Rip Rap Calculator

Work out the sloped area (bank length × slope length), multiply by the layer thickness for the volume, then by the placed density for the tonnage. A 33 ft long, 13 ft run 2:1 slope (10 m × 4 m run) is about 480 sq ft (45 m²) of face; at a 12 in (300 mm) light-class layer that is roughly 18 yd³ (14 m³), or about 25 US tons (22 t) at 1.6 t/m³ with a 10% voids allowance. The calculator does the slope maths and the units for you.

Riprap covers the sloped face, which is longer than the flat ground it sits on — the “slope factor”. A 2:1 slope has about 12% more surface area than its footprint, a 1.5:1 slope about 20% more, and a 1:1 slope over 40% more. Enter the run and the slope ratio and the calculator scales the area up automatically, so you do not come up short.

A common rule is 1.5–2 times the median stone size (D50): about 12 in (300 mm) for 6 in stone, 18 in (450 mm) for 9 in stone, 24 in (600 mm) for 12 in stone. The calculator’s class presets set a typical thickness; on a designed job the D50 and thickness come from the water velocity, so follow the drawing.

Bigger, faster water needs bigger stone. A common first-pass is the Isbash equation: D50 = V² ÷ (C² × 2g × (Sg − 1)), where V is the flow velocity, Sg the rock’s specific gravity (~2.65), g gravity, and C an Isbash coefficient set by turbulence (about 1.2 for a plain bank, 0.86 for turbulent flow near a structure). Switch the calculator to “Size from flow”, enter the velocity, turbulence and specific gravity, and it works out the required D50 and layer thickness, then the tonnage. It is an estimate for straight, uniform banks — steep chutes, bridge scour or big rivers need HEC-11 / USACE methods and an engineer.

D50 is the median stone size — half the rock by weight is larger, half smaller. Riprap is sold in graded classes (by D50 or by DOT class names) so the mix of sizes locks together and resists being washed out. Bigger, faster water needs a larger D50 and a thicker layer. Pick the class and the calculator uses its typical thickness.

Almost always. A geotextile filter fabric (or a graded bedding-stone layer) goes under the riprap so the soil beneath cannot wash out through the gaps between the stones, which would undermine the armour. Also key the bottom row into a toe trench so the layer cannot slide down the slope. The calculator includes the fabric area in the quote hand-off.