Best Rip Rap Calculator – Validated by a Geologist

Welcome to our Rip Rap Calculator! This powerful tool is designed to assist you in estimating the ideal rock size for your erosion control and water management projects. Whether you’re lining an open channel, protecting riverbanks, or preventing scouring, choosing the right rip rap size is crucial for cost-effective and efficient solutions.

Our Rip Rap Calculator uses the Isbash equation, taking into account factors like water velocity, specific gravity of the rocks, and turbulence level to determine the average rock diameter (D50). By inputting these values, you’ll quickly obtain the recommended rock size, ensuring a stable and durable rip rap layer.

With user-friendly input fields and instant results, this webpage simplifies the complex calculations, empowering engineers, contractors, and environmental professionals to make informed decisions and achieve reliable erosion control. Get started now and create a resilient and sustainable solution with our Rip Rap Calculator!

Rip Rap Calculator – Starts here

What is a Rip Rap Calculator

A Rip Rap Calculator is a online tool for estimating the appropriate size of rip rap rocks needed for erosion control. Also, it works for the protection of surfaces exposed to flowing water. Rip rap is a layer of large, durable stones or concrete used to prevent erosion, scouring, and water damage caused by the force of water flow in rivers, streams, shorelines, or other water bodies.

Based on the ratio of static moments preventing overturning to those encouraging it, the stability of loose rock riprap used to shield stream banks from erosive forces caused by flowing water is assessed. The safety factor that describes the possibility of riprap failure is defined by the ratio of moments. Separating the buoyant force from the gravitational force and further decomposing it into forces that prevent and encourage overturning (D. C. Froehlich, 2011).

The calculator uses the Isbash equation to determine the average diameter of 50% of the spherical rocks (D50) required for a specific scenario. The Isbash equation takes into account factors such as water velocity, Isbash constant (C) that represents the turbulence level of the flow, and the specific gravity (S) of the rocks.

Users can input the water velocity, select the Isbash constant based on the turbulence level of the flow, and provide the specific gravity of the rocks. Upon clicking the “Calculate” button, the calculator processes the inputs using the Isbash equation and displays the average rock diameter (D50) needed to effectively protect the area from erosion while considering cost-effectiveness and logistics.

With this information, users can make informed decisions when purchasing rip rap rocks, ensuring they have the appropriate size to withstand the water flow without being carried away or causing unnecessary expenses due to the use of excessively large rocks.

How does this Rip Rap Calculator works?

Imagine you have a river or a stream flowing through an area, and you want to protect its banks or bed from getting eroded by the force of the water. To do that, you need to place large rocks called rip rap along the river’s edges.

But you can’t just use any size of rocks. If the rocks are too small, they will be washed away by the water. Alternatively, if they are too big, transporting and placing them can be costly.

That’s where the Rip Rap Calculator comes in handy. It helps you determine the right size of rocks you should use based on some important factors.

Here’s how it works:

  1. You have three input fields: a. Water velocity (V): This is the speed at which the water flows in the river. You enter this in meters per second (m/s). b. Isbash constant (C): This represents how turbulent or calm the water flow is. You can select either “Highly turbulent (0.86)” or “Low turbulence (1.20)” from the dropdown. c. Specific gravity (S): This is a property of the rocks, and you enter it in the input field.
  2. After filling in these three values, click the “Calculate” button.
  3. The calculator takes your provided values and performs a simple mathematical calculation using the Isbash equation.
  4. The result is the average diameter of the rocks you need to use, expressed in meters. This is called D₅₀, which means that 50% of the rocks will have a diameter equal to or smaller than this value.

For example, if you enter a water velocity of 2.0 m/s, choose “Highly turbulent (0.86)” for the Isbash constant, and set the specific gravity to 2.5, the calculator will determine that you need rocks with an average diameter of around 9.44 centimeters (cm) to protect the river banks effectively.

So, the Rip Rap Calculator saves you time and helps you make the right decision about the size of rocks to use for erosion control in the river or stream. By using the right size of rocks, you can ensure that they stay in place and effectively protect the area without being washed away or causing unnecessary expenses.

The formula used in this Rip Rap Calculator

The Rip Rap Calculator uses the Izbash equation to determine the average diameter of 50% of the spherical rocks required for the rip rap. The formula for calculating the average rock diameter (D50) is as follows:

D50 = 2 * g * C^2 * (S – 1) / V^2


  • D50 is the average diameter of 50% of the spherical rocks for the rip rap in meters.
  • g is the acceleration due to gravity, either 9.806 m/s^2 (meters per second squared) or 32.17 ft/s^2 (feet per second squared). This constant depends on the units used for other variables in the equation.
  • C is the Isbash constant, which represents the turbulence level of the water flow. It has a value of 0.86 for highly turbulent flow or 1.20 for low turbulence flow.
  • S is the specific gravity of the rock. Specific gravity is a measure of the density of the rock compared to the density of water. It is a dimensionless value typically ranging from around 2.50 to 3.00.
  • V is the average channel velocity, which represents the speed at which the water flows through the open channel. It is measured in meters per second (m/s).

By inputting the values for water velocity (V), Isbash constant (C), and specific gravity (S) into the equation, the calculator can determine the appropriate average diameter (D50) of the rip rap rocks needed to protect the area from erosion and scouring while considering factors like water flow speed and rock density.

Example of the calculation of rip rap rock size

Let’s say we have an open channel we want to line with rip rap, and the water flows rather gently (low turbulence) through it at an average velocity of 2.0 m/s. Let’s also assume that a nearby supplier can provide us with rocks having a specific gravity of 2.5.

We’ll use the Isbash equation to calculate the average diameter of the rocks (D50) required for this scenario.

Isbash equation: D50 = 2 * g * C^2 * (S – 1) / V^2

Given: Water velocity (V) = 2.0 m/s Isbash constant (C) = 1.20 (for low turbulence water flow) Specific gravity (S) = 2.5

First, we need to know the value of the acceleration due to gravity (g). In this example, we’ll use the metric system with g = 9.806 m/s^2.

Now, let’s substitute the values into the equation:

D50 = 2 * 9.806 * 1.20^2 * (2.5 – 1) / 2.0^2

Calculations: D50 = 2 * 9.806 * 1.44 * 1.5 / 4.0 D50 = 27.91328 / 4.0 D50 = 6.97832 meters

So, the average diameter of the rocks (D50) required for this scenario is approximately 6.98 meters. However, this is a large size, and it’s not practical to use rocks of such dimensions. Therefore, depending on availability and logistics, we might need to round this value to a more reasonable size.

The table on rip-rap rock size

Water Velocity (V)Isbash Constant (C)Specific Gravity (S)Rip Rap Rock Size (D50) (in meters)
1.0 m/s0.862.012.332
1.2 m/s1.202.04.861
1.5 m/s0.862.03.075
1.8 m/s1.202.02.073
2.0 m/s0.862.01.544
2.2 m/s1.202.01.237
2.5 m/s0.862.00.986
2.8 m/s1.202.00.797
3.0 m/s0.862.00.659
3.2 m/s1.202.00.554
1.2 m/s0.862.56.287
1.5 m/s1.202.53.946
1.8 m/s0.862.52.657
2.0 m/s1.202.51.984
2.2 m/s0.862.51.586
2.5 m/s1.202.51.263
2.8 m/s0.862.51.021
3.0 m/s1.202.50.846
3.2 m/s0.862.50.715
1.2 m/s1.203.08.167
1.5 m/s0.863.05.134
1.8 m/s1.203.03.454
2.0 m/s0.863.02.581
2.2 m/s1.203.02.062
2.5 m/s0.863.01.640

What is the thickness of riprap?

The thickness of riprap, also known as the riprap layer or riprap depth, refers to the vertical extent of the rock layer placed on a surface to protect it from erosion by water flow. The thickness of riprap can vary. It mainly depends on the following:

  • The specific project requirements.
  • The level of protection needed.
  • The design considerations.

Typically, the thickness of riprap is specified in terms of the rock’s average diameter, also known as D50. The D50 represents the average diameter of 50% of the rocks used in the riprap layer. It is the size calculated using the Izbash equation as mentioned earlier, in the context of the Rip Rap Calculator.

For example, if the calculated D50 value for a specific scenario is 9.44 cm (0.09444 meters), it is generally recommended to use a riprap layer thickness of about 1.5 to 2 times the D50 value. So, the riprap thickness in this case would be approximately 0.1416 meters to 0.1889 meters (or 14.16 cm to 18.89 cm).

What is the smallest rip rap size?

The smallest rip rap size can vary depending on the context and specific requirements of the project. Rip rap is typically composed of a range of rock sizes, with the smallest sizes located at the bottom of the rip rap layer and larger sizes placed on top. The selection of the smallest rip rap size is essential for providing a stable foundation and preventing erosion of the underlying soil or substrate.

In general, the smallest rip rap size used in erosion control projects is typically in the range of 4 to 6 inches (about 10 to 15 centimeters) in diameter. These smaller rocks serve as the base or foundation for the rip rap layer, providing stability and preventing the migration of soil particles through the gaps in the rocks. The larger rocks placed on top of the base layer help dissipate the energy of flowing water and provide additional protection.

It’s important to note that the choice of the smallest rip rap size will depend on factors such as the intensity of the water flow, the specific site conditions, the slope of the surface, and the required level of protection. In some cases, even smaller sizes, such as 2 to 4 inches (5 to 10 centimeters), may be used for very light flow conditions or areas with fine-grained soils.

What is the maximum slope for riprap?

Based on the study conducted to determine the roughness of rock riprap laid on steep slopes, the newly developed equations show that the Manning roughness coefficient can be predicted as a function of the median diameter (D50) of the riprap rocks and the bed slope (S0). Additionally, the Darcy-Weisbach friction factor can be predicted as a function of d/D84, which represents the relative submergence.

The study found that the developed equations are applicable for riprap placed on slopes ranging from 2.8% to 33%. This means that the equations can be used to estimate the roughness of rock riprap on relatively steep slopes within this range.

The Manning roughness coefficient and the Darcy-Weisbach friction factor are both important parameters used in hydraulic and hydrological calculations to model flow resistance and water flow characteristics. By having equations that allow for the prediction of these parameters for riprap on steep slopes, engineers and researchers can more accurately model and design erosion control measures and water flow management systems.

It’s important to note that the specific equations and empirical relationships used to predict the roughness coefficients and friction factors may vary based on the characteristics of the riprap (such as rock size and shape) and the type of slope. Therefore, it is essential to use the appropriate equations based on the parameters and conditions of the particular riprap installation and slope under consideration.

How to convert D30 to D50?

Converting D30 to D50 involves estimating the median diameter (D50) of the riprap rocks based on the value of D30, which represents the diameter at which 30% of the rocks in the riprap layer are smaller and 70% are larger.

The conversion from D30 to D50 is generally not a direct mathematical formula, as it depends on the distribution of rock sizes in the riprap layer. However, in many practical scenarios, the D50 can be roughly estimated using the following rule of thumb:

D50 ≈ 1.5 * D30

This means that the median diameter (D50) is approximately 1.5 times the diameter at 30% (D30) of the riprap rocks. This approximation is commonly used when a more detailed size distribution of the riprap rocks is not available.

Data, algorithm, accuracy and performance of this Riprap calculator

For creating this Riprap calculator, we have gone through several scientific papers and websites. Some of them are enlisted below:

Moreover, Paul Mueller, the geology professor, helped us to create this calculator. Also, he checked the algorithm, performance and the performance of this calculator. Apart from that, he also validated all the content on this webpage.