Rip Rap Calculator results turn a bank or shoreline area and a riprap class into tons, cubic yards, cost and truck loads, or size the stone from channel velocity using FHWA HEC-11.
Measure width along the slope face, from toe to top of the rock.
Stone, Order and Hauling
Rip Rap Class Chart
| Class | D50 | D100 | Min Layer | D50 Weight |
|---|---|---|---|---|
| Class I | 6 in | 12 in | 12 in | 11 lb |
| Class II | 9 in | 18 in | 18 in | 37 lb |
| Class III | 12 in | 24 in | 24 in | 87 lb |
| Class IV | 15 in | 30 in | 30 in | 169 lb |
| Class V | 18 in | 36 in | 36 in | 292 lb |
| Class VI | 21 in | 42 in | 42 in | 464 lb |
| Class VII | 24 in | 48 in | 48 in | 693 lb |
| Class VIII | 30 in | 60 in | 60 in | 1,353 lb |
| Class IX | 36 in | 72 in | 72 in | 2,338 lb |
| Class X | 42 in | 84 in | 84 in | 3,712 lb |
Rip Rap Calculator for Banks, Shorelines and Channels
Rip rap is a layer of large, angular stone placed on a bank to stop flowing water or waves from carrying the soil away. The job comes down to two questions, which size of stone stays put in that flow, and how many tons of it the layer needs.
Contractors use the tonnage to order stone and book trucks for creek banks, pond shorelines and ditch linings. Engineers and site designers use the sizing mode to check the stone class against the design velocity before it goes into a spec.
The Rip Rap Calculator handles both. It gives tons, placed cubic yards, stone cost, truckloads and the median stone weight, or the required median stone size, class, layer thickness and stone weights from the channel hydraulics.
Two Modes in the Rip Rap Calculator
Tons, Volume and Cost takes the bank length and the width measured along the slope face, or a known area. Pick a riprap class from I to X, or Custom Thickness, and the layer thickness follows from that class.
Stone Size (HEC-11) takes the average velocity and average depth of the main channel at the design flow, the bank slope as H:V and a stability factor. It returns the median stone size and rounds it up to the next standard class.
The accordion holds the stone specific gravity, the share of voids in the placed layer, a waste margin, your stone price per ton, the truck capacity and whether the rock goes in the dry or underwater. US mode works in feet, inches and short tons, and metric mode in meters, centimeters and tonnes.
How Many Tons of Rip Rap a Layer Needs
The placed volume is the area times the layer thickness, plus the waste margin. With the thickness T in feet, dividing cubic feet by 27 gives cubic yards.
$$V_{cy} = \frac{A \times T}{27} \times (1 + w)$$
Placed rock is not solid, because gaps open between the stones. The Rip Rap Calculator takes the weight of solid stone from its specific gravity and removes the void share n, which defaults to 40% as an assumption to confirm with your quarry.
$$\gamma = S_s \times 62.4 \times (1 – n)$$
$$Tons = V_{cy} \times \frac{27 \times \gamma}{2000}$$
At a specific gravity of 2.65 and 40% voids, the layer weighs about 99 lb per cubic foot, or 1.34 tons per cubic yard. The default 100 ft bank, 12 ft along the slope with Class II stone, needs 73.3 cubic yards and 98.2 tons with 10% extra, or 6 truckloads of 18 tons.
The common mistake is measuring the slope width on the level instead of along the face. On a 2:1 bank the face is about 11.8% longer than its horizontal run, and on a 3:1 bank about 5.4% longer, so a level measurement orders that much too little stone.
Plan for a geotextile or a granular filter under the rock, as the tool’s notes advise, and key the toe of the layer into the ground. The Crushed Stone Calculator covers a bedding stone layer, and the Stone Weight Calculator weighs a single large boulder.
Riprap Classes and Layer Thickness
The ten classes follow the riprap gradations in NCHRP Report 568, named by median stone size D50 from 6 to 42 inches. The largest stone, D100, may be no more than twice the D50 in each class.
For layer thickness, section 4.3 of the FHWA HEC-11 manual calls for at least the D100 stone or 1.5 times the D50, whichever is greater, and never less than 12 inches. Because NCHRP 568 lets D100 reach twice the D50, the D100 term governs every class, and the Rip Rap Calculator sets the layer at 2 × D50 or 12 inches.
| Class | D50 | Max D100 | Min Layer | D50 Stone Weight |
|---|---|---|---|---|
| Class I | 6 in | 12 in | 12 in | 11 lb |
| Class II | 9 in | 18 in | 18 in | 37 lb |
| Class III | 12 in | 24 in | 24 in | 87 lb |
| Class IV | 15 in | 30 in | 30 in | 169 lb |
| Class V | 18 in | 36 in | 36 in | 292 lb |
| Class VI | 21 in | 42 in | 42 in | 464 lb |
| Class VII | 24 in | 48 in | 48 in | 693 lb |
| Class VIII | 30 in | 60 in | 60 in | 1,353 lb |
| Class IX | 36 in | 72 in | 72 in | 2,338 lb |
| Class X | 42 in | 84 in | 84 in | 3,712 lb |
The stone weights treat the D50 stone as a sphere at a specific gravity of 2.65, the way the tool converts size to weight. Class sizes come from NCHRP 568, while the layer column applies the HEC-11 rule for rock placed in the dry.
HEC-11 adds 50% to the layer where riprap is placed underwater, to cover the uncertainty of placing rock you can’t see. Pick Underwater (+50% Layer) under Placement and the default Class II layer grows from 18 to 27 inches, taking the order from 98.2 tons and 6 truckloads to 147.3 tons and 9.
The same section suggests another 6 to 12 inches of thickness, with larger stone, where floating debris, ice or boat wakes attack the layer. Some state manuals, such as Colorado’s bank protection chapter, write the D50 term as 2.0 times D50 instead of 1.5, which gives the same layers for these classes.
Sizing the Stone With the HEC-11 Equation
HEC-11 sizes revetment riprap from the average channel velocity and depth, with a correction for the bank angle. In US units, with the median size in feet, the equation reads as follows.
$$D_{50} = \frac{0.001 \times C \times V_a^3}{d_{avg}^{0.5} \times K_1^{1.5}}$$
The bank factor K1 compares the bank angle θ with the rock’s angle of repose φ, and the correction C adjusts for specific gravity and stability factor. Both reach 1.0 for a standard 2.65 stone at a stability factor of 1.2.
$$K_1 = \sqrt{1 – \frac{\sin^2\theta}{\sin^2\phi}}$$
$$C = \frac{2.12}{(S_s – 1)^{1.5}} \times \left(\frac{SF}{1.2}\right)^{1.5}$$
At the Rip Rap Calculator’s defaults of 10 ft/s, a 5 ft depth and a 2:1 bank, the median stone works out to 8.4 inches, which rounds up to Class II. Because velocity is cubed, raising it to 12 ft/s pushes the stone to 14.5 inches and Class IV.
Bank slope matters almost as much. The same flow on a 1.5:1 bank needs 12.8 inch stone, while a 3:1 bank needs only 6.5 inches, and HEC-11 recommends 2:1 as the steepest face for riprap, so flattening the slope is often cheaper than buying a larger class.
The bank must be flatter than the rock’s angle of repose, which defaults to 42°, or the Rip Rap Calculator rejects the slope. HEC-11 covers channel banks in steady flow, so bridge piers, culvert outlets and wave attack need their own checks, and real stability also depends on the filter, the toe and how the stone is placed.
Rip Rap Class and Tonnage Questions
How many tons of rip rap per cubic yard?
At a specific gravity of 2.65 and 40% voids, placed rip rap weighs about 1.34 tons per cubic yard. Denser stone or tighter placement raises it, so a 2.7 stone with 35% voids comes out near 1.48 tons per yard.
Quarries often quote their own tons per yard for each gradation, and that figure is the one to trust. Enter their numbers in the accordion and the Rip Rap Calculator recalculates the tons and truckloads.
What does Class 2 rip rap mean?
It depends on whose spec you follow. Under NCHRP 568, Class II has a 9 inch median stone, but the state-by-state chart from Martin Marietta shows Minnesota Class II at 3 to 12 inches and Virginia Class II dry riprap at 150 to 500 lb per stone.
Check which agency’s class system your plans or permit use before ordering. If the spec gives a median size instead of a class, pick Custom Thickness and enter twice that size, or at least 12 inches, as the layer.
What is 6 inch or 12 inch rip rap?
Where riprap is named by size, such as Colorado’s Riprap 6 and Riprap 12, the number is the median stone size D50 in inches. That matches NCHRP Class I for 6 inch and Class III for 12 inch stone.
The largest stones run up to twice that size, so 6 inch rip rap needs a 12 inch layer and 12 inch rip rap needs a 24 inch layer in the dry under HEC-11. A Class I layer weighs about two-thirds as much per square foot as Class II at the same voids.
How much does rip rap cost per square foot?
Cost per square foot follows from your price per ton and the layer thickness. At the tool’s placeholder price of 40 dollars per ton, Class II at 18 inches with 10% extra costs about 3.27 dollars per sq ft, and Class III at 24 inches about 4.37 dollars.
Those figures cover stone only. Delivery, filter fabric, bedding stone and placement are extra, so get a delivered price from the quarry and add placement from your contractor.
Can I calculate rip rap tons in Excel?
Yes. With the length in feet in column A, the slope width in feet in column B and the thickness in inches in column C, enter =A2*B2*(C2/12)/27*1.1*1.34 for tons with 10% extra at 1.34 tons per yard.
Change the 1.1 for a different waste margin and the 1.34 for your quarry’s tons per yard. The HEC-11 stone sizing needs trig on the bank angle, which is easier to run in the calculator than in a sheet.