Free tool
How much concrete does this slab need?
Volume first, bags second. The bag count depends on the yield printed on the bag, so check it before ordering.
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Tools
Volume and bags
Volume in cubic feet is length times width times depth, with depth converted from inches. One cubic yard is 27 cubic feet. Bag counts use typical yields of 0.45 cubic feet for a 60 lb bag and 0.60 cubic feet for an 80 lb bag, rounded up to whole bags.
Weight uses 145 lb per cubic foot, a common density for cured concrete. A delivered truck is priced by the yard and usually has a minimum order, which can make bags cheaper for small pours.
Order a little extra
Ground is never perfectly level and formwork moves. For slabs, five to ten percent extra is normal; for footings, more. Running short mid-pour creates a cold joint that cannot be fixed afterwards.
Frequently asked questions
Does the depth go in inches?
Yes. A typical driveway slab is four inches; a shed base may be the same, while a heavier pad can be six.
Is bag yield always the same?
No. It varies by mix and brand. The calculator uses common yields; read the bag for the exact figure and adjust if needed.
Should I add water to the bag count?
No, water is not part of the volume. Add only the mix water specified on the bag, and never extra to make it easier to place.
When is ready-mix cheaper?
Usually once the pour exceeds roughly a cubic yard, and sooner if the site is accessible to a truck. Compare the delivered price including the minimum charge.