Enter the hole diameter and depth, the post width and how many posts you are setting. The calculator subtracts the space the post occupies.
How it is calculated
Concrete per hole = (hole area - post area) x depth. Hole area is pi x (diameter / 24)^2 square feet; post area is (post width / 12)^2.
Worked example
A 4x4 post in a 10 in wide, 30 in deep hole uses about 1.1 cubic feet, so roughly 2 bags of 80 lb mix per post.
Frequently asked questions
How deep should a fence post hole be?
About one third of the post length above ground, and below your frost line where required.
How many bags per fence post?
Usually 1 to 3 bags of 80 lb mix, depending on hole size.