Rip Rap Calculator
Calculation Results
How It Works
Quantity Estimation
The quantity estimator calculates the volume, weight, and cost of rip rap needed for your project based on:
- Volume Calculation: Length × Width × Depth = Volume
- Weight Calculation: Volume × Density = Weight
- Cost Calculation: Weight × Cost per unit = Total Cost
Engineering Sizing
The engineering sizing calculator determines the appropriate rock size (D₅₀) for hydraulic stability using the Isbash equation:
D₅₀ = (V²) / (2g(SG-1)SF)
Where:
- V = Flow velocity
- g = Acceleration due to gravity
- SG = Specific gravity of rock
- SF = Safety factor
Example Calculation
Quantity Estimation Example
For a shoreline protection project:
- Length: 50 ft
- Width: 10 ft
- Depth: 1.5 ft
- Rock Density: 165 lb/ft³
- Wastage: 10%
Calculations:
- Volume: 50 × 10 × 1.5 = 750 ft³
- Weight: 750 × 165 = 123,750 lb = 61.9 tons
- With wastage: 61.9 × 1.1 = 68.1 tons
Engineering Sizing Example
For a channel with:
- Flow velocity: 8 ft/s
- Specific gravity: 2.65
- Safety factor: 1.5
Calculation:
D₅₀ = (8²) / (2 × 32.2 × (2.65-1) × 1.5) = 64 / (64.4 × 1.65 × 1.5) = 64 / 159.39 = 0.40 ft = 4.8 inches
Material Density Table
| Rock Type | Density (lb/ft³) | Density (kg/m³) | Typical D₅₀ Range (inches) |
|---|---|---|---|
| Granite | 165-170 | 2640-2720 | 6-36 |
| Limestone | 160-165 | 2560-2640 | 6-30 |
| Sandstone | 140-150 | 2240-2400 | 6-24 |
| Basalt | 175-185 | 2800-2960 | 8-36 |
| Quartzite | 165-170 | 2640-2720 | 6-30 |
Frequently Asked Questions
What is rip rap?
Rip rap is a layer of large, durable stones or concrete pieces used to protect soil from erosion in areas of concentrated flow or wave energy, such as shorelines, bridge abutments, and stormwater channels.
What does D₅₀ mean?
D₅₀ is the median stone diameter, meaning 50% of the stones (by weight) are larger than this size and 50% are smaller. It’s a key parameter in rip rap design for hydraulic stability.
How thick should a rip rap layer be?
Typically, the rip rap layer thickness should be 1.5 to 2 times the D₅₀ stone diameter. For most applications, a thickness of 12-24 inches is common.
What is the typical wastage percentage for rip rap?
Wastage typically ranges from 5% to 15%, depending on the site conditions, placement method, and rock shape. 10% is a common default value.
How do I select the right safety factor?
Safety factors typically range from 1.2 to 2.0. Use higher values (1.5-2.0) for critical applications or where flow conditions are uncertain, and lower values (1.2-1.5) for less critical applications with well-defined flow conditions.