Calculators for this guide
Typical slab thickness by use
| Slab | Typical thickness | Notes |
|---|---|---|
| Sidewalks and paths | 4 in | Foot traffic only |
| Patios | 4 in | Thicker under hot tubs or heavy features |
| Shed and small outbuilding bases | 4 in | Thickened edges are common |
| Driveways (cars) | 4–5 in | 5 in is a sturdier default |
| Driveways (trucks, RVs) | 6 in | Plus reinforcement and a good base |
| Garage floors | 4–6 in | Depends on vehicle and equipment loads |
These are typical residential figures. Footings and structural slabs are set by your local building code and the loads they carry — check with your building department or an engineer.
How thickness changes the order
Volume scales directly with thickness, so going from 4 in to 6 in adds 50% more concrete. For a 20 × 20 ft driveway pad:
| Thickness | Cubic yards (no waste) | 80 lb bags |
|---|---|---|
| 4 in | 4.94 | 223 |
| 5 in | 6.17 | 278 |
| 6 in | 7.41 | 334 |
At this size, ready-mix delivered by truck is far more practical than bags. Add 5–10% for waste and uneven ground before ordering.
Don't skip the base
A slab is only as good as what's under it. Most residential slabs sit on 4–6 in of compacted crushed stone, which spreads loads, improves drainage and reduces cracking from soil movement. Remove topsoil and organic material first, compact the subgrade, then place and compact the base in layers.
Reinforcement and control joints
- Reinforcement — welded wire mesh or rebar — helps hold cracks tight. Driveways and garage floors usually include it; check local practice.
- Control joints are cut or tooled grooves that make the slab crack in a straight line where you want it to. A common rule is to space them no more than 2–3 times the slab thickness in feet (so 8–12 ft for a 4 in slab).
- Thickened edges along the perimeter add strength where slabs are most likely to crack.