Concrete slab calculator: how many bags of concrete do I need for a slab?
56 bags
A 10 by 10 ft slab at 4 inches thick needs 1.23 cubic yards of concrete, which is 33.3 cubic feet. In bags that is 56 bags of 80 lb mix, because an 80 lb bag yields 0.60 cubic feet.
1.23 cubic yards · 33.3 cubic feet · for 10 × 10 ft at 4 in
Run your own numbers
Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
56 bags
1.23 cubic yards · 33.3 cubic feet
A worked example
A 13 by 9 ft pad at 4.5 inches thick needs 1.63 cubic yards, or 74 bags of 80 lb. Past about 1 cubic yard, ready-mix delivered by truck is usually cheaper than bags.
What else it is called
Bagged mix is sold as Quikrete, Sakrete and under store brands, and the yields are printed on the bag. Past roughly one cubic yard, ready-mix delivered by truck is usually cheaper than bags and a great deal less work.
Common sizes
| Area | 2 in deep | 3 in deep | 4 in deep | 6 in deep |
|---|---|---|---|---|
| 8 × 8 ft | 0.4 | 0.59 | 0.79 | 1.19 |
| 10 × 10 ft | 0.62 | 0.93 | 1.23 | 1.85 |
| 10 × 12 ft | 0.74 | 1.11 | 1.48 | 2.22 |
| 12 × 12 ft | 0.89 | 1.33 | 1.78 | 2.67 |
| 12 × 16 ft | 1.19 | 1.78 | 2.37 | 3.56 |
| 16 × 20 ft | 1.98 | 2.96 | 3.95 | 5.93 |
| 20 × 20 ft | 2.47 | 3.7 | 4.94 | 7.41 |
Cubic yards. Round up to what your supplier will deliver.
Where the numbers come from
- Bag yields: Quikrete Concrete Mix technical data sheet, Division 3, 03 31 00 Structural Concrete, document 1101. A 40 lb bag yields 0.30 cu ft, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 and 90 lb 0.675. Read 26 September 2026. Data sheet (PDF)
- Volume is geometry: 1 cubic yard = 27 cubic feet, exact by definition.
Bag counts are rounded up. Half a bag of mix cannot be saved once it is open, and a slab poured short is a slab poured twice.
Last checked September 26, 2026. How this site is made.