Concrete Calculator
Calculate the volume of concrete needed for a slab, and how many bags of ready-mix you'll need.
Why thickness matters more than you'd expect
Because volume scales directly with thickness, small changes make a bigger difference than they might seem — going from a 4-inch to a 6-inch slab increases the concrete needed by 50%, not a small amount. Standard residential slabs (walkways, patios) are commonly 4 inches, while driveways supporting vehicle weight are often poured at 5-6 inches.
It's standard practice to order slightly more concrete than the calculated minimum — a 5-10% buffer accounts for uneven subgrade, spillage, and slight measurement variance, and running short mid-pour is a much bigger problem than having a small amount left over.
Ready-mix bags vs. delivered concrete
For small projects, bagged concrete mix is practical — this calculator estimates both 60 lb and 80 lb bag counts, the two most common sizes. For larger pours (typically over 1-2 cubic yards), ready-mix delivered by truck is usually more cost-effective and easier to work with than mixing dozens of bags by hand.
One cubic yard of concrete equals roughly 45 bags of 60 lb mix or 40 bags of 80 lb mix, which is why most contractors switch to truck delivery once a project crosses that threshold.
Frequently asked questions
Should I order extra concrete beyond the calculated amount?
Yes — a 5-10% buffer is standard practice to account for uneven ground, spillage, and measurement variance, since running short during a pour is far more disruptive than having a small surplus.
At what point should I switch from bags to delivered ready-mix?
Once a project needs more than roughly 1-2 cubic yards of concrete, delivered ready-mix is generally more practical and cost-effective than mixing dozens of individual bags.