Concrete Slab Thickness: 4" vs 5" vs 6" (and the Bag Count)
Updated July 2026
A 6" slab uses 50% more concrete than a 4" slab, because the number scales straight with thickness. For 100 square feet that is 84 vs 56 bags of 80 lb mix. Pour 4" for patios, walkways and shed floors; 5 to 6" anywhere a vehicle drives or parks.
How thick, by use
| What it holds | Thickness |
|---|---|
| Walkways, patios, garden pads | 4" |
| Shed floors, AC and generator pads | 4" |
| Driveways: cars and light trucks | 5–6" |
| Garage floors | 5–6" |
| RV pads, heavy trucks, equipment | 6"+ with rebar |
Four inches is the floor for almost anything structural. A 3" driveway slab cracks under the first loaded pickup, and the concrete you saved is a fraction of what a tear-out costs. When in doubt on a load-bearing pour, thicker plus reinforcement beats thinner, and your local code sets the real minimum.
Bags by thickness, per 100 square feet
Counts below are placed volume with no waste, straight from the printed bag yields (80 lb → 0.60 ft³, down to 40 lb → 0.30). Add 5 to 10% for waste on a real pour.
| Thickness | Volume | 80 lb bags | 60 lb bags | 50 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 4" | 1.23 yd³ | 56 | 75 | 89 | 112 |
| 5" | 1.54 yd³ | 70 | 93 | 112 | 139 |
| 6" | 1.85 yd³ | 84 | 112 | 134 | 167 |
Read across one row for your pour, or use the interactive slab calculator to plug in your own length, width and thickness. The pattern holds at any size: going from 4" to 6" always adds half again as much concrete, so the thickness choice moves your bag pile more than most people expect.
What thickness changes besides the bag pile
Thicker slabs carry more load, but they lean harder on the base and the steel. Two things move with thickness:
- Reinforcement: flat 4" patios often ride on wire mesh or fiber; 5 to 6" slabs that take vehicles want a rebar grid. Count bars and sheets with the rebar and wire-mesh calculator.
- Control joints: space them in feet at about 2 to 3× the thickness in inches, so a 4" slab wants joints every 8 to 10 ft and a 6" slab can run a bit wider.
The base does not change with thickness: you still want about 4" of compacted gravel under any of these. Most cracked DIY slabs failed in the dirt, not the mix.
When thickness pushes you to a truck
A 6" driveway is where bags stop making sense fast. At 100 square feet a 6" pour is 84 × 80 lb bags, nearly two cubic yards once you add waste, and that is a full day of mixing before the first batch starts setting. Past about a yard, price a short load: see bags vs ready-mix. Footings and piers are the opposite case; they pour hole by hole, so bags stay reasonable well past a yard.