How Much Gravel Under a Concrete Slab? Depth & Tons
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Before you order concrete, you need to know how much gravel goes underneath it. The base decides how evenly the slab is supported, how well water drains away, and whether it cracks. This guide covers how deep the base should be, which stone to use, and how many yards and tons you need for common slab sizes.
Why a slab needs a gravel base
Concrete is strong in compression but weak when the ground under it moves. A gravel base does three jobs. It spreads the load evenly so one soft spot doesn't become a crack. It lets water drain away from the underside of the slab instead of sitting against it. And it breaks the capillary path that draws moisture up from the soil, which also helps in freezing climates, where water trapped under a slab expands and lifts it.
Many building codes require at least 4 in of clean aggregate under garage and basement floors unless the soil already drains well. Patios and walkways are often held to a lighter standard, but the same logic applies: a compacted base costs far less than fixing a cracked or settled slab.
How deep should the gravel be?
Four inches of compacted stone is the standard answer, and it is the number the slab thickness guide uses. The right depth shifts with the load and the ground.
| Project | Typical gravel base | Notes |
|---|---|---|
| Walkway or step landing | 3–4 in | Light loads; 4 in is easiest to keep consistent |
| Patio or shed pad | 4 in | Add more on soft or poorly drained soil |
| Garage or basement floor | 4 in minimum | Often paired with a vapor retarder; follow local code |
| Driveway slab | 4–6 in | Vehicle loads call for the deeper end |
| Clay, silt, or wet soil | 6–8 in or more | Consider geotextile fabric under the stone |
| Freezing climates | Deeper, as local practice requires | Ask a local contractor or the building department |
These are typical ranges, not code requirements. Local conditions and building rules take priority.
Gravel needed for common slab sizes
Volume is area times base depth, converted to cubic yards by dividing by 27. The table adds 10% for settling and an uneven subgrade, and converts to tons using crushed stone at about 2,600 lb per cubic yard. Use the Gravel Calculator for other sizes and stone types.
| Slab | Area | 4 in base | Weight | 6 in base | Weight |
|---|---|---|---|---|---|
| Step landing (4 × 4 ft) | 16 ft² | 0.22 yd³ | 0.3 tons | 0.33 yd³ | 0.4 tons |
| Small shed pad (10 × 10 ft) | 100 ft² | 1.36 yd³ | 1.8 tons | 2.04 yd³ | 2.6 tons |
| Shed or hot tub pad (12 × 12 ft) | 144 ft² | 1.96 yd³ | 2.5 tons | 2.93 yd³ | 3.8 tons |
| Patio (16 × 12 ft) | 192 ft² | 2.61 yd³ | 3.4 tons | 3.91 yd³ | 5.1 tons |
| Large patio or garage bay (20 × 20 ft) | 400 ft² | 5.43 yd³ | 7.1 tons | 8.15 yd³ | 10.6 tons |
| Two-car garage (24 × 24 ft) | 576 ft² | 7.82 yd³ | 10.2 tons | 11.73 yd³ | 15.3 tons |
Volumes include 10% extra. Weights are for crushed stone; denser mixes weigh more.
Which stone to use
The goal is a base that compacts into a hard, uniform layer. Angular crushed stone does that because the pieces lock together. Round stone does not.
| Material | Good for | Caution |
|---|---|---|
| Crusher run (dense-graded, stone with fines) | The most common slab base; compacts very firmly | Drains slowly; fines hold some water |
| Clean crushed stone (such as #57) | Where drainage matters most, or under footings | Compacts less tightly, so it may need a thin layer of finer stone on top |
| Sand and gravel mix | Light-duty patios and walks on stable ground | Quality varies; confirm it is clean and graded |
| Pea gravel or river rock | Not recommended | Rounded stone shifts and doesn't lock together |
Stone names vary by region, so describe what you want to your supplier: a three-quarter-inch angular crushed stone, with or without fines. The gravel types and sizes guide explains the common grades.
Worked example: a 16 × 12 ft patio slab
A 16 × 12 ft patio is 192 ft². With a 4 in slab on a 4 in gravel base, the concrete and the gravel take the same raw volume of 2.37 yd³ each.
- Dig depth: 4 in of base plus 4 in of concrete means excavating about 8 in below the finished surface.
- Gravel: 2.37 yd³ plus 10% is 2.61 yd³, or about 3.4 tons of crushed stone.
- Cost: at an example price of $40 per ton, the stone comes to about $136 before delivery.
- Concrete: 2.61 yd³ with 10% waste. Enter the same dimensions in the Concrete Calculator to see the order and the cost.
Preparing and placing the base
- Mark and excavate. Strip sod, topsoil, and roots down to firm soil, digging to the slab thickness plus the base depth. Extend the base a few inches beyond the forms on every side.
- Compact the subgrade. Tamp or use a plate compactor over the bare soil. Dig out and replace any soft or organic spots with gravel.
- Add the gravel in layers. Spread about 2 to 4 in at a time and compact each layer, so the density is even all the way down. Wet the stone lightly if it is dusty.
- Check the grade. Slope the surface about 1/8 to 1/4 in per foot away from buildings so water drains. The base should be flat and uniform, within about half an inch.
- Add the vapor retarder if needed. Slabs inside heated spaces usually get a 6-mil polyethylene sheet on top of the stone. Outdoor slabs don't need it.
- Set forms and reinforcement, then pour. Keep reinforcement lifted into the middle of the slab on supports, not lying on the gravel.
Common mistakes
- Skipping compaction. Loose stone settles after the pour, leaving voids that crack the slab.
- Building on topsoil or fill. Organic soil keeps compressing for years. Remove it, or bridge it with a deeper base.
- Using rounded stone. Pea gravel feels firm underfoot but shifts under a load.
- Uneven base depth. Variations change the slab thickness and send concrete into low spots. A level base also keeps your concrete order close to the calculation.
- Ignoring drainage. Without a grade away from the slab, water collects at the edge and undermines the base.
Related guides and calculators
Frequently asked questions
A 10 × 10 ft slab on a 4 in gravel base needs about 1.23 yd³ of gravel, or 1.36 yd³ with a 10% allowance. That is roughly 1.8 tons of crushed stone.