How Many 80lb Bags of Concrete Do I Need?
Quick answer: volume ÷ 0.6 cu ft per bag. One 80lb bag makes about 0.6 cu ft. A 10×10 slab 4 inches thick is 33.3 cu ft, so it needs 56 bags — 33.3 ÷ 0.6 = 55.5, round up.
- Bag yield: 0.6 cu ft per 80lb bag
- Volume math for slabs
- Bags by slab size
- Bags per cubic yard
- Footings and posts
- Worked example: 10×10 slab
- FAQ
Bag yield: 0.6 cu ft per 80lb bag
An 80lb bag of concrete mix makes about 0.6 cubic feet of concrete. A 60lb bag makes about 0.45 cu ft; a 40lb bag about 0.3 cu ft. These are the standard yields for Quikrete and similar bagged mixes — the exact number is printed on the bag.
Volume math for slabs
Convert thickness to feet (4 in = 0.33 ft), then length × width × thickness = cubic feet. A 10×10 slab at 4 inches: 10 × 10 × 0.33 = 33.3 cu ft. Divide by 0.6 for the bag count.
| Slab size | 4 in thick (cu ft) | 80lb bags | 6 in thick (bags) |
|---|---|---|---|
| 4×4 | 5.3 | 9 | 14 |
| 8×10 | 26.7 | 45 | 67 |
| 10×10 | 33.3 | 56 | 84 |
| 10×12 | 40 | 67 | 100 |
Bags per cubic yard
One cubic yard is 27 cu ft — about 45 eighty-pound bags (27 ÷ 0.6). When a job approaches a full yard, ready-mix delivery almost always beats hauling and mixing 45+ bags by hand.
Footings and posts
For footings and fence post holes, compute the hole volume the same way and divide by bag yield. A typical 10-inch-diameter post hole 24 inches deep holds about 1.2 cu ft — roughly 2 eighty-pound bags per hole.
Worked example: a 10×10 slab, 4 inches thick
- Volume = 10 × 10 × 0.33 = 33.3 cu ft
- 33.3 ÷ 0.6 = 55.5 → 56 bags
- + 5% extra → 59 bags for uneven ground
FAQ
Calculate the volume in cubic feet and divide by 0.6 — one 80lb bag makes about 0.6 cu ft. A 10×10 slab 4 inches thick is 33.3 cu ft, so it needs 56 bags.
About 0.6 cubic feet per 80lb bag. A 60lb bag makes about 0.45 cu ft, and a 40lb bag about 0.3 cu ft.
About 56 bags for a 4-inch-thick 10×10 slab. Volume = 33.3 cu ft; 33.3 ÷ 0.6 = 55.5, round up to 56. A 6-inch slab needs about 84 bags.
About 45 bags per cubic yard. One cubic yard is 27 cu ft; 27 ÷ 0.6 = 45. Consider ready-mix near a full yard.
Yes — add 5–10% for uneven ground, spillage and sub-base variations. Bags are the cheapest insurance on a small job.