Gravel & aggregate glossary

The words suppliers use — #57, CA6, crusher run, dense-grade, riprap, fines — in plain English, so you order the right material and read a quote without guessing.

Sizes and grades

Aggregate: The industry word for any crushed stone, gravel, or sand used in construction and landscaping. Gravel is one kind of aggregate.
AASHTO number (#57, #8, #4): A sieve-size grade from the AASHTO M43 standard. The lower the number, the larger the stone: #1 is 2.5–4 inch rip-rap, #4 is ~1.5 inch, #57 is ~3/4 inch, #8 is ~3/8 inch, #10 is stone dust. Full size chart.
#57 stone: The most-stocked US crushed stone — angular fragments around 3/4 inch with no fines. Drains well and is the standard driveway top course, French-drain fill, and paver-base stone. About 2,750 lb per cubic yard. #57 stone calculator.
#8 stone: Crushed stone about 3/8–1/2 inch — smaller than #57. Used as paver bedding and as a fine leveling course over a larger base.
#4 stone: Larger crushed stone, roughly 1.5–2.5 inch. A deep drainage or sub-base layer under #57 on heavy-traffic driveways; too large to walk on comfortably.
Screenings / stone dust: The finest crushed product (#10) — sand-to-dust sized particles left after screening. Used as paver joint filler and a tight leveling layer.
MOT Type 1: A UK sub-base spec: graded crushed stone with fines, 0–40 mm, that compacts hard under driveways and paving. The closest US equivalent is CA6 / 21AA / Class 5.

Base materials

CA6 / 21AA / Class 5: The same dense-grade base material under three regional names (Illinois, Michigan, Minnesota) — crushed stone blended with fines that compacts to near-concrete hardness. The right material under a driveway, paver patio, or shed pad. About 2,650 lb per cubic yard. 21AA / CA6 calculator.
ABC (Aggregate Base Course): The Southern US name for a dense-grade base aggregate with fines. Functionally the same as CA6 and crusher run — a compactable driveway base.
Crusher run / crush and run: Crushed stone straight from the crusher with all the fines still in it, so it compacts hard. A common, low-cost driveway base. Same idea as ABC and CA6. Crusher run calculator.
Dense-grade aggregate (DGA): Any aggregate blended across a range of sizes down to fines so it packs tight with few voids. Dense-grade materials (CA6, ABC, 21AA) are for compactable bases, not drainage.
Class A / Class 5 gravel: DOT names for dense-grade base blends. Numbering is set by each state, not nationally, so 'Class 5' or 'Class A' means a compactable base in one region and something else in another — confirm the local spec.
Bank run / pit run: Unprocessed gravel dug straight from a pit, unscreened, with mixed sizes and often sand or clay. Cheap fill, but inconsistent — not a spec base material.

Loose and decorative stone

Pea gravel: Smooth, rounded stones about 3/8 inch. Comfortable underfoot and popular for paths, play areas, and decorative beds, but it rolls and doesn't compact — never use it as a driveway or structural base. Pea gravel calculator.
River rock: Smooth, water-rounded stone 1–3 inches, used decoratively in beds and dry creeks. It does not compact, so it stays loose. River rock calculator.
Crushed stone: Mechanically crushed bedrock with angular, sharp faces that interlock and compact — unlike naturally rounded gravel. The numbered grades (#57, #8, #4) are all crushed stone. Crushed stone calculator.
Decomposed granite (DG): Crushed granite worn to a sandy, fine texture. Compacts to a firm, permeable path surface; common in the western US.
Riprap: Large stone (4 inches and up) placed for erosion control on banks, shorelines, and steep slopes. Not a surface you walk or drive on.
Lava rock: Lightweight, porous volcanic stone used as a decorative mulch substitute. At ~1,500 lb per cubic yard it weighs far less than crushed stone, so it's easy to over-order by weight.

Drainage and site prep

Open-graded aggregate: Stone of a single size with the fines washed out, leaving large voids for water to move through. Clean #57 is the classic open-graded drainage stone — the opposite of a dense-grade base.
Fines: The sand- and dust-sized particles in an aggregate. Fines make a material compact hard (good for a base) but block drainage (bad for a French drain).
Geotextile fabric: A woven or non-woven fabric laid between soil and gravel. It separates the two so the stone doesn't sink into soft ground, and it roughly doubles the life of a driveway or path.
Subgrade: The native soil beneath your gravel. Below about 6 inches of stone, the subgrade — not the gravel — carries the load, which is why soft subgrade needs fabric or excavation, not just more stone.
Sub-base: The compacted base layer between the subgrade and the finished surface. On a driveway it's the dense-grade course (CA6/ABC) under the top stone.
Compaction / compaction factor: How much loose gravel settles once it's tamped or driven on. Dense-grade base drops roughly 15–20% from loose to compacted, so order extra; open-graded #57 barely compacts. Compaction guide.

Units and math

Cubic yard: A volume of 3 ft × 3 ft × 3 ft = 27 cubic feet. Bulk gravel is planned in cubic yards; one yard of crushed stone weighs about 1.4 tons. Yards ↔ tons converter.
Short ton vs. metric tonne: A US short ton is 2,000 lb. A metric tonne is 1,000 kg (2,205 lb) — about 10% heavier. Most US quarries sell by the short ton; the UK, EU, and Australia use the tonne.
Density: The weight of a material per unit of volume, usually pounds per cubic yard (or kg/m³). Density is what converts your cubic-yard estimate into the tons a supplier bills — and it changes with material, grading, and moisture.

Know the term, need the number? Use the gravel calculator to turn any material and area into cubic yards, tons, and cost.