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Concrete Calculator

How many bags for a slab or post holes — includes 10% extra.

4" is standard for patios and walkways

How many bags of concrete a slab or a set of post holes actually takes — including the extra you should order, because running short mid-pour is a genuine disaster.

How it works

A slab's volume is length × width × thickness, converted to cubic feet and then to cubic yards by dividing by 27. Post holes are cylinders — π × radius² × depth — with the volume of the post itself subtracted, since the post displaces concrete. Ten percent is added to whatever you order, and the total is converted into bags using the yield printed on the bag (an 80-pound bag makes about 0.6 cubic feet).

Example

A 10 × 10 foot slab, 4 inches thick, is 33 cubic feet — about 1.4 cubic yards with waste, or roughly 60 eighty-pound bags.

Tips & common mistakes

  • Order 10 percent extra, always. Concrete has no second chances: if you run out partway through a pour, the joint between yesterday's concrete and today's is a permanent weak seam.
  • Somewhere around 40 to 50 bags, ready-mix delivered by truck starts to be both cheaper and vastly less work than mixing bags by hand. Get a quote before you commit to a weekend of mixing.
  • Concrete does not care about your schedule. Once it starts to set, it sets — have every tool, every form, and every helper in place before you add water to the first bag.
  • Damp-cure the slab for a few days rather than letting it dry in the sun. Concrete gains strength by hydrating, not by drying, and a slab that dries too fast cracks.

Frequently asked questions

How many bags of concrete do I need for a 10x10 slab?

About 60 eighty-pound bags for a 4-inch-thick slab, including a 10 percent overage. At that quantity it is worth pricing a ready-mix delivery.

How many 80 lb bags are in a cubic yard?

About 45. An 80-pound bag yields roughly 0.6 cubic feet, and there are 27 cubic feet in a cubic yard.

Should I use concrete or gravel for fence posts?

Concrete for anything that takes a load or catches wind — gates and fence posts especially. Gravel drains better and is fine for light-duty posts, but it does not hold nearly as well.