Quick Answer
- Residential patio / walkway: 4 inches of compacted gravel
- Garage floor / driveway (cars): 4–6 inches
- Heavy truck / RV pad: 6–8 inches
- On clay / poor soil: Add 2 extra inches + geotextile fabric
Slab + Base Cross-Section
Gravel Base Depth by Slab Type
| Slab Application | Concrete Thickness | Gravel Base | Total Excavation |
|---|---|---|---|
| Sidewalk / walkway | 4" | 4" | 8" total |
| Residential patio | 4" | 4" | 8" total |
| Garage floor | 4–5" | 4–6" | 8–11" total |
| Driveway (cars) | 4–6" | 6" | 10–12" total |
| Heavy truck pad / RV | 6–8" | 6–8" | 12–16" total |
| On clay / soft soil | Same + rebar | Add 2" extra | +2" vs above |
Calculate Gravel for the Base
Gravel Cubic Yards = (L ft × W ft × Depth in) ÷ 324
20×20 ft patio at 4" base:
20 × 20 × 4 ÷ 324 = 4.94 yd³ of base gravel
(Same formula as the concrete layer itself — use it twice)
20×20 ft patio at 4" base:
20 × 20 × 4 ÷ 324 = 4.94 yd³ of base gravel
(Same formula as the concrete layer itself — use it twice)
| Slab Size | Gravel at 4" (yd3) | Gravel at 6" (yd3) |
|---|---|---|
| 10×10 ft | 1.23 | 1.85 |
| 12×16 ft | 2.37 | 3.56 |
| 20×20 ft | 4.94 | 7.41 |
| 24×24 ft | 7.11 | 10.67 |
| 12×40 ft driveway | 5.93 | 8.89 |
Best Gravel for Concrete Base
- #21A / #411 (processed gravel): Best choice — angular stone compacts to a stable base
- Crushed limestone #57: Good drainage; widely available
- Bank gravel: Acceptable if well-compacted; less drainage than crushed stone
- Avoid: Round river gravel — does not compact or interlock well under slabs
âš ï¸ Why you can't skip the gravel base
Without a compacted gravel base, concrete slabs settle unevenly, crack, and heave with freeze-thaw cycles. Repairs cost more than the gravel you saved. The base is the most important part of a long-lasting slab.
Calculate Gravel for Your Slab Base
Enter slab dimensions and base depth for exact cubic yards of gravel needed.
Open Gravel Calculator →FAQ
Do I really need gravel under concrete?
Yes — compacted gravel provides drainage, prevents frost heave, and gives the concrete a stable base. Skipping the base is the most common cause of cracked and sunken slabs.
What kind of gravel to put under concrete?
Use #21A crushed stone, #411 processed gravel, or #57 limestone. Angular (not round) stones compact to form a stable interlocking base. Compaction in 2–3 inch lifts is critical.