Concrete Slab Calculator — Yards, Bags & Cost
Who this is for: For contractors and serious DIYers pricing a pour this week — shed pads, patio slabs, and one-car garage floors where you need the yardage and the bag math before the truck shows up.
Enter slab length, width, and thickness — get cubic yards, 80-lb bag count, and a cost ballpark. Built for shed pads, patios, and garage floors.
Quick answer: Slab volume = length × width × thickness ÷ 27. A 10×12 ft slab at 4 inches is 1.48 cubic yards (order 1.6 with waste) — about 50 eighty-pound bags per yard, so pours over roughly 1 yard usually go ready-mix.
4 in patios & sheds · 5-6 in vehicle slabs
10% standard · 15% broken forms
| Formula | L ft × W ft × thickness in ÷ 12 ÷ 27 = cubic yards |
|---|---|
| 80-lb bag yield | 0.60 ft³ per bag (≈45 bags/yd³) |
| 60-lb bag yield | 0.45 ft³ per bag (≈60 bags/yd³) |
| Default waste | 10% |
| Compiled | September 2026 |
How does this concrete slab calculator work?
Volume for a rectangular slab is length × width × thickness, divided by 27 to reach cubic yards — the unit ready-mix plants sell by. The calculator adds your waste factor (10% covers uneven subgrade and spillage) and converts the result into bag counts using published yields: an 80-lb bag yields about 0.60 ft³, a 60-lb about 0.45 ft³. Rule of thumb from the output: a 10×12 slab at 4 inches is about 1.5 yards — roughly 67 eighty-pound bags, which is exactly where bag-carrying stops making sense and a ready-mix truck (or a trailer of volumetric mix) pays for itself.
Common uses
- Sizing a shed pad or patio pour before ordering ready-mix
- Comparing the bag count against a truck's minimum-load charge
- Checking a subcontractor's yardage figure against your own numbers
- Budgeting material cost with a defensible waste factor