Crushed stone numbers, decoded
Quarries grade crushed stone by number, and the number tells you the size and the job. #57 (3/4 inch, clean) is the workhorse: driveways, drainage, concrete mix. #8 (3/8 inch) is pipe bedding and asphalt. #3 and #2 (1.5 to 3 inch) handle entrances, heavy mud and railroad style ballast. Crusher run, also sold as 2A modified, ABC or road base depending on your region, mixes stone with fines so it compacts nearly solid, which is why every base layer under pavers, slabs and driveways starts with it. Screenings or stone dust are the finest cut, used as the leveling layer under pavers.
Compaction changes your order
Crusher run compacts 10 to 20 percent from its loose delivered volume. If a plan calls for 4 finished inches of compacted base, you need roughly 4.6 loose inches of material. This calculator uses the compacted density of 115 lb per cubic foot for crusher run, which builds that reality into the tonnage; keep the 10 percent waste allowance on top for subgrade irregularities. Compact in lifts of no more than 4 inches at a time with a plate compactor, wetting lightly between passes.
Clean stone versus stone with fines
The single most useful distinction: clean stone (washed, no fines) stays permeable and is what drainage wants; stone with fines locks solid and is what structure wants. Use #57 clean stone in a French drain and it drains for decades. Use crusher run in the same trench and it seals up like a road. Reversed, a driveway base of clean #57 never stops shifting. Match the material to the function and half of hardscape failure modes disappear.
Base depths that hold up
Under pavers for foot traffic: 4 inches of compacted crusher run, then 1 inch of screed sand (our paver base calculator does both layers). Under a residential concrete slab: 4 inches. Gravel driveway base: 4 to 6 inches before the surface stone. Shed and hot tub pads: 4 to 6 inches. Doubling the base depth costs less than redoing a heaved patio; when your subgrade is soft clay, go deep.