Fence Post Concrete Calculator

Work out how many bags of concrete each fence post needs, subtracting the volume of the post itself.

About 3× the post width.
Rule of thumb: one-third of the post's above-ground height, below the frost line where required.

50 lb bags

31

  • Bags per post3.07
  • Concrete volume11.51 ft³

Estimates only. Round up when you order, and confirm structural details with your local building code.

How to use it

  1. Enter the number of posts.
  2. Enter hole diameter and depth.
  3. Pick the post size and bag size.

The formula

Concrete per post = π × (hole diameter ÷ 2)² × depth − post volume

Bags = total ÷ bag yield

Worked example

Number of posts10
Hole diameter10 in
Hole depth30 in
Post size4×4 wood (3.5 in)
Bag size50 lb

Result

50 lb bags31
Bags per post3.07
Concrete volume11.51 ft³

Tips before you order

  • Put 4–6 in of gravel at the bottom of each hole for drainage.
  • In frost areas, the hole must reach below the frost line.

Questions

How many bags of concrete per fence post?

A 10 in hole 30 in deep around a 4×4 needs about 1.15 ft³ — just over 3 bags of 50 lb, or 2 bags of 80 lb.