Concrete Slab Calculator
Calculate the concrete needed for a slab in cubic feet and cubic yards, with 80 lb bag count, wastage, and estimated cost.
ft
ft
in
%
$
Results
Concrete Needed (with wastage)
1.09 yd³
Slab Area
80 ft²
Volume (base)
0.99 yd³
Volume (cubic feet, with wastage)
29 ft³
80 lb Bags Needed
49 bags
Total Cost
$141.23
How is Concrete Volume Calculated?
Concrete volume is the slab's length × width × thickness. In imperial units, thickness in inches is converted to feet (÷ 12), then the cubic footage is divided by 27 to get cubic yards — the standard unit for ordering concrete. A wastage percentage accounts for spillage, uneven subgrade, and seams.
Example
A 10 ft × 8 ft slab at 4 inches thick is 10 × 8 × (4/12) = 26.67 cubic feet, or about 0.99 cubic yards. Adding 10% wastage gives ~1.09 cubic yards — roughly 49 standard 80 lb bags — and at $130/yd³ it costs about $141.
Common Use Cases
- Estimating concrete for patios, driveways, and shed pads.
- Comparing ready-mix delivery vs. mixing bags yourself.
- Budgeting material cost for a slab project.
FAQs
How much does an 80 lb bag of concrete cover?
A standard 80 lb bag yields about 0.6 cubic feet of concrete. At 4 inches thick, one bag covers roughly 1.8 square feet, so a 10 ft × 8 ft slab needs around 49 bags.
How much wastage should I add?
A 5–10% allowance is typical for small slabs. Add more for uneven ground, multiple pours, or complex shapes where seaming and spillage increase losses.
What is a standard slab thickness?
Interior floors and small pads are often 4 inches thick, while driveways and heavier loads usually call for 5–6 inches. Always confirm against your local building code.