# Rip Rap Calculator

URL: https://speedycalculator.com/calculators/construction/rip-rap/
Last updated: 2026-10-25

> The rip rap calculator sizes rock armour for channels and outlets with the Isbash equation – median stone diameter D₅₀ and weight from the water velocity – and estimates the layer thickness, volume and tonnage for the area.

## Inputs

- Water velocity (m/s, ft/s, km/h, mph)
- Specific gravity of rock
- Isbash coefficient
- Length of area (ft, m, in, yd, cm)
- Width of area (ft, m, in, yd, cm)
- Rock price (/US ton, /t, /lb, /kg)

## Results

- Median stone diameter D₅₀
- Median stone weight W₅₀
- Layer thickness (2 × D₅₀)
- Rock volume
- Rock tonnage
- Cost

## Formula

```
D₅₀ = V² ÷ (2 g C² (S − 1))
W₅₀ = π/6 × D₅₀³ × S × ρ_water
Layer thickness = 2 × D₅₀; tonnage = area × thickness × bulk density
```

## Explanation

Rip rap is large angular rock placed on banks, culvert outlets and shorelines to stop erosion. The stones must be heavy enough that fast-flowing water cannot move them. The Isbash equation, used by the U.S. Army Corps of Engineers and FHWA, relates the median stone size to the velocity, using a coefficient of 0.86 for highly turbulent flow and 1.20 for low turbulence.

### How to use the Rip Rap Calculator

- **Water velocity**
- **Specific gravity of rock**
- **Isbash coefficient**
- **Length of area**
- **Width of area**
- **Rock price**
- Every result field is editable: type into **Median stone diameter D₅₀** and **Water velocity** plus every other field update to match.

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter the design velocity, the specific gravity of the rock, the turbulence level and the area to protect. The calculator finds D₅₀ and the median stone weight, sets the layer thickness to twice D₅₀ and estimates the volume and tonnage with 40% voids.

### Tips for an accurate estimate

- Sketch the job and split irregular areas into rectangles, circles and triangles before you measure.
- Add a waste allowance – typically 5–10% for simple layouts and 15–20% for diagonal, curved or heavily cut work.
- Check the coverage or yield printed on the product, because sizes and densities vary between manufacturers.
- Buy all units of a finished material (tile, brick, siding) from the same batch so colours match, and keep a few spares for repairs.
- Confirm local prices – quarry, lumber and steel prices vary widely by region and season.

### Common mistakes to avoid

- Ordering exactly the calculated amount with no allowance for breakage, offcuts and settlement.
- Forgetting to subtract doors, windows and other openings – or subtracting small ones that still need cutting around.
- Confusing US short tons (2,000 lb) with metric tonnes (1,000 kg) when comparing quotes.

### Inputs explained

FieldWhat to enterWater velocityUnits: m/s, ft/s, km/h, mphSpecific gravity of rockA numberIsbash coefficientChoose one: 0.86 – high turbulence, 1.20 – low turbulenceLength of areaUnits: ft, m, in, yd, cmWidth of areaUnits: ft, m, in, yd, cmRock priceUnits: /US ton, /t, /lb, /kg

### What each result means

- **Median stone diameter D₅₀** = [Water velocity]^2÷(2×9.80665×[Isbash coefficient]^2×([Specific gravity of rock] - 1))
- **Median stone weight W₅₀** = pi÷6×[Median stone diameter D₅₀]^3×[Specific gravity of rock]×1000
- **Layer thickness (2 × D₅₀)** = 2×[Median stone diameter D₅₀]
- **Rock volume** = [Length of area]×[Width of area]×[Layer thickness (2 × D₅₀)]
- **Rock tonnage** = [Rock volume]×[Specific gravity of rock]×1000×0.6
- **Cost** = [Rock tonnage]×[Rock price]

### Example values

How the main result changes when **Water velocity** changes and everything else stays at the example values above:

Water velocity (m/s)Median stone diameter D₅₀ (in)1.53.72.258.33314.83.7523.134.533.31659.22

## Worked example

Inputs — Water velocity: 3 m/s; Specific gravity of rock: 2.65; Isbash coefficient: 0.86 – high turbulence; Length of area: 30 ft; Width of area: 10 ft; Rock price: $50 /US ton.

Results — **Median stone diameter D₅₀ = 14.8 in**; Median stone weight W₅₀ = 162.6 lb; Layer thickness (2 × D₅₀) = 29.6 in; Rock volume = 27.41 yd³; Rock tonnage = 36.74 US ton; Cost = $1,836.81.

## Key terms

- **D₅₀**: The stone diameter that half of the rock (by weight) is smaller than.

## FAQ

**What is rip rap?**
Large, angular stone placed to armour banks and outlets against erosion.

**What size rip rap do I need?**
It depends on the velocity: about 6 in D₅₀ for 6 ft/s and 12 in for 8–9 ft/s in turbulent flow. Use the calculator for your velocity.

**Why is the layer twice D₅₀?**
A layer about 1.5–2 times the median stone size lets stones interlock and protect the filter below.

**What formula does the Rip Rap Calculator use?**
D₅₀ = V² ÷ (2 g C² (S − 1)).


## Sources

- [FHWA HEC-11 – Design of Riprap Revetment](https://www.fhwa.dot.gov/engineering/hydraulics/)