A concrete slab needs about 4 inches of compacted, well-graded crushed stone underneath it. The base spreads the load evenly, drains water away so frost cannot heave the slab, and gives you a firm, level surface to pour on. Skipping it is the most common cause of cracked, settled slabs.
Concrete is only as good as what it sits on. A slab poured on soft or uneven dirt loses support in spots and cracks; a slab on a compacted stone base stays supported across its whole footprint.
Why a base matters
Three jobs: uniform support so the slab does not settle unevenly and crack; drainage so water does not pool under the slab and, in cold climates, freeze and heave it; and a level working surface so your slab is a consistent thickness. A thin spot in the slab is a weak spot, so a level base pays off in strength too.
What gravel to use
Use a well-graded crushed aggregate with a mix of stone sizes and fines — commonly sold as crusher run, road base, or 3/4-inch minus. The mix of sizes lets it compact tight and lock together. Avoid smooth pea gravel or single-size stone for the base; it shifts and will not compact firmly.
How deep and how to compact
Four inches of compacted base is standard for slabs and walkways; go to 6 inches or more for driveways and anywhere with soft soil or heavy loads. Compact it in 2-inch lifts with a plate compactor, wetting it lightly so the fines pack — loose-dumped gravel is not a base. You should not leave footprints in a properly compacted surface.
Vapor barrier for interior slabs
For a slab inside a building (a garage, shop, or basement floor), put a 6-mil or thicker polyethylene vapor barrier over the compacted base before pouring, to stop ground moisture from wicking up through the slab. Outdoor slabs usually skip it. Once your base is set, size the pour with the slab calculator below.
Common questions
How much gravel goes under a concrete slab?
About 4 inches of compacted crushed stone for slabs and walkways; 6 inches or more for driveways or soft soil.
What kind of gravel for under concrete?
Well-graded crushed stone with fines — crusher run, road base, or 3/4-inch minus. It compacts tight. Avoid smooth pea gravel for the base.
Do you need a vapor barrier under a slab?
For interior slabs, yes — a 6-mil poly sheet over the base stops ground moisture from wicking up. Outdoor slabs typically don't need one.