Concrete Calculator
How it works
Length and width in feet, times depth converted from inches to feet, gives the slab's volume in cubic feet. Dividing by 27 (the number of cubic feet in a cubic yard) gives the figure ready-mix concrete is usually ordered in. Dividing the cubic-foot volume by how much one bag yields, then rounding up to the next whole bag, gives the bag count for bagged mix instead.
Frequently asked questions
Why does this show volume in both cubic yards and cubic feet?
Because the two buying methods use different units. Ready-mix concrete delivered by truck is typically ordered by the cubic yard, while a bagged mix's own instructions are typically written per cubic foot. Showing both means whichever way you end up buying, the number you need is already on the page.
Why are bags always rounded up?
Because a partial bag is not something you can buy, and coming up short mid-pour — with concrete already curing — is a real problem, not a minor inconvenience. Rounding up to the next whole bag means a small amount of leftover mix at worst, rather than a second trip to the store at best.
Does this account for waste or spillage?
No — treat this result as a minimum, not a final number. In practice, some concrete is lost to spillage, an uneven subgrade, or overfilled forms, so many pours buy a small extra margin beyond the calculated volume. This tool shows the calculated need only; adding a waste margin on top is a judgment call for the specific site.