HesapHanem

Post Hole Concrete Calculator

Concrete bags needed to set your fence or deck posts.

How it is calculated

Hole diameter, hole depth and post width are in inches, and the bag yield is in cubic feet. For each hole the tool finds the cylinder π × (d/2)² × depth, then subtracts the post, taken as a square post width² × depth (all converted to feet). The remaining concrete per hole is multiplied by the number of holes, and the bag count is the total divided by the bag yield, always rounded up to a whole bag.

All five boxes must be filled in, and holes, diameter, depth and bag yield must be greater than zero. Post width may be 0, which means the whole hole is filled. A round post is therefore treated as a square of the same width, which slightly overestimates the post and underestimates the concrete. The bag yield is printed on the bag, and varies by product and weight. The tool allows nothing for waste or spillage, and the volume is in cubic feet.

Worked example

Ten holes, 10 in wide and 36 in deep, with 3.5 in square posts and bags that yield 0.6 ft³. One hole: π × (5/12)² × 3 ft = 1.636 ft³. The post takes (3.5/12)² × 3 = 0.255 ft³, leaving 1.381 ft³ per hole. For ten holes: 13.81 ft³. Bags: 13.81 / 0.6 = 23.02, rounded up to 24 bags. With no post (post width 0) the volume would be 16.36 ft³ and 28 bags.

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Common mistakes

  • Entering the hole size in feet or centimetres. Diameter, depth and post width are inches, and the output is cubic feet.
  • Using the bag's weight as its yield. The yield is the volume in cubic feet when mixed, shown on the bag, such as 0.6.
  • Leaving the post width blank. All five boxes are needed, so type 0 if you want no allowance for the post.
  • Entering the actual post length instead of the buried depth. Only the part in the hole matters.
  • Buying exactly the bag count shown. The tool rounds up but adds no waste, and uneven hole walls or a wider hole at the top take more concrete.

About this calculator

This calculator finds the concrete volume for a cylindrical post hole — hole volume minus the post's own volume — then converts that to the number of pre-mixed concrete bags needed, using each bag's standard yield (once mixed, a 60 lb bag typically yields about 0.45 cubic feet and an 80 lb bag about 0.6 cubic feet, roughly 0.013 and 0.017 cubic meters).

Fence installers, deck builders and DIYers use it before a single trip to the hardware store, since underestimating bags means a wasted second trip and overestimating wastes money — the rule of thumb of "hole diameter three times the post width, depth one-third to half the post's above-ground height" is a good guide for choosing the hole diameter and depth you enter here.

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Comments (1)

  • Editorial team

    Post width is the real size, not the nominal one: a lumber 4x4 measures about 3.5 inches, so entering 3.5 rather than 4 matters. With six holes of 10 inch diameter and 24 inch depth, and a bag yield of 0.6 cubic feet from the bag label, the tool gives 5.52 cubic feet, 10 bags. Which post sizes do you set most often?

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