Estimate how many tons of rip rap you need and its coverage per ton from area and layer thickness, using a typical placed density you can change.
Rip Rap Formula
- L is the length of the area to be armored (ft)
- W is the width of the area (ft)
- T is the rip rap layer thickness (in)
- d is placed bulk density including voids, in US short tons per cubic yard. The 1.5 planning default is about 1,779.83 kg/m³; obtain a project-specific value from the supplier. This is not solid-rock specific gravity
- w is the waste and settling allowance (%)
For a bank of vertical height R and H horizontal units per unit rise, the true slope length is shown below. Multiply it by the length along the bank for surface area. Layer thickness is measured normal to that surface; a vertical thickness is a different geometry. The quantity model does not establish whether a bank slope is stable or suitable for rip rap.
The separate Isbash screening calculation uses the following coefficient convention. In this convention, D50 is the equivalent spherical diameter of the median stone by weight, not the largest stone or a complete gradation. Velocity, coefficient selection and applicability must come from the relevant project design method.
- V is the average water velocity at the rip rap (ft/s or m/s)
- g is the calculator’s reference gravitational acceleration, 9.80665 m/s² (about 32.1740 ft/s²)
- C is the Isbash constant (1.20 for low turbulence, 0.86 for high turbulence)
- S is the specific gravity of the stone (typically 2.5 to 3.0)
Quantity mode computes volume and mass from true surface area, normal layer thickness and placed bulk density, then adds a stated allowance and optional stone cost. Coverage reverses mass ÷ density ÷ normal thickness; an entered strip width can convert area to strip length. Stone-size mode reports the simplified Isbash D50 and arithmetic multiples of 1.5 and 2. These multiples are comparisons, not minimum or preferred design thicknesses; the governing method may also depend on maximum stone size, gradation and placement.
Coverage Per Ton and Slope Multipliers
This table shows geometric coverage per US short ton at the stated depth and an assumed placed bulk density of 1.5 US short tons/yd³. It does not prescribe layer thickness or suitability for a particular shoreline, flow or application.
| Layer depth | Coverage per ton |
|---|---|
| 6 in | 36 sq ft |
| 9 in | 24 sq ft |
| 12 in | 18 sq ft |
| 18 in | 12 sq ft |
| 24 in | 9 sq ft |
The second table is the slope surface multiplier. A common mistake is measuring a bank on a map or from the top of the slope and ordering off that flat footprint. The true rock-covered surface is longer than the horizontal footprint, and the steeper the bank, the bigger the shortfall. Multiply the flat plan-view area by the factor below for your slope to get the real surface area.
| Bank slope (H:V) | Surface multiplier | Extra rock needed |
|---|---|---|
| 4:1 (very gentle) | 1.03 | +3% |
| 3:1 (gentle) | 1.05 | +5% |
| 2:1 (common shoreline) | 1.12 | +12% |
| 1.5:1 (steep) | 1.20 | +20% |
| 1:1 (very steep) | 1.41 | +41% |
Example Problems
Example 1: shoreline quantity. You want to armor a 50 ft stretch of shoreline 12 ft wide with a 12 in layer of rip rap. The area is 50 * 12 = 600 sq ft. The volume is 600 * 1 ft = 600 cubic feet, which is 600 / 27 = 22.2 cubic yards. At 1.5 tons per cubic yard, the base weight is 22.2 * 1.5 = 33.3 tons. Adding a 10% waste allowance gives 33.3 * 1.10 = 36.7 tons, so you would order about 37 tons.
Example 2: Isbash arithmetic. With 8 ft/s, C = 1.20 and stone specific gravity 2.65, the stated equation estimates D50 at about 5.02 in. Twice that value is about 10.05 in. Neither value selects a universal R-class, establishes a final layer thickness or approves a design. Supplier classes and governing hydraulic/gradation requirements must be checked separately.
Rip Rap Calculator FAQs
How many square feet does a ton of rip rap cover? At an assumed placed bulk density of 1.5 US short tons/yd³, one short ton covers 18 ft² at 12 in normal thickness, 36 ft² at 6 in, or 9 ft² at 24 in. Actual coverage depends on the supplied bulk density, layer geometry and placement; obtain the appropriate supplier/project data rather than applying a universal coverage range.
Do I need fabric under rip rap? A properly designed granular filter or compatible geotextile can prevent loss of the underlying soil through the armor. Selection depends on soil retention, permeability, installation and project criteria; a generic fabric choice or a predicted service-life claim is not established by this calculator.
What size rip rap should I use? The Isbash result is one simplified velocity-based screening value. Final selection also depends on the applicable flow velocity definition, turbulence, bank and bed slopes, rock properties, gradation, layer thickness, filter compatibility, scour/toe protection and placement. Universal velocity-to-size ranges or R-class names cannot replace the governing design guidance.
