Gravel Base Under a Concrete Slab: Depth and Tons

Gravel base under a concrete slab gives the concrete a firm, even, well-drained surface to sit on. For most patios, walks, and shed slabs, a compacted layer of about 4 inches of crushed stone is typical. Driveways and slabs on soft or poorly drained soils often get 4 to 6 inches or more. The base does not make up for bad soil on its own, but it evens out the subgrade, helps drainage, and makes the slab easier to pour at a consistent thickness.

Volume and tonnage calculator →

Typical base depth

Slab	Common compacted base depth
Patio or walkway	About 4 inches
Shed or hot tub pad	4 to 6 inches
Driveway	4 to 6 inches, more on clay or wet soil
Garage floor	About 4 inches, often with a vapor barrier

Local practice and soil conditions vary. Some well-drained sandy sites need little or no added base, while clay soils that swell when wet benefit from more.

Which stone to order

Ask for a crushed, angular stone with fines, often sold as crusher run, 3/4-inch minus, road base, or by a regional name. The mix of sizes locks together and compacts tightly. Rounded pea gravel does not compact well and is a poor choice under a slab. Some contractors place clean crushed stone under interior slabs for drainage, then add a vapor barrier.

How many tons you need

Multiply area by compacted depth to get cubic feet, divide by 27 for cubic yards, then multiply by about 1.4 to 1.5 tons per yard for compacted crushed stone. Order 10 to 15 percent extra for compaction and waste.

Slab size	4-inch base	6-inch base
10 x 10 feet	About 1.8 to 1.9 tons	About 2.6 to 2.8 tons
12 x 12 feet	About 2.5 to 2.7 tons	About 3.7 to 4 tons
20 x 20 feet	About 7 to 7.4 tons	About 10.4 to 11 tons
24 x 24 feet	About 10 to 10.7 tons	About 15 to 16 tons

Figures include no waste allowance. For other sizes, the base volume and tonnage calculator does the math.

Preparing and compacting

Strip topsoil and organic material down to firm subgrade.
Grade the subgrade so water drains away from buildings.
Compact the subgrade before adding stone.
Place stone in lifts of about 2 to 4 inches and compact each with a plate compactor, misting it lightly if dry. See compacting gravel base in lifts.
Check the grade with a string line so the slab will be a consistent thickness.

Vapor barriers and interior slabs

For heated spaces, garages, and basements, a polyethylene vapor retarder is commonly placed over or under the base depending on the design and local code. Outdoor patios and driveways usually skip it.

Common mistakes

Pouring on fluffy, uncompacted fill.
Using topsoil or clay as the base.
Leaving soft spots under the slab.
Making the base uneven, which leaves the slab thin in spots.

Bottom line

A gravel base under a concrete slab is usually about 4 inches of compacted crushed stone for patios and sheds, and 4 to 6 inches or more for driveways and soft soils. Order angular stone with fines, compact it in lifts, and figure tonnage at roughly 1.4 to 1.5 tons per cubic yard plus extra.

Questions people ask

How much gravel goes under a concrete slab?

About 4 inches of compacted crushed stone is common for patios and sheds, with 4 to 6 inches or more for driveways or soft soil.

Do you need gravel under a concrete slab?

Not always on well-drained, firm soil, but a compacted base helps drainage and gives an even surface on most sites.

What type of gravel is best under concrete?

Crushed angular stone with fines, such as crusher run or 3/4-inch minus, compacts best.

How many tons of gravel for a 12x12 slab?

About 2.5 to 2.7 tons for a 4-inch compacted base, before adding extra for waste.

Should gravel be compacted before pouring concrete?

Yes. Compact it in lifts with a plate compactor to avoid settlement and cracking.

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