Concrete Slab Calculator
Estimate concrete for a slab in cubic yards and pre-mixed bags.
Concrete needed
1.36 cu yd
- Area
- 100 sq ft
- Without waste
- 1.23 cu yd
- Pre-mixed 80lb bags
- 62 bags
Estimated material cost
$224
How we calculate this
Slab volume = length × width × thickness ÷ 27 for cubic yards of ready-mix, or divide total cubic feet by 0.6 for the number of 80 lb pre-mixed bags (each bag yields about 0.6 cubic feet of concrete).
Example
Measuring tips
Standard slab thickness is 4 inches for patios and walkways, and often 5–6 inches for driveways carrying vehicle loads. Confirm thickness requirements with your local code.
Common mistakes
- Ordering ready-mix by the bag count instead of cubic yards for large pours (bags are for small jobs only).
- Not adding rebar or wire mesh reinforcement — use our Rebar Calculator alongside this one.
- Underestimating waste from spillage and form imperfections.
Frequently asked questions
How many bags of concrete do I need for a slab?
Divide your total cubic feet by 0.6 for 80 lb bags — this calculator does that automatically once you enter your slab dimensions.
How thick should a concrete slab be?
4 inches is standard for patios and sidewalks; driveways typically use 5–6 inches.
Related calculators
Estimates are provided for planning purposes. Actual material requirements may vary based on supplier specifications, compaction, site conditions, installation method, waste, and local code.