How to use this calculator
- Pick the shape. Rectangle/square for slabs and pads; circle for round pads and pad footings.
- Enter dimensions and thickness. Thickness accepts inches or feet (US) and millimetres, centimetres, or metres (metric) — the units convert and stay consistent.
- Add a waste allowance for spillage and uneven subgrade — 5–10% is common for small pours.
- Estimate one or both purchase options: bags using the yield printed on the bag, and/or ready-mix at your local price per cubic yard or cubic metre.
The formulas
Circle: volume = π × (diameter ÷ 2)² × thickness
Required volume adds the waste percentage. Bagged concrete divides that by the yield per bag and rounds up to whole bags; ready-mix multiplies the required volume by the price per cubic yard (or cubic metre). The two options are alternatives — compare them, never add them.
Worked example
A 10 × 6 ft walkway slab, 4 inches thick, 10% waste, 0.6 cu ft per 60-lb bag at $6.50, ready-mix at $155 per cubic yard:
| Step | Result |
|---|---|
| Volume: 10 × 6 × (4 ÷ 12) | 20 cu ft = 0.74 cu yd |
| Required with waste: 20 × 1.10 | 22 cu ft = 0.81 cu yd |
| Bags: 22 ÷ 0.6 = 36.7 → round up | 37 bags ≈ $240.50 |
| Ready-mix: 0.81 × $155 | ≈ $126 |
Same pour, two supply methods — for a job this size, ready-mix is often both cheaper and far less labor. For a single footing, bags win.
Cubic feet, cubic yards, cubic metres
Bags are labelled by the volume they yield (cubic feet in the US, litres or cubic metres elsewhere); trucks sell by the cubic yard in the US and the cubic metre almost everywhere else. The calculator displays all three units at once so either kind of supplier quote can be checked directly.
Common mistakes
- Entering thickness in inches while the calculator assumes feet — always check the unit next to the field.
- Using a generic bag yield instead of the one printed on your bag — 60-lb and 80-lb bags differ substantially.
- Adding bagged and ready-mix results together when comparing them.
- Skipping the waste allowance on uneven ground — subgrade irregularities consume real volume.
Structural concrete, reinforcement, foundations, and any load-bearing design require assessment by a qualified engineer or professional. This calculator is for planning small non-structural projects only.
FAQ
How many 60-lb bags make one cubic yard?
One cubic yard is 27 cubic feet. A 60-lb bag yields roughly 0.45 cu ft, so 27 ÷ 0.45 ≈ 60 bags. That is why bagged concrete suits small pours: a half-cubic-yard job already means about 30 bags to mix by hand.
How thick should my slab be?
Common non-structural work such as walkways and shed pads is often planned at around 4 inches, but the right thickness depends on use, soil, and local building codes. This calculator accepts whatever thickness you enter — it does not tell you what thickness is safe. Confirm with your local requirements for anything structural.
Can I combine bagged concrete and ready-mix?
They are alternative purchasing methods, not additions. Estimate both, compare the totals, and order one. The calculator labels every result with its option letter (A for bags, B for ready-mix) so the numbers never get mixed up.
Does the estimate include reinforcement or finishing?
No. Rebar, mesh, forms, curing compounds, and finishing tools are not part of the volume calculation. Structural reinforcement decisions in particular require a qualified professional.