It's tempting to skip the gravel base and pour concrete straight onto graded soil — it saves a delivery and a day of compaction work. It's also one of the most common reasons DIY slabs crack, heave, or settle unevenly within a few years.
A compacted gravel layer does two jobs: it drains water away from beneath the slab (concrete sitting on saturated clay is a recipe for frost heave in cold climates), and it creates a stable, evenly-compacted surface so the slab doesn't settle unevenly into soft spots over time.
4–6 inches of compacted crushed stone or crushed concrete is standard under a residential slab or driveway. Compact it in 2-inch lifts rather than dumping the full depth at once — uncompacted gravel settles under the slab's weight later, which defeats the purpose.
Both work. Recycled crushed concrete is often cheaper and just as effective for base material; virgin crushed stone gravel compacts slightly more predictably. Either way, the depth and compaction matter more than which material you choose.
Size your base with the Gravel Calculator or the Crushed Concrete Calculator, then move to the Slab Calculator for the concrete layer on top.