Do I Need a Gravel Base Under Concrete?

For almost any permanent slab, the answer is yes — and skipping the base is one of the most common reasons slabs crack and settle. A compacted crushed-stone base does three jobs at once: it spreads the slab's load over the soil, it drains water away from the underside of the concrete, and it gives a firm, level surface to pour on. Concrete poured straight onto soft or uneven ground has nothing stable to rest on.

Why each job matters

Load spreading: concrete weighs about two tons per cubic yard, and a base distributes that weight so the soil doesn't settle unevenly under it. Drainage: a free-draining stone layer keeps water from pooling under the slab, which is critical in freeze-thaw climates where trapped water expands and heaves the concrete. A flat surface: a compacted base lets you pour a uniform thickness, so the slab is as strong as you designed it everywhere, not just where it happened to be thick.

How deep should it be?

Four inches of compacted base is the standard under a residential slab. Over soft, clay-heavy or poorly draining soil, go deeper — 6 inches or more — and build it in layers, often a coarser stone below a finer levelling layer. The key word is compacted: tamp it in lifts, because loose gravel keeps settling and defeats the purpose.

Estimating the stone

Crushed stone weighs about 1.5 tons per cubic yard (roughly 3,000 lb), and suppliers often sell by the ton, so you'll want both numbers. A 4-inch base under a 20 × 20 ft slab is about 5.4 cubic yards, or roughly 8 tons. Add about 10% because crushed stone compacts when you tamp it, so you need more loose volume than the finished depth suggests.

The gravel base calculator gives both cubic yards and tons and includes the compaction allowance; for driveways and larger areas, the gravel calculator handles any depth. Pair the base with the right slab thickness and you've built the foundation for a slab that lasts.