How this concrete calculator works
The calculator multiplies the length and width of your pour by its thickness to get the volume, in cubic yards or cubic meters — the units ready-mix suppliers quote in. A waste allowance is added on top, because some concrete is always lost to uneven subgrade, spillage and over-excavation. For most residential slabs a 10% allowance is the standard rule of thumb; increase it to 15% if your subgrade is rough or unformed.
How thick should a concrete slab be?
A typical patio, sidewalk or shed slab is 4 inches (10 cm) thick. Driveways that carry regular passenger vehicles should be 5 to 6 inches (12–15 cm), and slabs that will support heavy equipment or RVs are usually 6 inches or more, often with rebar or wire mesh. Footings are sized by local code — check frost depth requirements in your area before pouring.
Bags or ready-mix truck?
As a rule of thumb, bagged concrete makes sense up to about 1 cubic yard or 0.75 m³ — roughly 45 bags of 80 lb mix, or 60 bags of 25 kg mix. Beyond that, mixing by hand becomes slow and inconsistent, and a ready-mix delivery is usually cheaper per yard and produces a stronger, more uniform slab. Most suppliers have a minimum delivery and charge a short-load fee for small orders, so get a quote before deciding.
Yield of common bag sizes
An 80 lb bag of concrete mix yields about 0.60 cubic feet, a 60 lb bag about 0.45 and a 40 lb bag about 0.30 cubic feet — roughly 45 × 80 lb bags per cubic yard. In metric, a 25 kg bag yields about 12.5 liters (0.0125 m³), so one cubic meter takes about 80 bags. The calculator rounds bag counts up, since you can't buy a fraction of a bag.