Post Hole Concrete Calculator

A post hole is a cylinder: volume = π × (diameter ÷ 2)² × depth. A 12 in wide, 36 in deep hole is π × 0.5² × 3 = about 2.36 cu ft. Multiply by the number of holes, add a waste allowance for holes that bell out at the bottom, and divide by the yield of your bag size.

If a post will sit in the hole, select it above and the calculator subtracts the concrete the buried post itself displaces (a 4x4 in a 12 in hole displaces about 11% of the hole's volume). Most calculators ignore this and quietly over-sell you concrete; the worked examples on this page are figured with the hole empty.

Bag yields are the manufacturers' published figures. Quikrete and Sakrete publish the same yields, and the 50 lb figure is Quikrete's: an 80 lb bag makes 0.60 cu ft, a 60 lb bag 0.45, a 50 lb bag 0.375 and a 40 lb bag 0.30 cu ft of placed concrete.

Two field rules worth knowing: dig the hole about three times the post's width (a 4×4 post wants roughly a 12 in hole), and set the depth below your local frost line. A third of the post in the ground is the usual starting point. The Concrete Bag Calculator handles flat pours like slabs and patios. Setting the posts for a fence run? The Fence Calculator counts the posts, rails and pickets.