How Many Bags of Concrete Do I Need? Quick Calculator
An 80-lb bag makes 0.6 cubic feet. Here is a quick chart for common projects.
The formula: Length (ft) × Width (ft) × Depth (ft) = cubic feet. Divide by 0.6 for 80-lb bags or 0.45 for 60-lb bags. Always add 10% for waste.
Quick Reference Chart
Fence post hole (10 inches × 3 ft deep): 2 bags per post. Mailbox post: 3-4 bags. 4×4 ft pad (4 inches thick): 9 bags. 8×10 ft patio (4 inches thick): 45 bags. Basketball hoop: 8-10 bags. Small retaining wall footing (20 ft long): 15-20 bags.
When to Use a Ready-Mix Truck Instead
Over 30 bags (about 0.67 cubic yards), a ready-mix delivery is faster and often cheaper. Mixing 30+ bags by hand takes 3-4 hours and significant physical effort. A ready-mix truck delivers mixed concrete ready to pour at $130-160 per cubic yard with a typical 1-yard minimum.