Concrete volume is length × width × thickness, so with the footprint held fixed, thickness is the only thing that changes the amount. Going from 4 to 6 inches is a 50% increase in thickness and therefore a 50% increase in concrete.
Volume ratio = new thickness ÷ old thickness
6 in ÷ 4 in = 1.5×
Extra per inch (cu ft) = slab area (sq ft) ÷ 12
The same 10 × 10 ft slab at three thicknesses
| Thickness | Volume | Calculated | Buy |
|---|---|---|---|
| 4 in | 1.23 yd³ (33.3 cu ft) | 55.56 × 80 lb bags | 56 × 80 lb bags |
| 5 in | 1.54 yd³ (41.7 cu ft) | 69.44 × 80 lb bags | 70 × 80 lb bags |
| 6 in | 1.85 yd³ (50 cu ft) | 83.33 × 80 lb bags | 84 × 80 lb bags |
This example assumes
- 80 lb bags at 0.60 cu ft each
- No extra allowance, so the comparison shows the thickness effect only
Each extra inch on a 100 sq ft slab adds about 8.3 cu ft, which is roughly 14 more 80 lb bags. That is the quick check for any area: area ÷ 12 gives cubic feet per inch, and dividing by the bag yield gives bags per inch.
It scales with area too
On a larger slab the same inch costs more concrete, because it spreads across more area. A 20 × 20 ft slab at 4 inches is 4.94 yd³ (133.3 cu ft), and at 6 inches it is 7.41 yd³ (200 cu ft).
Quantity is not the design question
This guide stops at how much concrete a given thickness takes. It does not say which thickness is right. That depends on how the slab will be used, the ground under it, local requirements and whether it is reinforced, and it is a question for the plans you are working from or a qualified professional.
What changes the estimate?
- The thickness you have decided on, measured as the finished depth
- Variation in depth across the slab, which a single average hides
- Allowance for an uneven base, which affects thick and thin slabs alike
- Bag size or a ready-mix order, which changes how the volume is bought