Material estimator

Post Hole Concrete Calculator

Estimate concrete bags for fence posts, deck posts, mailbox posts, and other round post holes.

How This Calculator Works

Post holes are repetitive, so small rounding errors multiply fast. This calculator estimates concrete around square posts using hole diameter, hole depth, post width, and post count.

Formula

Concrete per hole = cylinder volume of the hole minus square post displacement. Total volume = per-hole volume x number of holes x overage.

Assumptions

The calculator assumes straight-sided holes. Bell-shaped footings, loose soil, frost-depth requirements, and gravel drainage bases may change the quantity.

Example

Eight 10 inch diameter holes, 36 inches deep, around 4x4 posts with 10% overage need about 12.15 ft^3 of concrete, or 21 bags of 80 lb mix.

Before You Buy

Confirm hole depth against local frost depth, fence height, and post type before buying material.
If you add gravel below each post, subtract that depth from the concrete depth or buy concrete separately from drainage stone.
For irregular holes, increase waste. Hand-dug holes are rarely perfect cylinders.

Common Questions

How wide should a post hole be?

A common rule of thumb is about three times the post width, but fence height, soil, and local code matter.

Should I subtract the post from the hole volume?

Yes for a closer estimate. The post takes up space that concrete will not fill.

Do I need gravel at the bottom?

Many post installations use gravel for drainage. Gravel is not counted as concrete volume in this calculator.