About the Concrete Calculator
Ordering concrete for a slab means knowing the volume you need, not just the surface area - depth matters just as much as length and width, and it's easy to underestimate.
How It Works
The calculator multiplies length by width by depth (converted from inches to feet) to get cubic feet, then converts to cubic yards - the unit concrete is typically ordered by - and estimates how many standard 80lb bags of pre-mixed concrete that volume would take.
Examples
A 10x10 patio slab
10 ft × 10 ft × 4 in depth needs about 1.23 cubic yards, or roughly 56 bags of 80lb pre-mixed concrete.
A thicker driveway slab
The same 10x10 footprint at 6 inches deep needs about 1.85 cubic yards - proportionally more since depth increased by 50%.
Advantages
- Shows both cubic yards (for ready-mix orders) and bag count (for DIY pours)
- Compares multiple depths side by side in the results table
- Simple three-field input for a fast estimate
Common Mistakes
- Forgetting to add extra for spillage and uneven subgrade
- Confusing cubic feet with cubic yards when placing a ready-mix order
- Not accounting for a ready-mix truck's minimum order size on small pours
Edge Cases to Watch For
- Ready-mix trucks deliver by the cubic yard with typically a minimum order (often 1 yard), so very small slabs may cost the same as a slightly larger one due to the minimum charge.
- An 80lb bag yields about 0.6 cubic feet of mixed concrete - the bag count in this calculator assumes standard pre-mixed bags, not a custom mix ratio.
- Uneven or sloped subgrade means actual volume needed can exceed the flat-surface calculation - contractors commonly add 5-10% for this.
Common Use Cases
- Ordering ready-mix concrete for a slab, patio, or footing
- Estimating how many bags of pre-mixed concrete a DIY project needs
- Comparing material cost across different slab thicknesses