Gravel Under Concrete Slab Calculator
Enter your slab dimensions and base depth, and this calculator tells you how much gravel you need under the concrete slab — in cubic yards and tons, with a compaction allowance built in so you don't come up short after tamping.
4″ standard · 6″ garages, driveways, clay soil
Typical crusher run: $25–$60 per ton delivered
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Enter a price per ton to estimate cost.
You might also need:Concrete Calculator·Gravel Calculator
How much gravel goes under a concrete slab?
The standard gravel base under a residential concrete slab is 4 inches, compacted. Step up to 6 inches for garage slabs, driveways, and any slab over clay or poorly draining soil. The math is the same as any bulk material — with one extra step, because gravel shrinks when you compact it:
order yd³ = compacted yd³ × 1.10 (compaction allowance)
tons = order yd³ × 1.5 (crusher run)
For example, a 20 × 20 ft slab with a 4-inch base needs 20 × 20 × 0.333 = 133 cubic feet ≈ 4.9 cubic yards compacted in place. Add 10% for compaction and you should order about 5.4 cubic yards — roughly 8.1 tons of crusher run.
Gravel under slab quick-reference chart
Tons of crusher run to order (10% compaction allowance included):
| Slab size | 4″ base | 6″ base |
|---|---|---|
| 10 × 10 ft (100 sq ft) | 2.0 tons | 3.1 tons |
| 12 × 12 ft shed | 2.9 tons | 4.4 tons |
| 16 × 20 ft patio | 6.5 tons | 9.8 tons |
| 20 × 20 ft slab | 8.1 tons | 12.2 tons |
| 24 × 24 ft two-car garage | 11.7 tons | 17.6 tons |
| 30 × 40 ft shop | 24.4 tons | 36.7 tons |
Choosing the right base material
- Crusher run / road base: the default. Angular stone with fines that locks tight under a plate compactor. Cheapest per ton in most markets.
- #57 stone: clean ¾″ stone with no fines. Self-compacting and free-draining — preferred under basement slabs and wherever a vapor barrier sits directly on the base.
- Recycled concrete (RCA): performs like crusher run at a lower price where available; check that it's spec'd for structural base use.
- Avoid pea gravel: rounded stones never interlock, so the base stays "live" and lets the slab settle.
Building the base right
- Excavate to uniform depth — dig out topsoil and organics completely; a slab poured over sod pockets will settle and crack there.
- Compact in lifts: spread and tamp no more than 4 inches at a time. One thick lift compacted from the top leaves the bottom loose.
- Wet it lightly: damp (not soaked) gravel compacts noticeably tighter than bone-dry material.
- Vapor barrier goes on top of the compacted base, directly under the concrete, for interior and garage slabs.
- Check depth as you go with a string line — a base that wanders from 3″ to 5″ wastes concrete and gravel both.
Frequently asked questions
How much gravel do I need under a concrete slab?+
Most residential slabs need a 4-inch compacted gravel base. For a typical slab that works out to about 1.4 cubic yards (roughly 2 tons) of gravel per 100 square feet, including a 10% compaction allowance. Slabs on poorly draining or clay soil, and garage or driveway slabs, should go to 6 inches.
What kind of gravel goes under a concrete slab?+
Crusher run (crushed stone with fines, also sold as road base or CR-6) is the standard choice because it compacts into a dense, stable layer. Clean #57 stone is used where drainage matters most, such as under basement slabs with a vapor barrier. Avoid rounded pea gravel — it never locks together and shifts under load.
Do I really need gravel under concrete?+
On well-drained, undisturbed soil a slab can be poured on grade, but a gravel base is cheap insurance: it provides uniform support, breaks capillary moisture rise, and keeps the slab from settling unevenly and cracking. Building codes commonly require a minimum 4-inch base for interior slabs.
How much does gravel compact when tamped?+
Crusher run and road base typically lose 10–15% of their loose volume when compacted with a plate compactor. That is why this calculator adds a compaction allowance — if you order exactly the compacted volume, you will come up short.
Should the gravel base be compacted before pouring?+
Yes — always. Compact in lifts (layers) of no more than 4 inches with a plate compactor, lightly wetting each lift. A properly compacted base should not show footprints when you walk on it.