Concrete Calculator
Work out exactly how much concrete you need for a slab or footing — in cubic yards for ready-mix, or bags if you're mixing it yourself. Add your own price for a full cost estimate.
How this calculator works
The calculator finds the volume of each pour, adds a waste allowance for spillage and uneven ground, then converts to cubic yards or bags:
volume = length × width × thickness cubic yards = (volume × (1 + waste%)) ÷ 27 bags = (volume × (1 + waste%)) ÷ bag yield
A 60 lb bag yields about 0.45 cubic feet of concrete once mixed, and an 80 lb bag yields about 0.6 cubic feet — this calculator uses those standard figures.
Worked example
A 10 ft × 10 ft slab, 4 inches thick:
- Volume: 10 × 10 × (4 ÷ 12) = 33.3 cu ft.
- Add 10% waste: 33.3 × 1.10 ≈ 36.7 cu ft.
- In cubic yards: 36.7 ÷ 27 ≈ 1.36 cu yd of ready-mix.
- In 80 lb bags: 36.7 ÷ 0.6 ≈ 61.2 → 62 bags.
This is roughly the point where ready-mix starts to make more sense than bags — 62 bags is a lot of manual mixing, and most ready-mix suppliers can deliver just over a cubic yard, though minimum order sizes vary and are worth checking before you commit to a small pour.
Choosing the right slab thickness
| Use | Typical thickness | Notes |
|---|---|---|
| Garden path, stepping stones | 3" | Foot traffic only |
| Shed base, patio | 4" | Light structures, occasional foot traffic |
| Standard driveway | 5–6" | Passenger vehicles; reinforcement often required |
| Heavy-vehicle driveway | 6"+ | Trucks or RVs; check local code |
Always confirm the required thickness and any reinforcement rules (rebar or wire mesh) against your local building code before pouring anything load-bearing — this table is a general guide, not a substitute for code.
Ready-mix vs bags
Bagged concrete is practical for small jobs — fence post footings, a stepping-stone path, a small patch — up to roughly half a cubic yard. Past that, mixing by hand or with a rented mixer becomes a lot of physical work for not much time saved, and ready-mix delivered by truck is usually cheaper per unit of volume. Most ready-mix suppliers have a minimum order, so a very small pour may still be more practical in bags even if the per-yard cost looks worse on paper.
Frequently asked questions
How thick should a concrete slab be?
A garden path or stepping-stone slab is often around 3 inches thick, a shed base or patio is typically 4 inches, and a driveway carrying vehicle weight is usually 5 to 6 inches, sometimes more for heavier trucks. Always check your local building code for the specific use.
When should I use ready-mix instead of bagged concrete?
Bagged concrete is practical up to roughly half a cubic yard, such as a small slab, footing or fence post job. Beyond that, ready-mix delivered by truck is usually cheaper per unit of volume and far less physical labor, though most trucks have a minimum order size.
How long does concrete need to cure before use?
Concrete typically reaches enough strength to walk on after about 24 to 48 hours, but keeps curing for around 28 days before reaching its full design strength. Avoid parking vehicles or placing heavy loads on a new slab for at least a week, longer in cold weather.
Do I need rebar or wire mesh in a concrete slab?
Reinforcement is recommended for anything bearing vehicle weight, spanning more than a few feet unsupported, or sitting on soil prone to shifting, since it helps control cracking. A thin garden path slab may not need it, but check local code requirements for driveways, footings and structural slabs.