How This Calculator Works
Concrete volume is length times width times thickness, converted from cubic feet into cubic yards since concrete is ordered and priced by the yard. This calculator also adds your chosen waste factor to account for subgrade irregularities, spillage, and minor over-excavation, which is standard practice on real job sites rather than ordering the exact theoretical minimum.
The bag count estimate is based on standard 80 lb premix bags, which yield roughly 0.6 cubic feet of concrete each. This figure is useful for small pours like sidewalks or footings where ready mix delivery isn't practical.
Choosing Slab Thickness
- 3 to 4 inches: Walkways, patios, and shed floors
- 4 to 5 inches: Standard residential driveways
- 6 inches: Driveways expecting heavier vehicles or RV parking
- 6 to 8 inches: Commercial slabs or areas with heavy equipment traffic
Rebar Grid Spacing Explained
Rebar spacing refers to how far apart the steel reinforcement bars sit in both directions, forming a grid. Tighter spacing, like 12 inches, gives more reinforcement and is standard for driveways and slabs expected to bear vehicle weight. Wider spacing, like 24 inches, is common for light duty slabs like sheds or walkways where reinforcement is mostly there to control cracking rather than bear heavy loads.
Ordering Ready Mix vs Bagged Concrete
As a rule of thumb, pours needing more than roughly 1 cubic yard are usually more cost effective and less labor intensive with a ready mix truck delivery rather than dozens of bags mixed by hand. This calculator gives you both figures so you can decide which approach fits your project size and budget.