Riprap calculator

How the riprap calculator works

Riprap is large, angular rock placed to stop erosion on banks, ditches, shorelines and culvert outlets. The quantity math is simple; the important part is the layer thickness, which depends on the stone size.

area (sq ft) = length of bank × width along the slope face
cubic yards = area × thickness (in) ÷ 12 ÷ 27
tons = cubic yards × about 1.5

Worked example

A bank 50 ft long, 10 ft measured down the slope face, with riprap of 6 inch median size (largest stones about 12 inches):

• Area: 50 × 10 = 500 sq ft
• Thickness: 12 inches = 1 ft, so volume = 500 cu ft, plus 10% = 550 cu ft
• Cubic yards: 550 ÷ 27 = 20.4 cu yd
• Tons: 20.4 × 1.5 = 30.6 tons

Measure along the slope, not across the map

The width that matters is the length of the slope face the rock will sit on. If you only know the height of the bank and its steepness, use this table (H:V means horizontal run to vertical rise):

The face length is √(run² + rise²). Add the toe: riprap is usually carried down into a trench or apron at the bottom of the slope so the water cannot undercut it. Include that area too.

Layer thickness: the stone-size rule

Riprap is described by its gradation: D50 is the median stone size (half the stones by weight are smaller) and D100 is the largest. A widely used rule, found in U.S. Army Corps of Engineers and Federal Highway Administration guidance, is that the layer should be at least D100 thick and at least 1.5 × D50 thick, whichever is greater. With a typical gradation where the largest stones are about twice the median, that works out to about 2 × D50: the same “twice the stone size” idea used for landscape rock on the rest of this site.

Riprap is sold in classes or grades whose names and sizes vary by state transportation department and by quarry. Ask the supplier for the actual D50 and maximum stone size of the class they offer, then pick the closest row or type your own thickness.

Riprap stone sizes and weights

Classes are often specified by stone weight rather than size. A rough conversion for a stone of about 165 lb per cubic foot, treated as a ball:

For individual large stones, the boulder weight calculator gives a better estimate from three measurements.

Weight per cubic yard

Placed riprap weighs roughly 1.4–1.7 tons per cubic yard (2,800–3,400 lb). It is heavier than a loose pile of landscape rock because the angular stones are placed and keyed together, with smaller stones filling gaps. The calculator uses 1.5 tons. Many quarries sell riprap by the ton, so the ton figure is what you will usually order.

Before you place riprap

• Check whether you need a permit. Work in or next to streams, rivers, lakes and wetlands is regulated in many places. Talk to your local authority before you start.
• Get a design for anything that carries real flow. Stone size for a channel or culvert outlet depends on water velocity and is an engineering calculation. This calculator gives quantities once the size is chosen.
• Use a filter layer. Geotextile fabric or a graded gravel bedding under the stone keeps soil from washing out through the gaps.
• Key in the toe. A trench at the base of the slope, filled with riprap, stops the layer sliding or being undercut.

This is a quantity estimate. Riprap density, gradation and the required thickness vary with the stone and the design. Follow your engineer's or agency's specification where one applies.