Rip Rap Calculator
Enter your project area, placement depth, and rip rap class to get instant tonnage, volume in cubic yards, and estimated material cost. Switch to the Hydraulic Sizing tab to find the minimum median rock diameter (D50) for a given water velocity using the Isbash equation. Toggle between metric and imperial units throughout.
What is rip rap?
Rip rap (also spelled riprap or rip-rap) is a layer of large, durable angular stones or concrete rubble placed to protect soil surfaces from erosion caused by water flow. It is one of the most widely used erosion-control methods in civil and environmental engineering, appearing on stream banks, shorelines, road embankments, culvert outlets, and drainage channels. Unlike rounded river gravel, angular crushed stone interlocks under hydraulic loading, making it far more stable. Typical stone types include granite, limestone, basalt, and dolomite, all with specific gravities in the 2.60-2.70 range.
How to use this calculator
Choose Volume, Weight and Cost mode to estimate how much material you need. Enter the project length and width (or the area directly if you already know it), the intended placement depth, and the rip rap class that matches your project specification. Add a waste factor of 5-10 percent for typical projects. If you have a quarry price, enter it to see an instant material cost estimate. Switch to Hydraulic Sizing (Isbash) mode to determine the minimum median stone diameter (D50) needed to resist a given water velocity: enter the design velocity, turbulence condition, and stone specific gravity, and the calculator applies the Isbash equation. Toggle between imperial and metric units at any time.
The Isbash equation for hydraulic sizing
The Isbash equation calculates the minimum median stone diameter (D50) that can resist movement under a design flow velocity: D50 = V2 / (2 * g * C2 * (S - 1)), where V is the flow velocity (ft/s or m/s), g is gravitational acceleration (32.17 ft/s2 or 9.81 m/s2), C is the Isbash constant (0.86 for high turbulence, 1.20 for low turbulence), and S is the specific gravity of the stone. A higher velocity, lower turbulence constant (0.86 rather than 1.20), or lower specific gravity all increase the required D50. Once D50 is known, the full gradation is specified so that D85 is roughly 1.5-2 times D50 and D15 is about half of D50.
Volume and weight calculations
Volume is calculated as area multiplied by depth. For a rectangular project, area equals length times width. The resulting cubic footage is divided by 27 to convert to cubic yards. Weight is then found by multiplying volume by the bulk density of the chosen stone class: approximately 105 lb/ft3 for Class 1, 110 lb/ft3 for Class 2, 115 lb/ft3 for Class 3, and 120 lb/ft3 for Class 4. The waste factor accounts for material lost to settling, irregular voids, and over-excavation; a factor of 5-10 percent is standard. Divide total weight by 2,000 for short tons, or by 2,205 for metric tonnes. A standard tandem-axle dump truck carries about 18 short tons, so the truck-load figure is total short tons divided by 18.
Rip rap class and application guide
| Class | Typical size | Layer depth (min) | Typical use | Approx. density (lb/ft3) |
|---|---|---|---|---|
| Class 1 - Light | 6-12 in (15-30 cm) | 12-18 in | Light drainage channels, roadside ditches | 105 |
| Class 2 - Medium | 12-18 in (30-46 cm) | 18-24 in | Stream bank stabilization, culvert protection | 110 |
| Class 3 - Heavy | 18-24 in (46-61 cm) | 24-36 in | Shoreline armor, high-velocity channels | 115 |
| Class 4 - Extra Heavy | 24+ in (61+ cm) | 36-48 in | Spillway aprons, high-energy outlet protection | 120 |
Standard classifications for angular crushed stone. Minimum depth = 1.5 x maximum stone diameter. Actual project specifications may differ.
Frequently asked questions
What is the difference between rip rap classes?
Rip rap is grouped by typical stone size: Class 1 (light, 6-12 in) is used for roadside ditches and low-energy drainage; Class 2 (medium, 12-18 in) suits stream banks and culvert protection; Class 3 (heavy, 18-24 in) handles high-velocity channels and shoreline armoring; Class 4 (extra heavy, 24+ in) is reserved for spillways and very high-energy outlets. Each class also has a different bulk density because larger, more angular stones pack more tightly.
How deep should rip rap be placed?
A commonly used rule of thumb is that the minimum layer depth should be at least 1.5 times the maximum stone diameter in your gradation. In practice: 12-18 in for Class 1, 18-24 in for Class 2, 24-36 in for Class 3, and 36-48 in for Class 4. Thicker layers are required in high-velocity or high-wave-energy environments. Always confirm the placement depth with your project erosion-control specification or a licensed engineer.
Do I need geotextile fabric under rip rap?
Yes, in most cases. Geotextile filter fabric (or a properly graded bedding stone layer) is placed under rip rap to prevent fine soil particles from washing through the voids into the waterway, a process called piping or undermining. The fabric also helps distribute loads. Omit it only if the underlying soil is coarse gravel or if your specific project design documents say otherwise.
How many tons of rip rap do I need per square foot?
At a typical 18-in placement depth with Class 2 stone (110 lb/ft3) and a 10% waste factor, you need roughly 0.091 short tons per square foot (about 11 ft2 per ton). Heavier classes or thicker placements increase this significantly. Use the Volume and Weight mode above to get an exact figure for your specific combination of area, depth, and stone class.
What does D50 mean in rip rap sizing?
D50 is the median stone diameter: 50 percent of stones in the gradation are smaller and 50 percent are larger. It is the key sizing parameter in the Isbash equation and most hydraulic design methods. Specifications also define D15 (the smaller stones that prevent gaps) and D85 (the larger stones that provide structural interlocking). D85 is typically 1.5 to 2 times D50.
What is the Isbash constant and how do I choose it?
The Isbash constant C accounts for the turbulence of the flow. Use C = 0.86 (high turbulence) for locations with complex flow patterns such as channel bends, culvert outlets, and spillway aprons. Use C = 1.20 (low turbulence) for straight, uniform reaches of a channel or for bank protection in relatively uniform flow. A lower constant (0.86) requires a larger stone for the same velocity, providing a more conservative design.
How much does rip rap cost?
Material prices vary significantly by region and stone type. As a rough guide, expect $35-55 per short ton for Class 1-2 crushed limestone or granite at the quarry, and $55-85 per ton for larger Class 3-4 material. Delivered prices add $10-25 per ton depending on haul distance. Installation labor typically runs $25-45 per ton. Enter the price you have been quoted into the calculator to get a project-specific estimate.