Rip rap is sized by class, not by the dimensions you would use to order most other aggregate. A spec sheet calls for “R-4” or “medium stone fill,” and unless you work with it regularly, those labels do not tell you what is going to show up on the truck.
This is a plain-language starting point: roughly what each class measures, what it is generally used for, and how the Pennsylvania and New York naming systems line up. It is a guide, not a specification. Sizes and availability vary by quarry and by project requirement, so confirm the details with your NESL sales representative before you order.
Key Takeaways
- Rip rap is sized by class. In Pennsylvania, classes run R-3 through R-8; lower numbers are smaller stone.
- The classes that carry most orders: R-3 (roughly 2″–6″), R-4 (roughly 3″–12″), and R-6 (roughly 6″–24″).
- New York uses stone fill classes instead of R-numbers, and they map almost directly: fine stone fill ≈ R-3, light ≈ R-4, medium ≈ R-6, heavy ≈ R-7/R-8.
- No class of rip rap is stackable, R-3 through R-8. It is placed, not built with.
- Gabion baskets have their own sizing rule, separate from rip rap classes.
- NESL produces R-3 through R-8 across Pennsylvania and stone fill classes in Western New York, though not every class is stocked at every location.
- Sizes here are typical ranges for orientation. Always confirm the governing spec and current availability before ordering.
How Rip Rap Is Sized
Most construction aggregate is sized by sieve: a #57 stone passes a 1-inch screen and is retained on a #4. Rip rap is too large for that approach to be practical, so it is classified by a size range and a gradation specification instead.
In Pennsylvania, PennDOT uses the R-series: R-3 through R-8, with lower numbers indicating smaller stone. Each class has a defined gradation that controls how much of the material may fall above or below the target range. In New York, NYSDOT uses descriptive stone fill classes (fine, light, medium, and heavy) covering effectively the same material.
Both systems exist to answer the same question: is this stone large enough to stay put under the water flow it will face?
The Working Size Chart
| Class | NY equivalent | Top size (100% passing) | Working range | Typical use |
|---|---|---|---|---|
| R-3 | Fine stone fill | 6″ | 2″ – 6″ | Lining banks alongside roadways and creeks; gabion fill |
| R-4 | Light stone fill | 12″ | 3″ – 12″ | Bank lining where flow is heavier; channel protection |
| R-5 | not listed | 18″ | 4″ – 18″ | Transitional applications between R-4 and R-6 |
| R-6 | Medium stone fill | 24″ | 6″ – 24″ | “Bumper rock,” creek lining around bends |
| R-7 | Heavy stone fill | 30″ | 12″ – 30″ | Larger bumper rock, heavy creek lining |
| R-8 | Heavy stone fill | 42″ | 15″ – 42″ | Heaviest armoring applications |
Working ranges above are approximate and intended for orientation. The gradation specification, not the inch range, is what governs on a specified project.
The R-series starts at R-3. Pennsylvania does not recognize an R-1 or R-2 class; for material smaller than R-3, see the AASHTO stone size chart.
A note on New York sizing. NESL’s New York product sheet lists heavy stone fill at “36 inches.” That is a practical description rather than the spec figure. The gradation standard puts R-7 at a 30″ top size and R-8 at 42″. On specified work, order to the R-class and the gradation, not the descriptive range.
Two related products often get ordered alongside rip rap:
Surge stone (4″–12″) is used for the same kind of bank lining work but is a less strictly graded material. Where a spec does not require a defined R-class, surge is frequently the more economical choice.
Landscape stone (12″–36″) is sized like rip rap but selected for appearance rather than performance: borders, garden features, and decorative placement.
Choosing a Class
The class you need is driven by how fast the water moves and how much energy it carries. The notes below are general guidance for common situations. On engineered work, the class is set by the project drawings or the engineer, not by rules of thumb.
R-3 (2″–6″). The smallest class in common use. Appropriate for drainage swales, gentle bank protection, and culvert aprons where flow is modest. Also the standard fill for gabion baskets. Small enough to place by hand where a machine cannot reach.
R-4 (3″–12″). The general-purpose class. Most roadside bank lining, most residential and light commercial erosion control, most channel protection on moderate flow. If a project calls for rip rap without further specification, R-4 is often what is intended.
R-5 (4″–18″). The step between general-purpose and heavy protection. Specified where flow carries more energy than R-4 is meant for but does not yet call for R-6: steeper channels, outlet protection at moderate velocity, and bank sections taking more current than the straightaways. Top size 18 inches, machine-placed. New York’s stone fill classes have no direct equivalent, which is why R-5 appears without an NY match in the chart above.
R-6 (6″–24″). Where flow is fast or the bank is taking direct impact: outside of stream bends, culvert outlets discharging at velocity, or shoreline exposed to wave action. This is where “bumper rock” starts. It is placed with an excavator, and it does not stack; each stone sits where it lands.
R-7 (12″–30″) and R-8 (15″–42″). Heavy armoring for high-velocity flow, large culvert outfalls, and bridge scour protection. These are engineered placements, generally specified by a civil engineer rather than selected on site.
What “Not Stackable” Means
No class of rip rap is stackable. That applies across the full range, R-3 through R-8, and the distinction matters more than it sounds.
Rip rap is placed, not built with. The stones are irregular by design, and they resist movement through their own weight and their contact with the slope, not by keying together into a structure. Placing any class of rip rap, from R-3 up through R-8, produces an armoring layer, not a stack that can be built on.
Practically: do not plan to build a vertical or near-vertical face out of rip rap of any class. If a project needs a structural wall, that is a retaining wall or a gabion system, not a rip rap placement. Rip rap armors a slope. It does not hold one up.
Gabions
Gabion baskets use rip rap as fill, but the sizing rule is different. The stone must be large enough not to pass through the wire mesh, and small enough to fill the basket without leaving large voids.
Under PennDOT Section 626.2.a.1, the requirement is keyed to basket height:
| Basket height | Min. stone size | Max. stone size |
|---|---|---|
| Less than 1 foot | 3 inches | 5 inches |
| 1 foot and over | 4 inches | 8 inches |
In practice this means R-3 for most gabion work, with R-4 in taller baskets. If a spec calls for gabions and a rip rap class in the same sentence, the class is describing the fill.
Estimating Quantity
Rip rap is sold by the ton and placed by volume, so the conversion matters.
Rip rap runs heavier than standard crushed stone, at roughly 1.5 to 1.7 tons per cubic yard against about 1.4 for typical aggregate. Placement thickness is usually specified as a multiple of the stone size, commonly around twice the nominal diameter, which means larger classes require substantially more tonnage per square foot of coverage.
These figures are rough planning numbers, not design values. For a rough take-off: calculate the area to be covered, multiply by the specified placement depth in feet, divide by 27 for cubic yards, then multiply by 1.6. Add 10 percent, because rip rap placement wastes more than graded aggregate does. The stones do not nest.
The NESL product calculator handles the volume-to-tonnage step. Most rip rap projects also need a geotextile filter fabric underneath. Without it, water moving through the stone pulls fine soil out from below and the placement settles unevenly. Confirm the fabric requirement before ordering stone.
Gradation Requirements
Each class carries a defined gradation, meaning the percentage of material permitted to pass each sieve size. Classification follows NCSA size numbers, which PennDOT adopts.
The table below is reproduced for reference. Specifications are revised periodically, so check the current governing publication for your project before relying on these figures.
| Rock size (mm) | Rock size (in) | R-8 | R-7 | R-6 | R-5 | R-4 | R-3 |
|---|---|---|---|---|---|---|---|
| 1070 | 42 | 100 | |||||
| 760 | 30 | 100 | |||||
| 610 | 24 | 15–50 | 100 | ||||
| 460 | 18 | 15–50 | 100 | ||||
| 380 | 15 | 0–15 | |||||
| 300 | 12 | 0–15 | 15–50 | 100 | |||
| 230 | 9 | 15–50 | |||||
| 150 | 6 | 0–15 | 15–50 | 100 | |||
| 100 | 4 | 0–15 | |||||
| 75 | 3 | 0–15 | 15–50 | ||||
| 50 | 2 | 0–15 | |||||
| 37.5 | 1.5 | ||||||
| 25 | 1 | ||||||
| 19 | 0.75 | ||||||
| 2.5 | 0.1 |
Values are percent passing. Blank cells are not specified for that class.
How to read it. Take R-4. One hundred percent of the material passes a 12-inch screen, so 12 inches is the top size. Between 15 and 50 percent passes a 6-inch screen, which is where the bulk of the material sits. No more than 15 percent passes a 3-inch screen, which caps how much undersized material is allowed. That combination is what makes a load R-4 rather than R-3 or R-5.
On PennDOT-funded work, the supplier provides documentation confirming the delivered material meets the specified gradation from an approved source. If your project requires that documentation, request it when you place the order rather than after the truck is loaded.
Where to Get It
NESL produces R-3 through R-8 at quarries across Pennsylvania, including locations serving the Altoona, Chambersburg, Harrisburg, Lancaster, Lehigh Valley, Lewisburg, and Scranton areas. Western New York locations carry the equivalent stone fill classes.
Not every class is stocked at every location, and availability changes with production and demand. Check with the plant nearest your project before planning around a particular class.
Because rip rap is heavy and freight is a large share of delivered cost, the nearest producing quarry usually determines the economics of a project. Find your closest NESL location to check what is stocked in your area.
Ready to Order?
Know your class and quantity? Request a quote and a NESL representative will follow up.
Not sure which class fits your project? Describe the application, including flow conditions, slope, and whether the work is state-specified, and our dispatch team will recommend a class.
Before You Order
A few things worth repeating:
This article is a general guide. It is written to help you understand what the class numbers mean and roughly what each one is for. It is not a specification document, and it should not be used in place of one.
The project spec governs. On any PennDOT, NYSDOT, municipal, or engineered project, the governing specification, the project drawings, and the engineer of record determine what material is required. Where anything here differs from those, follow those.
Specifications change. Gradation standards and DOT publications are revised periodically. The figures here were accurate to the sources available when this was written; confirm against the current publication for your project.
Availability varies. Production differs by quarry and changes over time. A class listed here may not be stocked at the location nearest you.
When in doubt, call. Describe the application, including flow conditions, slope, and whether the work is state-specified, and our sales and dispatch teams will help you land on the right class. That conversation takes a few minutes and is considerably cheaper than a load of the wrong stone.