Riprap Calculator

Compute stable rock size. River channel erosion control, scour prevention. Isbash equation


Register to enable "Calculate" button in riprap rock sizing calculator.
Water velocity, V: 
Rock diameter, D: 
Isbash constant, C:    Enter 0.86 or 1.2. See below
Rock specific gravity, S:    Typically 2.65. See below
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Units: ft=foot, knots=nautical miles per hour, m=meter, mph=miles per hour.

Riprap in a River

riprap photo

Riprap Calculator
Riprap is used for erosion control, to prevent scour, and to minimize sediment transport in rivers and streams. A stable riprap rock size is desired. The Isbash equation computes the smallest diameter stone D having specific gravity S that if dropped in water flowing at velocity V will settle and remain stationary on the channel bed (Blevins, 2003, p. 218; NCHRP, 2006, p. 14).

riprap equation

The riprap calculator will also solve the equation in reverse. In that case, riprap rock size D is entered. The computed water velocity V is the maximum water velocity at which the riprap will remain in place.

The variables in the riprap calculator are (units shown in SI, but our calculation allows a variety of units):
C = Isbash constant. Per references below, C=0.86 for highly turbulent conditions or C=1.2 for low turbulence.
D = Median diameter of spherical stone or rock. Also known as D50 (m).
g = Acceleration due to gravity, 9.8066 m/s2.
S = Specific gravity of stone or rock. Typically varies from 2.56 to 2.92 depending on the rock material. A commonly used value is 2.65.
V = Water velocity approaching the riprap (m/s).

Blevins, Robert D. 2003. Applied Fluid Dynamics Handbook. Krieger Publishing Co.

National Cooperative Highway Research Program [NCHRP]. 2006. Riprap Design Criteria, Recommended Specifications, and Quality Control. Report 568. Transportation Research Board. Retrieved from


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Please contact us for consulting or questions about riprap sizing.

LMNO Engineering, Research, and Software, Ltd.
7860 Angel Ridge Rd.   Athens, Ohio  45701  USA   Phone: (740) 707-2614


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