Rip Rap Calculator
Estimate stable median rock size for flowing water, or calculate riprap volume, weight, and material cost for a defined coverage area.
Project inputs
Live results
Hydraulic sizing checkpoints
| Item | Value | Unit | Interpretation |
|---|---|---|---|
| Approach velocity | 2.00 | m/s | Entered design velocity |
| Median diameter D₅₀ | 0.0944 | m | Estimated stable median stone size |
| Layer thickness guide | 0.1888 | m | Two times D₅₀ planning check |
How to use this rip rap calculator
What this calculator does. This tool supports two related planning tasks. In Rock sizing from water velocity mode, it estimates the median stone diameter, D₅₀, with the Isbash equation for rock exposed to flowing water. In Rip rap quantity and cost mode, it converts a coverage area and layer depth into placed volume, bulk weight, and an optional material cost. It is suitable for preliminary channel, bank, outlet, ditch, dike, and erosion-control estimates. It does not design a complete revetment, confirm a graded filter, account for wave attack, determine toe geometry, or replace an engineer's site-specific review.
When to use it. Use the sizing mode when comparing candidate stone gradations for a known design velocity, checking how turbulence changes the median size, or preparing an early supplier inquiry. Use quantity mode when developing a takeoff for a mapped area, comparing layer depths, adding a construction allowance, or estimating the material portion of a budget. For formal design, consult the FHWA Design of Riprap Revetment guidance and project criteria.
How to calculate. The calculator opens with a complete hydraulic example, so the first result and a validated example workbook are available immediately.
- Choose Calculation mode. The form shows the inputs relevant to sizing or quantity.
- Replace the demonstration values with project values. Results update live; no Calculate button is needed.
- Read the primary result, supporting cards, and checkpoint table. Review the assumptions beside the result before using it for ordering or design discussions.
- Select Download Excel to create a current-state workbook with Summary, Inputs, and Breakdown sheets.
- Select Reset to clear all demonstration data. Reset may disable Download Excel until every required field in the selected mode is complete and valid again.
Input guide. Calculation mode is required and selects one of the two models. In sizing mode, Water velocity is a required positive decimal in meters per second; for example, 2.0 m/s. A higher velocity increases D₅₀ with the square of velocity, so small velocity changes can materially affect stone size. Do not enter discharge in this field. Flow turbulence is required: low turbulence uses C = 1.20, while high turbulence uses C = 0.86 and produces a larger stone estimate for the same velocity. Choose based on hydraulic conditions, not desired output. Gravitational acceleration is required, in m/s²; 9.806 is the normal Earth value. Rock specific gravity is required, dimensionless, and must exceed 1; 2.5 is a realistic example. Higher specific gravity reduces the calculated diameter because denser stone has greater submerged resistance.
In quantity mode, Coverage area is required in square feet, such as 1,500 ft². Rip rap depth is required in inches, such as 12 in; doubling depth doubles volume and weight. Wastage allowance is required from 0% through 100%; 10% is a common planning allowance but must reflect site geometry and handling. Bulk rock density is required in pounds per cubic foot; 104 lb/ft³ is the opening example, but a supplier's tested or quoted value is preferable. Do not substitute solid mineral density for delivered bulk density. Material cost per US ton (optional) accepts a nonnegative dollar amount such as $48.00. Leave it blank when price is unknown; the physical takeoff still calculates.
Output guide. In sizing mode, Median rock diameter D₅₀ is the estimated diameter at which half the rock mass is finer and half is coarser; it is shown in inches and centimeters. Minimum layer guide is two times D₅₀, a planning check rather than a universal specification. The cards also repeat the selected Isbash constant and Specific gravity. The checkpoint table reports approach velocity, D₅₀ in meters, and the two-D₅₀ layer check. In quantity mode, Total rip rap weight is the primary ordering estimate in US tons. Supporting outputs show net volume, volume including wastage, bulk weight in pounds, and estimated material cost when a unit price is supplied. The table separates base quantity, wastage, and total quantity so the allowance remains auditable.
Worked example. The opening sizing example uses 2.0 m/s water velocity, low turbulence C = 1.20, gravity 9.806 m/s², and rock specific gravity 2.5. The calculation is D₅₀ = V² ÷ [2 × g × C² × (S – 1)] = 4 ÷ [2 × 9.806 × 1.20² × 1.5] = 0.09442 m. That equals 9.44 cm or 3.72 in. The two-D₅₀ layer guide is therefore about 0.1888 m, 18.88 cm, or 7.44 in. These values match the first-open display and workbook checkpoints.
Learn more. Riprap performance depends on hydraulic loading, gradation, layer thickness, foundation preparation, filters, and placement. The USACE HEC-RAS riprap sizing documentation explains a more detailed rock-gradation workflow for river hydraulics.
Formula and planning assumptions
D₅₀ = V² / [2 × g × C² × (S – 1)]
The Isbash relation is an empirical screening model. This implementation uses C = 1.20 for low turbulence and C = 0.86 for high turbulence. The quantity model uses volume = area × depth, adjusted volume = volume × (1 + wastage), weight = adjusted volume × bulk density, and cost = weight in US tons × price per ton. The USACE hydraulic design manual for flood-control channels provides broader channel design procedures and riprap context.
Interpreting the estimate responsibly
A calculated D₅₀ is not the same as a complete gradation specification. Procurement may require minimum and maximum stone sizes, allowable percentages passing designated sieve or size limits, shape and durability criteria, and placement requirements. Likewise, a quantity estimate based on plan area assumes a reasonably uniform thickness. Irregular banks, trenches, keyways, toe aprons, transitions, and settlement can add material beyond a simple area-times-depth calculation.
Velocity alone may not capture local acceleration, bends, turbulence, drop structures, culvert jets, debris, wave loading, ice, or rapidly varied flow. The FHWA flexible channel lining guidance discusses lining selection and hydraulic stability for roadside channels. Confirm the design basis and applicable agency criteria before construction.