Step 1: volume
- Slab: length × width × thickness, all in feet. A 10 × 10 ft slab 4 in thick: 100 × 4 ÷ 12 = 33.33 cu ft.
- Post hole: π × (diameter ÷ 2)² × depth.
Step 2: the yield on the bag
Each bag states how much mixed concrete it makes. For example, Quikrete’s data sheet for its Concrete Mix lists approximately:
| Bag | Approximate yield |
|---|---|
| 40 lb | 0.30 cu ft |
| 60 lb | 0.45 cu ft |
| 80 lb | 0.60 cu ft |
Source: Quikrete Concrete Mix 1101 data sheet. Other products differ, including fast-setting post mixes; use the figure on your bag. The same data sheet says this mix is designed for pours 2 in thick or more.
Step 3: divide and round up
Bags = volume ÷ yield, rounded up. The 33.33 cu ft slab needs 33.33 ÷ 0.60 = 55.6, so 56 bags of that 80 lb product before any allowance. One cubic yard (27 cu ft) is 45 of those bags.
Post holes: subtract the post
The post takes up space in the hole. A nominal 4×4 is actually 3½ in square, a 6×6 is 5½ in. For a 10 in hole with 24 in of concrete:
- Hole: π × 5² × 24 = 1,885 cu in.
- 4×4 post: 3.5 × 3.5 × 24 = 294 cu in.
- Concrete: 1,591 cu in = 0.92 cu ft per hole. Ten holes: 9.21 cu ft = 16 bags at 0.60 cu ft.
Ignoring the post here would overstate the concrete by about 18%.
Allowance, and what this doesn’t cover
Forms bulge and ground is uneven, so many people order a little extra; how much is your call. Thickness, reinforcement, footing size and frost depth are design and code questions, and slabs that support structures may need a permit. Call 811 before you dig.