Post Hole Concrete Bag Calculator

Only the space around the post, not the full hole. The tool computes the cylinder volume of the round hole from its diameter and depth, subtracts the volume of the square post that occupies the middle, and converts what is left into cubic feet, then into bags at the yield you enter.

A widely used guide is a hole about three times the post width across and roughly a third of the post's above-ground height deep, and always below your local frost line. Deeper, wider holes hold a post more firmly but use more concrete, so set the dimensions to your own plan and let the tool follow them.

A 60 lb bag of standard premix yields about 0.45 cubic feet and an 80 lb bag about 0.6 cubic feet, which are the two common values to enter. Fast-setting and high-strength blends can differ, so check the yield printed on your bag and type that figure into the Bag yield box.

Concrete is sold only in whole bags, so any fraction of a bag rounds up to the next one. Buying that extra partial bag as a full one guards against a hole that dug out wider than planned and saves a mid-pour trip to the store. The cubic foot figures above are shown unrounded so you can see how close the last bag is.

Yes. A thicker post fills more of the hole and leaves less room for concrete, so it lowers the count. In the same hole a 6 by 6 uses noticeably less concrete than a 4 by 4, because the larger post displaces more of the volume you would otherwise fill.

The engine subtracts a square post based on the width you enter, so a genuinely round post will displace a little less than that. For a round post you can either accept the slightly conservative result, which leaves a small margin, or reduce the post width value to trim the estimate.

If the post volume meets or exceeds the hole volume, the concrete per hole is floored at zero rather than going negative, and the bag count comes out as zero. That is a signal that your hole is not wider than the post, so widen the hole before setting.

Many builders put a few inches of gravel under the post for drainage, which reduces the concrete depth. This tool fills the full hole depth you enter, so if you use a gravel base, subtract its thickness from the hole depth before calculating.

No. The whole estimate is computed in your browser from basic geometry, and none of your measurements are uploaded or stored. It works offline once the page has loaded.