How Much Gravel Under a Concrete Slab? (Depth, Yards & Tons)

The short answer

Put 4 inches of compacted gravel under patios, walkways, and shed pads, and 6 inches under driveways, parking pads, and garages that carry vehicles. The math is one formula: length × width × depth (in feet) ÷ 27 = cubic yards, then multiply yards by 1.4 for tons. A 20×20 slab at 4 inches needs about 5 yards (7 tons); at 6 inches it needs about 7.5 yards (10.5 tons). Order #57 stone or crusher run — never pea gravel — and compact it in lifts.

Depth by slab use

Depth is decided by load, not by preference. The gravel sits under the concrete — the slab itself (typically 4 inches for residential work) pours on top of the finished base:

Clay soil pushes everything one step deeper in practice: if your subgrade is clay, use the 6-inch column even for a shed pad, and lay woven geotextile fabric between the soil and the stone so the gravel does not sink into the mud over the seasons.

The math: one worked example

Convert depth to feet first — 4 inches = ⅓ foot, 6 inches = ½ foot — then it is a single multiplication:

20 × 20 slab at 4 inches: 20 × 20 × (1/3) ÷ 27 = 4.94 cubic yards, so order 5 yards (about 7 tons at 1.4 tons per yard). Add 10% overage for compaction and spillage and the practical order is 5.5 yards.
Same slab at 6 inches: 20 × 20 × 0.5 ÷ 27 = 7.41 cubic yards, so order 7.5 yards (about 10.5 tons); with overage, 8.25 yards.

Every entry in the tables below is built from that same formula. For an odd footprint, our gravel calculator runs it in one pass — and when you are ready to size the concrete itself, the concrete calculator handles the pour.

Yards and tons by footprint: 4-inch base

Tons use gravel's 2,800 lbs/yd³ (×1.4). The “order” column adds 10% overage, rounded to what suppliers sell:

Yards and tons by footprint: 6-inch base

Priced per ton at the quarry? Our gravel-to-tons calculator converts either direction so you can compare a per-yard quote against a per-ton one.

Which gravel to order — and which to refuse

Two products are correct for a slab sub-base. #57 stone — clean, angular crushed rock about ½ to 1 inch — drains freely and locks together under compaction; it is the default under most residential slabs. Crusher run (also sold as road base or dense-grade aggregate) is the same angular stone plus crusher dust, which packs into an even harder, tighter layer; choose it where the subgrade is soft or the slab carries vehicles. The product to refuse is pea gravel: its rounded stones shift under load and never interlock, so the base migrates and the slab settles unevenly. Sand alone is wrong for the same reason — it pumps and washes out from under the slab edge.

Building the shed first and the slab later? Our shed-base gravel guide covers the pad-only version of this job, where the gravel is the finished surface rather than a sub-base.

Vapor and moisture barrier note

For any interior slab — garage, basement, workshop — lay 10-mil polyethylene sheeting over the compacted gravel before the pour, with seams lapped 6 inches and taped. Ground moisture wicks up through concrete by capillary action, and the poly breaks that path; skip it and you get damp floors, musty smells, and flooring that fails to bond. For exterior slabs like patios and walkways, do the opposite: no vapor barrier. Outdoors it traps water under the slab instead of excluding it — drainage through the gravel base is the moisture strategy there.

Compaction in lifts — the step that decides everything

Gravel only becomes a base once it is compacted, and compaction only reaches a few inches deep per pass. Place the stone in 3–4 inch lifts: spread a lift, run a plate compactor over it until it stops leaving marks, then spread the next. Dumping the full 6 inches at once and rolling the top compacts nothing below the surface — the bottom stays loose, settles over the first wet season, and the slab above cracks along the settlement line. Rent the plate compactor for the day; it is the cheapest tool on the job and the one most skipped. Water each lift lightly before compacting — damp stone packs tighter than dry.

What the gravel base costs

Typical U.S. ranges for the stone itself, before the concrete:

That is usually under 10% of the total slab project — concrete, forming, and labor dominate. It is also the part of the job you cannot redo after the pour, which is exactly why skimping it is the most expensive saving in concrete work. Wondering what a cubic yard of anything actually weighs? Our cubic-yard weight guide covers every common material on one chart.

Common mistakes

Pea gravel as sub-base. Rounded stone migrates — angular #57 or crusher run only. One deep lift. Compact in 3–4 inch passes or the bottom never densifies. Skipping fabric on clay. The stone sinks into the mud and the “base” becomes a rumor. Vapor barrier outdoors. It traps water under exterior slabs; drainage is the strategy there. Exact-yard ordering. Compaction eats 10–15% of loose volume; the second delivery fee always exceeds the extra half yard. Gravel but no drainage plan. The base moves water down and out — make sure “out” exists by grading soil away from the slab edge.