anchoring at the top and toe.
What size riprap should I use?
Stone size depends on flow velocity, wave action, and slope. Small ditches and gentle pond edges often use 4–8 inch stone, while stream banks and culvert outlets with faster water commonly need 9–18 inch or larger stone. For channels and outlets, use the size from your plans or an engineer.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.