What This Solves
Calculates the required riprap stone size (D50) for channel bank and bed protection using Isbash, USACE, HEC-11, or Maynord methods.
Best Used When
- You are designing channel bank protection with riprap for a stream or drainage channel
- You need to determine the median stone size (D50) and gradation for erosion protection
- You want to compare riprap sizing results from multiple standard 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)
- Flow velocity or shear stress is the primary design parameter
- The riprap blanket thickness is typically 1.5 * D50 or the D100 size, whichever is greater
- Filter fabric or granular bedding is provided beneath the riprap
- The channel geometry is stable and not actively migrating
Input Quality Notes
Design velocity or shear stress should be computed from a hydraulic analysis (not estimated). Use the velocity at the toe of slope or bank, which may differ from the average channel velocity.
Size riprap stone for channel banks, culvert outlets, and bridge scour protection. Enter your design velocity, flow depth, and stone properties to get the required median stone size (D50), full gradation, blanket thickness, and the matching standard riprap class — using the Isbash, USACE shear-stress, HEC-11, or Maynord method.
Standard Riprap Classes
| Class | D50 (in) | Application |
|---|---|---|
| Class I | 6-9 | Light duty, low velocity |
| Class II | 9-15 | Medium duty |
| Class III | 15-21 | Standard protection |
| Class IV | 21-27 | Heavy duty |
| Class V | 27-36 | Very heavy duty |
| Class VI | 36-48 | Severe conditions |
Isbash Coefficients
| 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.
How riprap sizing works
All four methods solve for the median stone size D50 — the stone size that resists the hydraulic forces trying to move it. They differ in whether they work from velocity, shear stress, or the Froude number. The calculator then builds the gradation, blanket thickness, and stone weight from that D50.
Isbash equation (velocity)
D50 = V² / [ 2·g·C²·(SG−1) ]
Relates stone size to the velocity needed to initiate motion. C is the Isbash coefficient (see table); a shape factor and flow-condition factor adjust the effective velocity. Widely used for bank protection.
USACE shear stress (Shields)
τ₀ = γw·R·S, then D50 = (τ₀·SF) / [ τ*c·(γs−γw)·Ks ]
Bed shear stress from hydraulic radius R and channel slope S, compared against the critical Shields parameter (τ*c = 0.047 for riprap). Ks reduces capacity on side slopes.
HEC-11 (Froude number)
D50 = Cs·y·Fr2.5·SF / (SG−1)1.5, with Fr = V / √(g·y)
FHWA comprehensive procedure. Uses the flow Froude number with a stability coefficient Cs = 0.30 (conservative). Good general method for channels and revetments.
Maynord (EM 1110-2-1601)
D30/y = Sf·Cs·Cv·Ct·Fr2.5; D50 ≈ 1.25·D30
USACE velocity-depth method for flood-control channels, using velocity-distribution and blanket-thickness coefficients. Solves for D30 and scales to D50.
Variable definitions
- D50 — median stone size (50% finer by weight)
- V — average channel (design) velocity
- y — flow depth; R — hydraulic radius (A/P); S — channel slope
- g — gravitational acceleration (32.2 ft/s² or 9.81 m/s²)
- SG — specific gravity of stone (default 2.65)
- γw, γs — unit weight of water (62.4 lb/ft³) and stone
- SF — stability (safety) factor; Fr — Froude number; Ks — side-slope reduction factor
From D50 the calculator derives the gradation (D15 = 0.4·D50, D85 = 1.8·D50, D100 = 2.0·D50), the minimum blanket thickness t = max(1.5·D50, D100), and the median stone weight from an equivalent-sphere volume, W = (4/3)πr³·SG·γw.
Standard riprap size classes (D50)
Typical AASHTO/DOT riprap classes by median stone size. Once the calculator returns a D50, it reports the matching class so you can specify a standard graded product rather than a custom gradation.
| Class | D50 range (in) | Typical application |
|---|---|---|
| Class I | 6 – 9 | Light duty, low velocity |
| Class II | 9 – 15 | Medium duty |
| Class III | 15 – 21 | Standard protection |
| Class IV | 21 – 27 | Heavy duty |
| Class V | 27 – 36 | Very heavy duty |
| Class VI | 36 – 48 | Severe conditions |
Isbash coefficient C by installation: dumped/loose-stable 1.2, placed-stable 1.4, keyed (embedded) 1.6 — embedment lets a smaller stone resist the same velocity. Shape factors range from 1.0 (very angular) to 0.75 (well rounded), and flow-condition factors increase the effective velocity from 1.0 (uniform) up to 1.75 (high velocity).
Frequently asked questions
What is the D50 of riprap?
D50 is the median stone size of a riprap gradation — 50% of the stone by weight is larger and 50% is smaller. It is the single value that riprap design methods solve for, because it characterizes the stability of the whole graded mixture. This calculator reports D50 in both feet/metres and the practical inches/mm used on plans, then derives the rest of the gradation (D15, D85, D100) from it.
Which riprap sizing method should I use?
Use the Isbash equation for velocity-based bank protection where you know the design velocity. Use the USACE shear-stress method when you have channel slope and hydraulic radius and want a Shields-criterion check. HEC-11 is a comprehensive Froude-number procedure good for general channel and revetment design, and the Maynord method (USACE EM 1110-2-1601) is the velocity-depth approach for flood-control channels. When in doubt, run more than one method and design to the more conservative (larger) D50.
How thick should a riprap layer be?
A common minimum is a blanket thickness equal to the larger of 1.5 times D50 or 1.0 times the maximum stone size (D100). For the standard gradation used here (D100 = 2.0 x D50), that works out to t = 2.0 x D50. The layer must be at least one full stone thick everywhere and is normally placed over a granular or geotextile filter to prevent the underlying soil from washing out.
What specific gravity should I assume for riprap stone?
A specific gravity of 2.65 is the standard assumption for sound granite, limestone, and most quarried stone, and it is the default in this calculator. Lighter stone needs to be larger to provide the same resistance to flow, so if your source rock has a lower specific gravity (verify with the supplier), enter the actual value — the required D50 increases as specific gravity drops.
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Last verified: February 2026