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.
Assumptions to Check
Before ordering materials, confirm the dimensions, product coverage, waste factor, and local installation requirements for your project. Manufacturer coverage tables and local code rules can change the final quantity, especially for structural work, exterior projects, and irregular layouts.
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