How Many Bags of Concrete Do You Need? (40, 60 and 80 lb Tables)
Bag counts for common slab sizes and post holes in 40, 60 and 80 lb bags, and when to switch to a ready-mix truck.
Premixed concrete comes in 40, 60 and 80 lb bags. To find how many you need, work out the volume in cubic feet and divide by the bag’s yield. The tables below have done that for common jobs, with 10% extra for waste.
Bag yields
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.6 cu ft | 45 |
Slabs, 4” thick
| Slab (ft) | Cu yd | 40 lb | 60 lb | 80 lb | Ready-mix |
|---|---|---|---|---|---|
| 4 × 4 | 0.22 | 20 | 14 | 10 | 0.25 yd |
| 4 × 8 | 0.43 | 40 | 27 | 20 | 0.5 yd |
| 6 × 6 | 0.49 | 44 | 30 | 22 | 0.5 yd |
| 8 × 8 | 0.87 | 79 | 53 | 40 | 1 yd |
| 10 × 10 | 1.36 | 123 | 82 | 62 | 1.5 yd |
| 10 × 12 | 1.63 | 147 | 98 | 74 | 1.75 yd |
| 12 × 12 | 1.96 | 176 | 118 | 88 | 2 yd |
| 12 × 16 | 2.61 | 235 | 157 | 118 | 2.75 yd |
To change thickness, recalculate the volume before rounding the bag count. A 6-inch slab has 1.5 times the volume of a 4-inch slab of the same area. A 5-inch slab has 1.25 times the volume.
Fence and deck post holes
Each hole has 6” of gravel in the bottom for drainage, and the volume of the post is subtracted.
| Hole (diameter × depth) | Post | 60 lb bags per hole | 80 lb bags per hole | 80 lb bags for 10 holes |
|---|---|---|---|---|
| 8" × 24" | 4×4 | 0.97 | 0.73 | 8 |
| 10" × 36" | 4×4 | 2.81 | 2.11 | 22 |
| 12" × 36" | 6×6 | 3.52 | 2.64 | 27 |
| 12" × 48" | 6×6 | 4.92 | 3.69 | 37 |
Round partial bags up per hole if you mix one hole at a time; buying for the whole fence at once wastes less.
The formula
- Volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12.
- Add waste: multiply by 1.1 for 10% extra.
- Bags = volume ÷ bag yield, rounded up.
Example: a 10 × 10 ft slab, 4” thick: 10 × 10 × 4 ÷ 12 = 33.3 cu ft. With 10% extra, 36.7 cu ft. Divided by 0.6, that’s 62 bags of 80 lb.
Bags or ready-mix?
Bags are fine for post holes, steps, small pads and repairs. Around 1 cubic yard (about 45 bags of 80 lb), most people switch to a ready-mix truck:
- One even batch means a stronger, more consistent slab.
- You avoid hours of mixing while the first batches start to set.
- Many plants charge a short-load fee for small orders, so get a delivered price and compare.
Some suppliers also rent small towable mixers with a yard or less already mixed, a handy middle option.
Which bag size?
The concrete is the same; only the weight changes. 80 lb bags cost less per cubic foot but are heavy to carry. 40 and 60 lb bags are easier on your back and better for small jobs where you’d otherwise open a big bag for a little concrete.
Use the Concrete Slab & Footing Planner for any size, thickness or number of holes, plus rebar, base gravel and forms.