Size riprap for channels, banks, and drainage protection. Review velocity, slope, density, and safety factors. Turn field assumptions into practical stone guidance for planning.
This page uses a simplified Isbash style screening method for riprap sizing.
1. Stone density = Stone specific gravity × 1000
2. Submerged specific gravity = (Stone density ÷ Water density) − 1
3. Side slope factor = 1 + (0.2 ÷ Side slope ratio H:1V)
4. D50 = (Safety factor × Turbulence factor × Side slope factor × Velocity²) ÷ (0.86 × Submerged specific gravity × 9.81)
5. Dmax = 1.5 × D50
6. Layer thickness = Thickness factor × D50
7. Shear stress = Water density × 9.81 × Hydraulic radius × Energy slope
8. Average stone weight = (π ÷ 6) × D50³ × Stone density
9. Rock tonnage = Area × Layer thickness × (1 − Void ratio) × Stone density ÷ 1000
Meaning of D50: D50 is the median stone diameter. About half the stones are smaller, and about half are larger.
| Scenario | Velocity (m/s) | Energy Slope | Area (m2) | D50 (mm) | Thickness (mm) | Rock Mass (tonnes) | Class |
|---|---|---|---|---|---|---|---|
| Drain outlet apron | 2.4 | 0.02 | 45 | 500.7 | 751 | 58.21 | Very Heavy Riprap |
| Roadside channel | 3.5 | 0.018 | 120 | 1277.8 | 1916.6 | 396.17 | Extra Heavy Riprap |
| High energy bank | 5.1 | 0.025 | 180 | 3468.4 | 5549.4 | 1806.99 | Extra Heavy Riprap |
A riprap size calculator helps estimate rock sizing for erosion control. It is useful for channels, culvert outlets, embankments, and stormwater protection. Early sizing supports planning, budgeting, and design comparison. This page turns common hydraulic inputs into practical stone guidance and quantity estimates.
Riprap that is too small can shift, roll, or wash away. Riprap that is too large can raise cost and slow installation. Better sizing improves stability and material control. It also helps teams compare supplier stone classes before ordering or writing a scope.
This riprap size calculator estimates median stone diameter, maximum stone diameter, layer thickness, underlayer thickness, average stone weight, boundary shear stress, and total rock tonnage. It also applies a turbulence factor, a side slope adjustment, and a safety factor. Those additions make the result more useful for real sites.
Flow velocity usually drives the result the most. Higher velocity often requires larger rock. Hydraulic radius and energy slope help describe boundary force. Stone specific gravity affects submerged stability. Void ratio affects quantity. Protected area changes tonnage and hauling needs. Thickness factor helps convert stone size into placed layer depth.
D50 is the median stone size. It is a common reference for riprap selection. Dmax gives a practical upper grading check. Layer thickness shows the minimum placed depth for the protective layer. Average stone weight helps field teams understand lifting, handling, and equipment needs before placement begins.
This tool is helpful for drainage swales, roadside ditches, outlet aprons, streambank patches, channel lining reviews, and temporary protection studies. It also supports option testing during concept design. Estimators can compare several cases quickly and create better material allowances for bids and procurement planning.
This calculator uses a simplified screening method. Final design may also depend on geotextile selection, filter layers, local riprap gradation rules, uplift risk, freeze effects, toe support, and construction tolerances. Always confirm the result with project criteria and qualified hydraulic or geotechnical review before construction starts.
Riprap is placed rock used to protect soil from erosion. It is common on slopes, channels, streambanks, culvert outlets, and shorelines where flowing water can remove fine material.
D50 is the median stone diameter. About half the stones in the gradation are smaller than this size, and about half are larger. It is a common design reference.
Yes. It works well for quick culvert outlet checks. Use realistic outlet velocity, slope, and turbulence values. Final apron length and local scour depth still need separate review.
Steeper side slopes reduce stability. Stones are more likely to move or roll. This calculator uses a side slope factor so the recommended riprap becomes more conservative.
A safety factor helps account for uncertainty in field conditions, input estimates, and construction variation. Higher values lead to larger recommended rock sizes and a more conservative result.
No. Void ratio mainly changes the quantity estimate. It affects how much solid rock exists inside the placed layer volume, so it influences tonnage and material planning.
No. This is a practical screening tool. Final design should consider local standards, quarry gradation, filter needs, toe details, and hydraulic conditions that may require a different approach.
Use the value from the quarry or material test when available. A common estimate for dense rock is around 2.65, but real material properties should guide the final value.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.