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How Much Gravel Do I Need Under a Concrete Slab?
A concrete slab needs a compacted gravel sub-base of about 4 inches (100 mm) — more on soft ground. Here is the right depth, the stone to use, and how much you will need by slab size.
How thick should the gravel base be?
For most slabs — sheds, garages, patios, walkways — lay 4 inches (100 mm) of compacted crushed stone over firm, well-drained ground. On soft, frost-prone or clay soil, increase the sub-base to 6–8 inches (150–200 mm) and compact it in lifts. The gravel spreads the load, stops the slab cracking, and lets water drain so frost cannot heave it.
Always compact the gravel before you pour. Loose stone settles and the slab will crack above the low spots.
What gravel goes under concrete?
Use 3/4 inch (19 mm) crusher run (also called Granular A or 3/4 minus) — angular crushed stone with stone dust that compacts to a tight, stable base. Some pros top it with a thin layer of clear 3/4 inch stone as a capillary break. Avoid rounded pea gravel, which will not lock together under load.
Gravel needed by slab size
Figures are for a 4 inch compacted base. Increase by about half for a 6 inch sub-base.
| Slab size | Area | Volume at 4" | Crushed stone |
|---|---|---|---|
| 10 x 10 ft | 100 ft² | ≈ 1.2 yd³ | ≈ 1.8 t |
| 12 x 12 ft | 144 ft² | ≈ 1.8 yd³ | ≈ 2.6 t |
| 20 x 20 ft (single garage) | 400 ft² | ≈ 4.9 yd³ | ≈ 7.1 t |
| 24 x 24 ft (double garage) | 576 ft² | ≈ 7.1 yd³ | ≈ 10.3 t |
Add 5–10% extra for compaction. For your exact slab, use the Crushed Gravel Calculator.
Then size the concrete
Once the base is set, work out the concrete itself with the Concrete Calculator — it gives cubic yards, cubic metres, pre-mix bags and cost for slabs, footings and columns. For other base projects, see the gravel depth guide.
Frequently asked questions
How much gravel do I need under a concrete slab?
Plan on a 4 inch (100 mm) compacted crushed-stone base over firm ground, or 6–8 inches on soft or frost-prone soil. A 10x10 ft slab needs about 1.2 cubic yards (roughly 1.8 tonnes) at 4 inches.
What kind of gravel goes under a concrete slab?
Use 3/4 inch crusher run (Granular A or 3/4 minus) — angular crushed stone with dust that compacts to a firm, stable, well-draining base. Avoid rounded pea gravel under load-bearing concrete.
Do you need gravel under a concrete slab?
Yes, in almost all cases. A compacted gravel sub-base spreads the load, provides drainage so frost cannot heave the slab, and gives a level surface to pour on, which greatly reduces cracking.
How deep should gravel be under a garage slab?
At least 4 inches of compacted crushed stone, and 6–8 inches on soft or clay soil. Compact in lifts before pouring so the base does not settle under the weight of the slab and vehicles.
Disclaimer: This guide gives general estimates for planning. Actual gravel quantities, weights and prices vary with material type, particle size, moisture, compaction, supplier and your province. Costs are material-only ballparks in Canadian dollars and exclude labour, delivery and taxes unless stated. Always confirm quantities and prices with your local supplier.