Quick answer
How is riprap size selected?
A riprap sizing calculation determines the required median stone size D50: half of the specified stone by weight is finer and half is coarser. Because drag, scour, geometry, and failure mechanisms differ, FHWA HEC-23 uses a separate relationship for each channel, bridge, culvert, overtopping, or wave application rather than one universal formula.
Enter the hydraulic inputs below and compare the required D50 with the next larger nominal FHWA class. The Design Guideline 17 wave example is the sole implemented class-band exception. Then use the reported thickness and geometry as a screening calculation and separately specify the full gradation, filter, toe, and extent requirements—not D50 alone. US customary and metric calculations are both supported.
FHWA Hydraulic Toolbox parity
Application-Specific Riprap Design
HEC-23 uses a different design equation for each application. Circular culvert aprons are handled by the HEC-14 Outlet Protection Calculator.
FHWA Results
Legacy / Comparative Sizing Methods
The methods below are retained for comparison. Use the application-specific FHWA section above when matching Hydraulic Toolbox results.
Legacy Isbash coefficients
These coefficients apply only to the comparative Isbash method below. Use the HEC-23 Table 4.1 classes for an FHWA class specification.
| Condition | C |
|---|---|
| Loose, minimum stability | 0.86 |
| Loose, average | 1 |
| Loose, stable | 1.2 |
| Placed, stable | 1.4 |
| Keyed (embedded) | 1.6 |
Ready to Calculate
Select a sizing method and enter flow parameters to determine riprap requirements.
For educational purposes only. Not a substitute for professional engineering judgment.
Interactive model
Riprap Layer and Gradation Explorer
Inspect a three-dimensional protection layer and connect median stone size, gradation, and blanket thickness to the design.
Open full model (opens in a new tab)FHWA application coverage
Each choice uses its own published HEC-23 relationship and layout rules. Circular-culvert outlet aprons use HEC-14 Equation 10.4 on the separate outlet-protection page.
| Application | FHWA basis | Design scope |
|---|---|---|
| Channel revetment | HEC-23 Eq. 4.1 | Bends, straight reaches, downstream transitions |
| Channel spur | HEC-23 Eq. 4.1 | Spur and end-dike protection |
| Bridge pier | HEC-23 Eq. 11.1 | Shape-adjusted pier velocity and scour depth |
| Abutment / guide bank | HEC-23 Eq. 14.1-14.2 | Setback-ratio velocity and Froude regimes |
| Embankment overtopping | HEC-23 Eq. 5.1-5.3 | Stone stability and through-flow thickness |
| Open-bottom culvert | HEC-23 Eq. 18.1 | Entrance protection, footing, and scour geometry |
| Wave attack | HEC-23 Eq. 17.8-17.13 | Hudson or Pilarczyk stone sizing and runup |
| Design wave | HEC-23 Eq. 17.1-17.7 | Fetch/depth estimate; wind duration requires a separate check |
| Filter screening | HEC-23 Design Guideline 16 | Selected granular interfaces or geotextile O95/conductivity criteria |
HEC-23 Table 4.1 riprap classes
Non-wave applications select the next standard class whose nominal D50 meets or exceeds the calculated requirement. The Design Guideline 17 wave path alone may select a lower nominal class when the calculated D50 remains within that class's permitted band, matching FHWA's worked example. Values below are the U.S. customary Table 4.1 dimensions; metric calculations use exact inch-to-metre conversions.
| Class | Nominal D50 (in) | Permitted D50 band (in) | D100 (in) |
|---|---|---|---|
| Class I | 6 | 5.7- 6.9 | 12 |
| Class II | 9 | 8.5- 10.5 | 18 |
| Class III | 12 | 11.5- 14.0 | 24 |
| Class IV | 15 | 14.5- 17.5 | 30 |
| Class V | 18 | 17.0- 20.5 | 36 |
| Class VI | 21 | 20.0- 24.0 | 42 |
| Class VII | 24 | 23.0- 27.5 | 48 |
| Class VIII | 30 | 28.5- 34.5 | 60 |
| Class IX | 36 | 34.0- 41.5 | 72 |
| Class X | 42 | 40.0- 48.5 | 84 |
This class reference is not a delivered-material gradation check. Use the Rock and Sediment Gradation Calculator with project specification limits and representative field or stockpile measurements.
Riprap layer-thickness checks
Layer thickness is application-specific. Always use the thickness returned by the selected FHWA design above; the summary below explains why a D50-only specification is incomplete.
| Application | Thickness basis used by this calculator |
|---|---|
| Channel revetment / spur | Greatest of 1 ft, 1.5 times design D50, or the selected class D100 |
| Bridge pier | Greatest of three times design D50, contraction scour plus long-term degradation, or bed-form trough depth; underwater placement adds 50% |
| Embankment overtopping | Hydraulic interstitial-flow check; the class escalates if the trial layer fails |
| Open-bottom culvert / wave attack | Application geometry plus D50/D100 minimums reported by the selected method |
Design limits
These equations size stone and the FHWA-prescribed protection geometry; they do not replace site-specific checks for flanking, uplift, ice or debris, constructability, geotechnical stability, or inspection after floods. Channel Equation 4.1 is limited to longitudinal slopes of 2% or less. The wave layout is limited to 1V:1.5H or flatter, and its wind-wave estimate does not check duration. Bottomless-culvert guidance requires free-surface flow through the 500-year event and flared wing walls, and must include contraction scour, long-term degradation, and lateral migration. A compatible granular or geotextile filter is integral to the design; passing the displayed filter criteria is not a complete material or installation approval.
Frequently asked questions
Which FHWA riprap application should I use?
Choose the application that matches the hydraulic mechanism: channel revetment or spur, bridge pier, bridge abutment or guide bank, embankment overtopping, open-bottom culvert, or wave attack. HEC-23 does not use one universal D50 equation for all of these cases. Circular culvert aprons use the separate HEC-14 Outlet Protection Calculator.
What is D50?
D50 is the median stone dimension for the specified gradation: 50 percent of the stone by weight is finer and 50 percent is coarser. Except for the documented Design Guideline 17 wave example, this calculator uses FHWA’s next-larger nominal D50 class rule. It does not measure or certify a delivered stockpile gradation.
Why are the older Isbash, shear-stress, HEC-11, and Maynord calculators still shown?
They remain available below the FHWA application section for comparison and legacy workflows. To reproduce FHWA Hydraulic Toolbox results, use the application-specific HEC-23 tools because they include the geometry, velocity, scour, wave, and filter rules particular to each design.
Does the calculator also design the filter?
It performs bounded filter screens, not a complete product or construction design. The granular option checks soil-to-filter retention and conductivity, filter-to-armor compatibility, and placement thickness. The geotextile option screens O95 retention, conductivity, and transition-layer need for open-channel or wave exposure; strength, survivability, seams, installation, and manufacturer conformance remain to be specified.
What This Solves
Screens application-specific FHWA HEC-23 D50, nominal class, thickness, geometry, and selected filter criteria, with legacy Isbash, HEC-11, USACE, and Maynord methods available for comparison.
Best Used When
- You need a preliminary HEC-23 screen for a channel, bridge, overtopping, bottomless-culvert, or wave application
- You need to determine the required median stone size (D50), next nominal class, and layer geometry
- You want to compare application-specific FHWA results with legacy sizing methods
Do NOT Use When
- You are designing outlet protection at a pipe or culvert outlet specifically — Use Outlet Protection Calculator
- You need to evaluate grass or other flexible lining adequacy instead of riprap — Use Channel Lining Calculator
Key Assumptions
- Stone sizing equations assume angular, durable rock with standard specific gravity (2.65)
- Each application remains within the limits stated beside its inputs; Equation 4.1 channels are 2% or flatter
- Non-wave applications select the next-larger nominal class; the DG17 wave example is the documented band exception
- Full stockpile gradation, stone durability, toe and termination details, and constructability are checked separately
- Displayed filter results cover only the named retention, conductivity, interface, and transition criteria
Input Quality Notes
Use site-specific hydraulic and scour results. Include contraction scour, long-term degradation, lateral migration, underwater placement, wind duration, and material testing wherever the selected application requires them; a passing screen is not a final construction specification.
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Last verified: August 2026