Concrete Calculator
For slab pours. Enter your slab dimensions. We'll show cubic yards and the equivalent in 60-lb and 80-lb bags.
Advanced options
v0 limit: slabs only. Footing, column, and stair calcs coming next — for now, use a slab equivalent as a rough estimator (footing = length × width × depth treated as a slab).
Order
0.00
cubic yards
- Area
- 0 sq ft
- Volume
- 0 cu ft
- 60-lb bags
- 0
- 80-lb bags
- 0
- Estimated cost
- —
How the concrete calculator works
For a slab, we calculate volume as length × width × (thickness / 12) in cubic feet, then divide by 27 to get cubic yards. A 10% waste factor covers spills, over-excavation, and the reality that formwork is never perfectly square. Concrete yards are ordered in quarter-yard increments, so we round up.
Bags vs. ready-mix truck
The break-even is around 0.5 cubic yards (~23 of the 80-lb bags). Below that, bags are almost always easier. Above that, a ready-mix truck delivers in an hour what it takes all day to mix from bags.
- 60-lb bag = 0.45 cu ft when mixed. 60 bags per cubic yard.
- 80-lb bag = 0.60 cu ft when mixed. 45 bags per cubic yard.
Slab thickness by project
- 3.5" — walkway or patio, light foot traffic only.
- 4" — the default for driveways, patios, and garage floors in mild climates.
- 5–6" — garage floors for heavy vehicles, workshop slabs, or freeze-thaw climates.
- 6"+ — structural slabs, commercial driveways, anything that'll support a vehicle turning while loaded.
Don't forget
- A 4-inch slab needs 4 inches of compacted gravel base under it. See the gravel calculator.
- Rebar or wire mesh for anything over 4 inches, or driveways of any thickness.
- An expansion joint every 10–15 feet to prevent cracking.
- A control joint cut into the slab within 24 hours of pouring.