What size rock bag do I need?
Enter the design flow velocity at your site and this will point you at a starting size, using the published grouped current resistance for each bag. It narrows the shortlist before you talk to an engineer — it does not replace one.
Grouped current resistance by size
These are the figures the calculator matches against. Each rating is the grouped current resistance — bags placed together as an armor layer, not a single bag standing alone.
| Size | Grouped current resistance | Volume | Diameter | Typically specified for |
|---|---|---|---|---|
| 1-Ton Rock Bag | ~13.1 ft/s | 0.6 m³ | 1.5 m | Light scour, small streambanks, residential and lakefront projects. |
| 2-Ton Rock Bag | 15.4 ft/s | 1.13 m³ | 1.9 m | Most-specified size — streambanks, shoreline revetment, and light-to-moderate scour. |
| 4-Ton Rock Bag | 17.4 ft/s | 2.71 m³ | 2.4 m | Heavy-civil size — bridges, riverbanks, and port toes in higher-energy conditions. |
| 8-Ton Rock Bag | 19.4 ft/s | 6.0 m³ | 3.0 m | Offshore, oil & gas, and severe-current coastal applications. |
Above 19.4 ft/s, or where loading is not steady current, the size comes out of a project-specific review — see custom rock bags.
The method behind the number
Each bag size carries a published grouped current resistance — the flow velocity an armor layer of that size is rated to hold. The calculator takes your design velocity and returns the smallest size whose rating meets or exceeds it, because oversizing costs money and undersizing loses the bank.
Velocity is the governing variable for steady-current scour, which is why it is the input. For the reasoning in full, including how to pick the design event, read sizing flow velocity to bag size.
What this does not account for
- Water depth, turbulence, and impinging jets at culvert and outfall discharges
- Wave action, wake energy, and combined wave-current loading on coastal and marine sites
- Bed material, slope angle, and whether geotextile underlayment is required
- Debris and ice impact, vessel contact, and seismic or scour-hole geometry
- Local agency or DOT specifications that mandate a particular unit or test standard
Treat the output as a shortlist, not a design. Final selection belongs to the engineer of record for your project. See also when not to use rock bags.
How FES lays the bags out
Picking a size is the first step. Turning it into a bag count and an arrangement follows three rules, and they are the same rules our engineering team applies when we size a job for you.
- Rule 1 · Coverage
85–90% of bag diameter
Each bag is counted as covering 85–90% of its own diameter, not 100%. That allowance is what sets the linear requirement — divide the run length you need to armor by the effective coverage to get bags per course.
- Rule 2 · Height
Stacks = requirement ÷ bag height
The number of stacked courses is the protection height your design calls for divided by the height of the chosen bag. A taller requirement is met by adding courses rather than by moving to a larger unit.
- Rule 3 · Arrangement
Slope sets vertical or stairstep
The slope of the bank decides whether the stack is built vertically or stepped back as a stairstep profile. Steeper banks step; a near-vertical face stacks straight.
These rules give a defensible starting quantity. They do not account for waste, transitions into existing protection, keyed-in toe courses, or the geometry of an existing scour hole — all of which change a real bill of quantities. Send the site details and we will work the layout through with you.
FES Solutions is a supplier, not the engineer of record. This calculator and the rules above are a starting point for discussion, not a site-specific design. Consult a qualified engineer for the design of your project — scour depth, foundation conditions, hydraulic loading, and the governing specification are their call, not ours.
Send us the design event, depth, and site conditions and our engineering team will confirm the size — or call +1 512-766-6608.