How Much Gravel Do You Need Under a Concrete Slab?

Definitions

The three layers under a slab (most guides blur these)

It helps to name the layers, because “the gravel” is really two or three different things doing different jobs. From the bottom up:

The subgradeis the native soil itself (sometimes improved or replaced). It's the only mandatory layer — a slab isn't self-supporting, so the prepared soil beneath is what actually carries it. It must be compacted, cleared of organic material, and graded to within about an inch and a half of the target level.

The subbase is the granular layer — your gravel — placed over the subgrade. On top of that, a thin base courseof finer material can be added to make a clean, flat surface to pour on. Most homeowners just say “the gravel base” for the whole thing, which is fine — but knowing the subgrade is a separate (and non-negotiable) layer is the difference between a slab that lasts and one that sinks.

Step 1

How deep should the gravel be?

The standard is 4 to 6 inches of compacted gravel over a compacted subgrade, and for most residential slabs — patios, sheds, garage floors — 4 inches is enough.

Step 2

Why it has to be compacted in lifts

Compaction is the step DIYers skip, and skipping it is what cracks slabs. Spread the gravel and pour on top without compacting, and the loose material keeps settling under the slab's weight for years — producing the exact cracks and sinking the gravel was supposed to prevent.

The technique is to work in lifts (layers): spread 2 to 3 inches, dampen it lightly, run a plate compactor (a vibratory compactor — rentable for roughly $75–100 a day) over it three or four times with overlapping passes, then add the next lift. Never try to compact more than about 3 inches at once — the machine can't densify the bottom of a deep pile, so it stays loose where it matters most.

To check it: walk across the finished surface. It should feel solid and leave no footprints. (Professional jobs verify 95% or more of maximum density with a Proctor-based test; the footprint test is the practical DIY version.) For areas under about 25–50 square feet a hand tamper works, but for anything larger, mechanical compaction is the only thing that reaches real density.

Step 3

Which gravel? (the part most people get wrong)

First rule: use angular crushed stone, not rounded or pea gravel. Angular pieces have sharp faces that interlock under compaction and resist shifting; rounded stone behaves like marbles — it rolls and migrates under load and never locks up. That single choice matters more than the brand or color of the rock.

Beyond that, there are two families of crushed stone that do opposite jobs, and the confusion between them is where slabs go wrong:

Dense-graded, for compaction — “crusher run” (also called crush-and-run, graded aggregate base/GAB, ABC, or #411). It's crushed stone plus stone dust and screenings. The fines fill the gaps between the stones, so when you compact it, it locks into a hard, low-void platform. This is your structural base.

Open-graded, for drainage — “#57” or “#67” stone (clean, washed, roughly ¾-inch, no fines). Water runs straight through it, which makes it an excellent capillary break(a layer that stops groundwater wicking up into the slab). But clean stone doesn't compact the way dense-grade does — it more or less settles where it's dumped — so as a thick standalone base under a slab it carries more settlement risk unless it's well contained.

Step 4

Water: drainage, capillary break, and the vapor barrier

A good gravel base is also a water-management system. Its capillary break function stops moisture wicking up from the water table into the slab, and its porosity moves surface water away — which reduces freeze-thaw cracking compared with pouring concrete straight onto clay or topsoil.

Step 5

Gravel over clay or soft soil

If your subgrade is clay, wet, or soft, the base needs help — otherwise the gravel slowly punches down into the mud and mixes with it, losing its structure.

Step 6

Don't skip the base — what failure looks like

It's worth stating plainly, because it's the whole point of this guide: dumping gravel into the hole and pouring concrete on top guarantees failure. Uncompacted base keeps settling under the slab, and the result is cracks, sinking, and an uneven surface — no matter how good the concrete or how much rebar you used.

Sizing

How much gravel will you need?

For planning, a 4-inch base needs about 1.25 cubic yards per 100 square feet, and a 6-inch base about 1.85 cubic yards per 100 square feet. Order 10–15% extra to cover an uneven subgrade and the volume you lose to compaction.

For your actual project, the gravel calculator takes your dimensions and base depth and returns the exact amount — in cubic yards and tons — with the waste allowance built in.

Questions

Gravel-under-slab FAQ

Receipts

Sources & methodology

Depths, lift thicknesses, and material roles are well-corroborated standards, given as ranges. The “best single base material” is genuinely debated in the trade, so this guide presents the trade-off rather than a false rule. The ~30% longevity gain and the rental cost are typical industry figures, labeled as such. Quantities are computed; the gravel calculator is the source for your number. Exact requirements vary by jurisdiction and supplier naming; local building code governs, and problem soils warrant an engineer. For the shared publish-our-receipts standard, see the methodology page.