How much gravel under a concrete slab?
Quick Answer
You need a minimum 4-inch compacted gravel base under a concrete slab for patios and driveways. In cold climates with freeze-thaw cycles, or on poor/clay soils, use 6 inches. The gravel should be compacted crushed stone (not pea gravel) to prevent settling.
Quick Reference Table
| Application | Recommended base depth |
|---|---|
| Patio slab (mild climate) | 4 inches crushed stone |
| Driveway (standard) | 4–6 inches crushed stone |
| Cold climate / freeze-thaw | 6–8 inches |
| Clay or expansive soil | 6–8 inches + geotextile fabric |
| Walkway / sidewalk | 4 inches |
How to Calculate It Yourself
- 1
Excavate to a depth of 4" gravel + 4" concrete (8" total for standard patio).
- 2
Spread 3/4" crushed stone (not pea gravel — it doesn't compact well).
- 3
Compact in 2" lifts with a plate compactor. Don't compact more than 2" at a time.
- 4
Check with a level; the base should slope 1/8" per foot away from structures for drainage.
- 5
Pour concrete within 24 hours of compacting to avoid the base shifting.
Pro Tip
Use 3/4" crushed angular stone, not pea gravel or sand. Angular stone locks together when compacted; round stones shift under load and cause slab cracking.
Need an exact number for your specific dimensions?
Use the Gravel Calculator →