How Much Concrete Do I Need? Quick Calculator for Any Project
An 80-lb bag makes 0.6 cubic feet. Here is how to calculate bags or yards for any slab, footing, or post hole.
The formula: Length (ft) × Width (ft) × Depth (ft) = cubic feet. Divide by 27 for cubic yards. Divide by 0.6 for 80-lb bags (or 0.45 for 60-lb bags). A 10×10 patio at 4 inches thick: 10 × 10 × 0.333 = 33.3 cubic feet = 1.23 cubic yards = 56 bags of 80-lb concrete. Always add 10% for waste and spillage.
Bag vs Ready-Mix Truck
Under 1 cubic yard (about 45 bags): bags are practical. Mixing 45 bags by hand takes 3-4 hours of intense labor. Over 1 cubic yard: a ready-mix truck is faster, easier, and often cheaper ($130-160 per cubic yard delivered, 1-yard minimum). The truck arrives pre-mixed and pours directly — no mixing, no bag disposal, done in 30 minutes.
Common Project Quantities
Fence post hole (10 inch diameter × 36 inch deep): 2 bags per post. Mailbox post: 3-4 bags. 4×4 slab: 9 bags. Sidewalk section (4×20 at 4 inches): 20 bags. Full driveway (10×40 at 6 inches): 3.7 cubic yards — definitely a truck job.