Riprap Stone Calculator

About the Riprap Stone Calculator

Riprap is large angular stone used to armor slopes, channels, shorelines, and structures against erosion from water flow. It's a permanent erosion control solution that outperforms blankets and vegetation in high-energy environments where water velocity or wave action would wash away lighter materials.

This calculator estimates the riprap tonnage needed based on the area to be protected and the stone layer thickness. Standard riprap classifications range from Class I (6-inch stone) to Class VI (36-inch stone), with layer thickness typically 1.5 to 2 times the median stone diameter.

Whether you're protecting a culvert outlet, lining a drainage channel, or stabilizing a pond bank, This calculator converts your area and thickness into the cubic yards and tons of riprap to order.

When This Page Helps

Riprap is sold by the ton, and the tonnage depends on area, thickness, and stone density. This calculator bridges those variables so you can get an accurate quote from your stone supplier.

How to Use the Inputs

1. Measure the area to be protected in square feet.
2. Determine the required stone layer thickness (in inches or feet).
3. Confirm the stone density (1.4–1.6 tons/yd³ typical).
4. Review the total volume in cubic yards.
5. Check the weight in tons for ordering.
6. Add 10% for irregular surfaces and waste.

Example Calculation

Tips & Best Practices

• Place geotextile filter fabric under riprap to prevent soil migration through the stone.
• Use angular, durable stone (granite, limestone) — avoid soft or rounded stone.
• Layer thickness should be at least 1.5× the maximum stone diameter.
• Toe the riprap into the channel bottom or slope base for anchoring.
• Specify the stone size class in your order — not just tonnage.
• Machine-placed riprap is more uniform than dump-and-spread methods.

Riprap Size Classifications

Riprap is classified by median stone diameter (D50): Class I (6 in), Class II (9 in), Class III (12 in), Class IV (18 in), Class V (24 in), Class VI (36 in). Each class has a grading range with minimum and maximum sizes for proper interlocking.

Application Areas

Common riprap applications: culvert outlets and inlets, channel linings, bridge abutments, shoreline protection, dam spillways, slope stabilization, and stormwater outfall protection. Each has specific sizing and thickness requirements.

Filter Layer Requirements

Beneath riprap, install either a non-woven geotextile fabric or a graded stone filter layer. Geotextile is standard for most applications. For high-velocity flows, a granular filter layer is preferred because fabric can tear if stones shift.

Calculating for Slopes

When riprap is placed on slopes, remember to calculate the actual slope area (not the plan-view area). Slope area = plan area ÷ cos(slope angle). For a 2:1 slope, the surface area is about 12% more than the plan-view area.

Frequently Asked Questions

• What size riprap do I need?

  Riprap size depends on water velocity. Low flows (under 5 fps): 6‒12 inch stone. Moderate flows (5‒10 fps): 12‒18 inch. High flows (10‒15 fps): 18‒24 inch. Very high flows or wave action may need 24‒36 inch stone.

• How thick should the riprap layer be?

  The layer thickness should be at least 1.5 times the maximum stone diameter, with a minimum of 12 inches. For example, if using 12-inch stone, the layer should be at least 18 inches thick.

• How much does riprap cost?

  Riprap costs $25–$75 per ton depending on stone size and location. Delivery adds $5–15 per ton. Class I (small) is cheapest; Class V–VI (large) is most expensive. Placement labor adds $30–60 per ton.

• Do I need filter fabric under riprap?

  Yes, almost always. A non-woven geotextile filter fabric prevents soil from migrating up through the stone voids, which would undermine the riprap layer. This is a standard engineering requirement.

• What is the density of riprap?

  Placed riprap (including voids) weighs 1.3–1.6 tons per cubic yard, depending on stone type and void ratio. Granite is heavier (~1.5 tons/yd³). Limestone is slightly lighter (~1.4 tons/yd³). Use your supplier's density.

• Can I use riprap for landscaping?

  Yes, riprap creates attractive, low-maintenance landscape features. Use it around ponds, along drainage swales, in dry creek beds, and as decorative borders. Smaller classes (6‒12 inch) work best for residential landscaping.