Riprap Calculator

Riprap volume is calculated by multiplying project length, width, and placement depth, then converting that volume into tons using the density of the specific rock type used, typically 1.4 to 1.7 tons per cubic yard. Placement depth depends on riprap class, with larger stone classes requiring a thicker layer to remain stable against water flow or slope pressure.

For example, a 50 foot by 10 foot slope with an 18 inch (1.5 ft) placement depth requires 750 cubic feet, or about 27.8 cubic yards, roughly 39 to 47 tons depending on stone density. This calculator handles the conversions automatically, so you only need to enter dimensions and pick a stone class.

Tonnage depends on volume and material density. Once you know the cubic yards required for your project, multiply by the density of your chosen riprap type to get total tons. Most suppliers sell and price riprap by the ton rather than by volume, making this the most useful figure for ordering and budgeting.

Because a single cubic yard can weigh anywhere from 1.4 to 1.7 tons depending on the stone, using the correct density matters on larger jobs. The calculator lets you choose a rock type or type the exact density from your supplier so the tonnage reflects the material you are actually buying.

Riprap is graded into classes based on average stone size, and the right class depends on the water velocity or slope conditions it needs to resist. Class I riprap uses smaller stone and suits low flow drainage ditches. Class II and III use progressively larger stone for moderate slopes and stream banks with higher flow. Class IV uses the largest stone and is reserved for high velocity applications like culvert outlets, where smaller stone would be washed away.

Placement thickness scales with class: roughly 12 inches for Class I up to 30 inches or more for Class IV. As a rule of thumb, the layer thickness should be at least the average diameter of the stone so the rock interlocks and stays in place.

Riprap slows water flow and prevents soil erosion on slopes, embankments, and shorelines by dissipating the energy of moving water across an irregular rock surface. Correct placement depth and stone size matter as much as total volume, since undersized stone on a steep or high flow slope can shift or wash out over time, defeating the purpose of the installation.

For slope work, measure the length along the slope face and the height (or width) of the area being protected to get the true surface area rather than the flat footprint, then apply the recommended thickness for your chosen class.

Yes, in most applications. Filter fabric (geotextile) sits between the soil and the riprap layer, preventing underlying soil from migrating up through the gaps in the rock over time. Skipping fabric is a common cause of riprap installations settling or losing effectiveness years after installation, even when the rock itself remains in place.

Fabric is sized to the project footprint plus an overlap allowance where sheets meet, so this calculator adds about 10% to the area when you turn filter fabric on.

A small waste allowance covers uneven subgrade, stones that do not nest perfectly, and losses during placement. Add about 5% for straightforward, uniform slopes and around 10% for irregular slopes or channel and ditch shapes where the rock has to conform to varying cross sections. You can override the waste percentage for your specific site conditions.