How Much Gravel Do You Need Under a Concrete Slab?
The short answer
A typical concrete slab sits on 4 inches (100 mm) of compacted gravel, which works out to about 1.2 cubic yards of crushed stone per 100 square feet (about 0.9 m³ per 9.3 m²). Go deeper when the load or the soil gets worse:
- Patio, sidewalk, shed slab - 4 in (100 mm)
- Garage slab - 4 to 6 in (100 to 150 mm)
- Driveway, or anything a vehicle parks on - 6 to 8 in (150 to 200 mm)
- Soft or clay soil - 6 in or more, over well-compacted subgrade
- Well-drained gravelly soil (Group I) - code lets you skip the base entirely (IRC R506.2.2 exception)
The math is simple: cubic yards = length (ft) × width (ft) × depth (ft) ÷ 27. Then order 10 to 20% extra, because crushed stone shrinks as you compact it.
Why gravel goes under concrete at all
The base layer does three jobs, and skipping it costs you all three:
- Drainage. Water passes through gravel instead of pooling under the slab. That means less frost heave in winter and less ground moisture wicking up into the concrete.
- Uniform support. Compacted angular stone spreads loads and bridges soft spots, so the slab settles evenly, or not at all. Uneven settling is what cracks slabs.
- A flat surface to pour on. A trimmed gravel base keeps the slab a consistent thickness. Pour straight onto rough dirt and every hump becomes a thin spot, and thin spots crack first.
Use an angular crushed stone that locks together when compacted: ¾-in minus, crusher run, or #57 stone all work (Concrete Network’s subbase guidance calls for a compactible, easy-to-trim granular fill). Smooth rounded river rock rolls instead of locking, so it never firms up.
What the code says
The US residential code (IRC R506.2.2) requires a 4-inch base course of clean graded sand, gravel, crushed stone, crushed concrete, or crushed slag passing a 2-inch sieve wherever the slab sits below grade. The one exception: soils that are already well-drained sand-gravel mixtures (Group I in Table R405.1) need no base at all.
Two related numbers while you are in that chapter: the slab itself must be at least 3½ in thick (R506.1), and slabs under heated space need a 10-mil vapor retarder between the gravel and the concrete (R506.2.3). Garages, patios, walks, and driveways are exempt from the vapor retarder.
Gravel for common slab sizes
Loose volume before compaction, with weight at 1,600 kg/m³ (about 1.35 US tons per cubic yard, the crushed-stone density our calculator uses). Order 10 to 20% over these numbers.
| Slab size | Area | 4 in base | 6 in base |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 1.2 yd³ (~1.7 t) | 1.9 yd³ (~2.5 t) |
| 10 × 12 ft | 120 sq ft | 1.5 yd³ (~2.0 t) | 2.2 yd³ (~3.0 t) |
| 12 × 12 ft | 144 sq ft | 1.8 yd³ (~2.4 t) | 2.7 yd³ (~3.6 t) |
| 12 × 16 ft | 192 sq ft | 2.4 yd³ (~3.2 t) | 3.6 yd³ (~4.8 t) |
| 16 × 20 ft | 320 sq ft | 4.0 yd³ (~5.3 t) | 5.9 yd³ (~8.0 t) |
| 20 × 20 ft | 400 sq ft | 4.9 yd³ (~6.7 t) | 7.4 yd³ (~10.0 t) |
| 24 × 24 ft (2-car) | 576 sq ft | 7.1 yd³ (~9.6 t) | 10.7 yd³ (~14.4 t) |
(1 yd³ = 0.76 m³; tons are US short tons. Gravel is usually sold by the ton or the cubic yard, so either column works at the supplier.)
Worked example: a 12 × 16 ft shed slab
The base. 12 × 16 = 192 sq ft. A 4-in base is a third of a foot deep, so 192 × 4 ÷ 12 = 64 ft³, and 64 ÷ 27 = 2.37 yd³ of loose crushed stone (1.8 m³). Add about 15% for compaction and you order roughly 2.7 yd³, call it 3.7 to 4 tons.
The concrete on top. Handy rule: at the same depth, the pour needs the same volume as the base. A 12 × 16 slab at 4 in thick is the same 64 ft³ = 2.37 yd³ of concrete, which is 107 bags of 80 lb mix. At that count, mixing by hand stops being fun, and a ready-mix truck starts making sense.
👉 The Gravel Calculator turns your footprint and base depth into cubic yards and tons for crushed stone, pea gravel, or sand, and the Concrete Calculator then sizes the pour on top in yards and 80/60/40 lb bag counts.
Watch-outs
- Compact in thin lifts. A rental plate compactor only densifies the top 2 to 3 in per pass. Spread 2 to 3 in, compact, repeat. Dumping 6 in and running the plate once leaves the bottom loose.
- Gravel does not fix bad subgrade. Strip the topsoil and anything organic first, then compact the exposed soil before the first lift goes down. Base over mush settles like mush.
- Skip the smooth stuff. Rounded river rock and pea gravel alone will not lock up under a slab. If you love the look, save it for paths and use angular crushed stone here.
- Local code wins. Frost depth, expansive clay, and seismic rules vary. The IRC numbers above are the national baseline; your building department can require more.
Related
- Concrete Calculator - slab, footing, and column volume in cubic yards, plus bag counts.
- Concrete tools - the full concrete toolset.
- How thick should a concrete slab for a shed be - the layer that goes on top of this base.
- How much does a 10×10 concrete slab cost - materials vs ready-mix pricing for a small pad.
- How many bags of concrete per fence post - the same bag-yield math, one hole at a time.