Use this chart to translate measured particle diameter into a consistent scientific size class. Supplier names such as “landscape rock,” “pea gravel” or “river rock” are commercial labels and may not match one universal diameter, so verify the nominal size listed for the material you are buying.
Rock and gravel size classes by particle diameter
| Size class | Diameter (mm) | Approx. diameter (in) | How to read it |
|---|---|---|---|
| Very fine pebble | 2 to <4 | 0.08 to <0.16 | Small gravel-size particles. |
| Fine pebble | 4 to <8 | 0.16 to <0.31 | Measure the nominal particle, not the bag or pile. |
| Medium pebble | 8 to <16 | 0.31 to <0.63 | Particle diameter doubles across the class. |
| Coarse pebble | 16 to <32 | 0.63 to <1.26 | Useful as a physical-size comparison, not a product name. |
| Very coarse pebble | 32 to <64 | 1.26 to <2.52 | Still within the pebble class in the cited scale. |
| Fine cobble | 64 to <128 | 2.52 to <5.04 | Larger individual stones. |
| Coarse cobble | 128 to 256 | 5.04 to 10.08 | Upper cobble range before boulders. |
| Boulder | >256 | >10.08 | Particles larger than the cobble range. |
This is a grain-size classification reference. It does not claim that a retailer or quarry uses these exact class names for a product. For ordering, use the supplier’s stated nominal size and grading.
Why product names and physical sizes are different
A scientific grain-size class describes particle diameter. A commercial landscape-rock name can also communicate color, source, shape, screening, finish or local trade usage. That is why a product name alone should not be converted into an exact diameter without supplier documentation.
When comparing two products, record the stated nominal size in inches or millimeters first. Then use the chart above to understand the physical scale of the particles.
Size does not determine quantity or weight by itself
Project quantity still depends on area and placed depth. Weight also requires an applicable bulk density. A rock-size label should therefore be treated as one input to material selection, not as a substitute for volume or density.
For quantity, use a calculator matched to the material task. For weight, keep moisture, grading and supplier density assumptions visible.
How to compare a supplier listing
- Find the supplier’s nominal particle size or screened size range.
- Convert inches to millimeters only if needed; do not infer size from a product nickname.
- Compare the measured or stated diameter with the USGS-derived classes in the chart.
- Use separate volume and weight inputs when estimating how much material the project needs.
Questions about landscape-rock sizes
Is “pea gravel” one exact size?
Do not assume an exact diameter from the name alone. Confirm the nominal size or range on the specific supplier listing.
Is a larger rock automatically heavier per cubic yard?
No single weight follows from particle diameter alone. Bulk weight also depends on material density, grading, void space and material condition.
Can this chart choose the best rock for drainage or a driveway?
No. The chart is a physical-size reference. Project-specific performance, base design and drainage decisions require additional project and material information.
Classification source: U.S. Geological Survey — Standardized method for logging drill core, grain-size table. Unit conversions are rounded for display.