Ordering too little concrete means a mid-pour trip to the hardware store. Ordering too much wastes money and creates disposal headaches. Getting it right requires calculating volume accurately, adding a waste factor, and converting to the right unit (bags or cubic yards).
The Basic Volume Formula
Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft) Cubic Yards = Volume (cu ft) / 27 A 10 × 12 foot patio at 4 inches thick: 10 × 12 × 0.333 = 40 cu ft = 1.48 cubic yards.
Common Project Calculations
| Project | Typical Dimensions | Volume | 80-lb Bags |
|---|---|---|---|
| Sidewalk (4×20 ft, 4 in) | 4 × 20 × 0.33 | 26.7 cu ft | 45 bags |
| Patio (10×12, 4 in) | 10 × 12 × 0.33 | 40 cu ft | 67 bags |
| Post hole (10 in dia, 42 in deep) | πr²h | 2.3 cu ft | 4 bags |
| Driveway (10×20, 6 in) | 10 × 20 × 0.5 | 100 cu ft | 3.7 cu yd (truck) |
Waste Factor
Always add 5–10% for waste. Concrete gets spilled, forms are imperfect, and ground is uneven. For irregular shapes, add 10–15%. A $20 extra bag is better than stopping mid-pour.
Bags vs Truck Delivery
- Under 1 cubic yard: Use premixed bags (60 or 80 lb). An 80-lb bag yields 0.6 cu ft.
- 1–3 cubic yards: Consider a ready-mix truck or trailer mix from a hardware store.
- Over 3 cubic yards: Truck delivery is almost always more economical and practical.
Calculate your exact needs with the Concrete Calculator, which handles slabs, holes, tubes, and stairs with automatic bag counts.
Key Takeaways
- Volume = L × W × D, then divide by 27 for cubic yards.
- Add 5–10% waste factor to your calculated volume.
- One 80-lb bag yields about 0.6 cubic feet of concrete.
- Over 1 cubic yard, consider truck delivery for cost and practicality.
Frequently Asked Questions
How many bags of concrete do I need for a 10x10 slab?
A 10x10 foot slab at 4 inches thick requires about 33 cubic feet, or approximately 56 bags of 80-lb premixed concrete. Add 5-10% for waste, so order 60-62 bags. At this volume, consider a trailer mix or small truck delivery instead.
How thick should a concrete slab be?
Sidewalks and patios: 4 inches. Driveways for cars: 4-6 inches. Driveways for heavy vehicles: 6-8 inches. Garage floors: 4-6 inches. Always check local building codes for minimum requirements.
What PSI concrete should I use?
Standard projects (sidewalks, patios): 3,000 PSI. Driveways and garage floors: 3,500-4,000 PSI. Heavy traffic or structural: 4,000-5,000 PSI. Higher PSI costs more but provides greater durability.