4-inch vs. 6-inch slabs: the material difference

Understand the change in volume before discussing the right design for your project.

For the same area, a 6-inch slab uses 50% more concrete than a 4-inch slab. That is a quantity comparison, not a thickness recommendation.

Compare equal areas

Depth is one of the three dimensions of volume. Increasing depth from 4 to 6 inches multiplies volume by 6 ÷ 4 = 1.5. Length, width and allowance must remain the same for this comparison.

20 × 20 ft slab	4 inches	6 inches
Net volume	4.94 yd³	7.41 yd³
With 10% allowance	5.43 yd³	8.15 yd³
Extra net volume	—	2.47 yd³

Separate design from arithmetic

Vehicle loads, soil support, reinforcement, joints and local requirements can change the design. A deeper slab does not automatically fix poor preparation. Have the required thickness specified for the project, then use the calculator to estimate its volume.

Compare material cost fairly

Multiply each allowance-adjusted volume by the same quoted price per cubic yard. Then add the delivery and order charges for each option. Supplier increments may change the final difference. Reinforcement, labor and excavation costs are separate from this material calculation.

Measure what will actually be poured

Thickened edges, footings or steps require their own volume. Add those sections only once and avoid counting any overlapping volume twice. Keep the geometric net quantity separate from extra material so the supplier can understand your estimate.

The same comparison at common slab sizes

The 50% increase holds at every size, but the practical effect grows with area. On a small pad the difference is a few dozen bags; on a two-car-garage-sized slab it is more than two cubic yards of ready-mix.

Slab	4 in net	6 in net	80-lb bags at 4 in*	80-lb bags at 6 in*
10 × 10 ft	1.23 yd³	1.85 yd³	62	92
12 × 12 ft	1.78 yd³	2.67 yd³	88	132
20 × 20 ft	4.94 yd³	7.41 yd³	245	367

*Bag counts include a 10% allowance and assume 0.60 ft³ per bag. Ready-mix and bags are alternatives.

When the bag count decides the method

At 6 inches, even a 10 × 10-foot slab needs about 92 of the example 80-lb bags, roughly 7,360 lb of dry product to move and mix. Larger or thicker slabs often shift the practical choice toward ready-mix delivery. Price both methods with the concrete cost calculator once the thickness has been specified.

Make it your project

Change the measurements and allowance to match your plan.

Open Concrete Slab Calculator

MaterialScope editorial · Updated September 29, 2026 · Calculation methodology