4-Inch vs 6-Inch Concrete Slab: How Much More Concrete?
Side-by-side cubic yards and bag counts for common slab sizes at 4 and 6 inches thick, plus the typical uses for each thickness.
Quick answer
A 6-inch slab uses exactly 50% more concrete than a 4-inch slab of the same size (6 ÷ 4 = 1.5). Per 100 ft² that is 1.23 yd³ at 4 in versus 1.85 yd³ at 6 in — about 56 vs 84 bags of 80 lb premix.
Formula
Per 100 ft²: 4 in → 100 × 4 ÷ 12 ÷ 27 = 1.235 yd³; 6 in → 100 × 6 ÷ 12 ÷ 27 = 1.852 yd³
80 lb bags = ft³ ÷ 0.60, rounded up
4-inch vs 6-inch: concrete needed by slab size
| Slab (ft) | yd³ at 4 in | yd³ at 6 in | Extra yd³ | 80 lb bags at 4 in | 80 lb bags at 6 in |
|---|---|---|---|---|---|
| 10×10 | 1.23 | 1.85 | 0.62 | 56 | 84 |
| 10×12 | 1.48 | 2.22 | 0.74 | 67 | 100 |
| 12×12 | 1.78 | 2.67 | 0.89 | 80 | 120 |
| 10×20 | 2.47 | 3.70 | 1.23 | 112 | 167 |
| 12×24 | 3.56 | 5.33 | 1.78 | 160 | 240 |
| 20×20 | 4.94 | 7.41 | 2.47 | 223 | 334 |
| 24×24 | 7.11 | 10.67 | 3.56 | 320 | 480 |
For other sizes, enter your dimensions in the concrete slab calculator (cubic yards for ready-mix) or the concrete bag calculator.
Which thickness do you need?
Typical residential practice is roughly:
- 4 inches — walkways, patios, shed floors and garage floors or driveways for ordinary passenger cars on a well-compacted base. This is the common minimum for residential slabs-on-ground.
- 5–6 inches — driveways that see pickups, RVs, delivery or trash trucks, slabs on weaker or less uniform soil, and pads for heavy equipment or hot tubs.
Thickness is only one part of strength: base preparation, concrete mix strength, joint spacing and reinforcement matter as much. Local building codes and the load you expect take precedence over any rule of thumb — when in doubt, ask your building department or a local concrete contractor. See why concrete cracks for the most common failure causes.
What the extra 2 inches costs
Because volume scales directly with thickness, material cost rises by 50% too. Labor and forming rise less, since the same area is finished either way, so the total installed price usually rises by less than 50%. Use the driveway calculator for an installed-cost range, or the driveway cost guide for a breakdown.
Assumptions
- Uniform thickness over the whole slab. Thickened edges (common on driveways and garage slabs) add volume — calculate the edge as a footing and add it.
- 80 lb premix bag ≈ 0.60 ft³; 60 lb ≈ 0.45 ft³; 40 lb ≈ 0.30 ft³.
- 1 cubic yard = 27 cubic feet.
FAQ
How much more concrete is a 6-inch slab than a 4-inch slab?
Exactly 50% more, because volume is proportional to thickness: 6 ÷ 4 = 1.5. Per 100 ft² it goes from 1.23 to 1.85 cubic yards.
Is 4 inches of concrete enough for a driveway?
For passenger cars on a well-compacted base, 4 inches is a common residential choice; 5–6 inches is typical where heavier vehicles drive. Follow local code and your contractor’s advice.
How many 80 lb bags for a 10x10 slab 6 inches thick?
10 × 10 × 6 ÷ 12 = 50 ft³; 50 ÷ 0.60 = 83.3, so 84 bags before waste.
Last updated: October 12, 2026.