How to Calculate Concrete Bag Quantities
Concrete volume is length × width × thickness (all in feet), then divide by 27 for cubic yards. Then divide your cubic feet by the bag yield to get bag count.
cubic feet = L (ft) × W (ft) × thickness (ft)
bags = cubic feet ÷ bag yield (cu ft per bag)Bag Yield Reference — Quikrete Specifications
| Bag size | Yield (cu ft) | Bags per cu yd | Notes |
|---|---|---|---|
| 40 lb | 0.30 | 90 | Good for small repairs, hand-mixing |
| 60 lb | 0.45 | 60 | Popular for post holes |
| 80 lb | 0.60 | 45 | Most efficient, most common |
Source: Quikrete Concrete Mix product specification
Slab Thickness Guide
| Application | Thickness | Notes |
|---|---|---|
| Sidewalk, patio (foot traffic) | 4 inches | Standard residential |
| Driveway (passenger cars) | 4–5 inches | 5 in for heavier vehicles |
| Garage floor | 4–6 inches | 6 in for trucks/RVs |
| Shed floor | 3.5–4 inches | Minimum 3.5 in |
| Footings (residential) | Varies by load | Consult IRC or local code |
Source: IRC (International Residential Code) Table R506.1; ACI 318
Worked Example: 12×20 ft Driveway Section
- Dimensions: 12 ft × 20 ft × 4 in (0.333 ft)
- Volume: 12 × 20 × 0.333 = 80 cu ft = 2.96 cu yd
- With 10% waste: 88 cu ft = 3.26 cu yd
- At 0.60 cu ft per 80 lb bag: 147 bags
- At ready-mix pricing (~$150–$200/yd): $490–$650 for concrete alone
Recommendation: At 3+ cubic yards, call a ready-mix supplier. Mixing 147 bags by hand takes 3–4 workers a full day and is rarely as consistent as transit-mixed concrete.
Fence Post Footing Depth
The standard rule is footing depth = 1/3 of total post length, minimum. In northern climates, footings must extend below the local frost line — which ranges from 0 inches in Florida to 60+ inches in northern Minnesota. Check your county's frost depth map. A post that heaves out of a shallow footing will lean in the first hard winter.