How Much Gravel Under a Concrete Slab?
Most concrete slabs need 4 to 6 inches of compacted gravel underneath, with driveways and heavy-load pads going up to 8 inches or more. The gravel base spreads the load, drains water away from the slab, and keeps the pour from cracking or heaving as the ground shifts. Below is how to pick the right depth, the volume math in cubic yards and tons, and what it costs by slab size.
Key takeaways
- A 4-inch gravel base is standard for patios and walkways; use 6 inches for driveways and 8 to 12 inches for heavy equipment pads.
- A 10x10 slab needs about 1.23 cubic yards (1.85 tons) of gravel at 4 inches, or 1.85 cubic yards (2.8 tons) at 6 inches.
- One cubic yard of gravel covers about 81 square feet at 4 inches deep, or 54 square feet at 6 inches.
- Crushed angular stone or road base compacts and locks together far better than smooth pea gravel under a slab.
- Gravel delivered and spread runs about 55 to 65 dollars per cubic yard in 2026, plus a delivery fee on small orders.
How much gravel you need under a concrete slab
Most residential slabs sit on 4 to 6 inches of compacted gravel over a firm, leveled subgrade. That range is the standard baseline cited in ACI 302 guidance for slab-on-ground construction, and it is what most local codes and contractors default to for patios, sheds, and garage floors.
The right depth still depends on the job. A light patio or walkway can get by on 4 inches, while a driveway or a pad carrying heavy equipment needs 6 to 8 inches to spread the load properly. Soft or clay-heavy soil, and regions with hard freeze-thaw cycles, push that number higher still, sometimes to 8 to 12 inches or more.
| Project | Gravel depth |
|---|---|
| Patio or walkway | 4 inches |
| Shed or garage floor | 4 to 6 inches |
| Driveway | 6 to 8 inches |
| Heavy equipment pad | 8 to 12 inches |
| Severe frost heave regions | 8 to 24 inches |
To size the gravel and the concrete together for your own dimensions, plug the footprint into the concrete slab calculator.
Why a slab needs a gravel base at all
Gravel does two jobs a concrete slab cannot do on its own. First, it spreads point loads across a wider area of soil, which keeps the slab from cracking under a parked vehicle, a heavy appliance, or seasonal ground movement. Second, it drains water away from the underside of the slab instead of letting it pool and soften the subgrade.
Skip the gravel and pour straight onto bare soil, and the slab is riding on whatever that soil does next. Expansive clay swells and shrinks with moisture, silty soil holds water and softens, and any organic material underneath rots and settles. All three cause the classic symptoms: hairline cracks, sunken corners, and a slab that rocks slightly underfoot.
A well compacted gravel layer, placed in lifts and tamped as it goes, gives the concrete a stable, self-draining shelf to sit on instead.
Gravel volume by slab size, in cubic yards and tons
Gravel volume uses the same formula as concrete: length times width times depth in feet, divided by 27 to get cubic yards. The only difference is the depth, since gravel goes down before the slab, not at the slab's own thickness.
Cubic feet = Length(ft) x Width(ft) x (Depth(in) ÷ 12)Cubic yards = Cubic feet ÷ 27Gravel is sold by the ton as often as by the yard. Most crushed stone and road base weighs about 1.4 to 1.5 tons per cubic yard, so multiply the cubic yard figure by roughly 1.4 to 1.5 to get tons, and add 10 to 15% on top for compaction loss.
| Slab size | Gravel at 4 in | Gravel at 6 in |
|---|---|---|
| 10 x 10 ft | 1.23 yd³ (1.85 tons) | 1.85 yd³ (2.8 tons) |
| 12 x 12 ft | 1.78 yd³ (2.65 tons) | 2.67 yd³ (4.0 tons) |
| 20 x 20 ft | 4.94 yd³ (7.4 tons) | 7.41 yd³ (11.1 tons) |
| 24 x 24 ft (driveway apron) | 7.11 yd³ (10.7 tons) | 10.67 yd³ (16.0 tons) |
As a quick reference, one cubic yard of gravel covers about 81 square feet at 4 inches deep, or 54 square feet at 6 inches. For a driveway pour, the full breakdown of concrete plus base is in how much a concrete driveway costs.
What gravel size and type to use under concrete
Use angular crushed stone, not smooth river gravel or pea gravel. Angular pieces interlock when compacted, which locks the base into a solid, load-bearing layer. Round, smooth stone just rolls and shifts under pressure, no matter how much you tamp it.
The common choices are crushed stone, road base (also sold as crusher run or #21A), which is a blend of crushed rock and stone dust that packs into a dense, hard surface. A 3/4-inch minus gravel is the typical size for a slab base, small enough to compact tightly but large enough to drain.
Compact in lifts of no more than 2 to 4 inches at a time with a plate compactor, checking each layer before adding the next. A gravel bed dumped in one thick pass and never tamped defeats the purpose, since it settles unevenly once the slab and its load sit on top.
What a gravel base costs installed in 2026
Gravel for a slab base runs about 55 to 65 dollars per cubic yard installed in 2026, which includes the material, delivery, and spreading. Bought separately, crushed stone and road base cost roughly 20 to 45 dollars per ton, or 35 to 70 dollars per cubic yard, with delivery adding 50 to 150 dollars per load on top.
Most suppliers set a delivery minimum of 5 to 10 tons, so a small patio job often pays close to the same delivery fee as a driveway-sized order. Combining the gravel delivery with the concrete order, or buying bagged gravel for a very small footprint, can avoid that minimum.
Common gravel base mistakes to avoid
The most frequent error is skipping compaction and hoping the weight of the concrete does the work. It does not. Loose gravel settles unevenly after the pour, and the slab cracks or dips as it does. Compact every lift, and check that the surface is firm underfoot before forming up.
The second mistake is going too thin on soft or clay-rich soil to save on material. If the subgrade is questionable, 4 inches is not enough, no matter what the slab is for. A soil test or a quick conversation with a local contractor settles that question cheaper than a cracked patio does.
The third mistake is skipping a gravel base entirely on a patio because it seems like overkill. It is not: even a small pad benefits from proper drainage, and it is far easier to add gravel before the pour than to fix a heaved slab after. Once the base and depth are set, use the concrete slab calculator to finalize the concrete order on top.
Frequently asked questions
Is 2 inches of gravel enough under a concrete slab?
What happens if you don't use gravel under concrete?
How much will a 50 lb bag of gravel cover?
How much gravel for a 30x40 concrete pad?
Can you pour concrete directly on dirt instead of gravel?
References
- ACI 302: Guide to Concrete Floor and Slab Construction
- Crushed stone (Wikipedia)
- NRMCA: Concrete Slab-on-Ground Construction
- QUIKRETE: All-Purpose Gravel spec sheet