Post Hole Concrete Calculator — Fence Posts & Footings

Fence runs and gate posts: enter hole count, diameter and depth, get total bags — split per hole.

Fence Post Concrete Calculator

The hole rule: 3× the post, ⅓ in the ground

A fence post hole should be about three times the width of the post (a 4×4 post wants a 12-inch hole) and buried at least one-third of the fence's height, plus 6 inches of gravel at the bottom for drainage. For a 6-foot fence that means a 2-to-3-foot hole — right at the frost line in most of the US, which is the real requirement in freeze country.

Bags are the right format for this job

A single 12-inch × 3-foot hole takes about 1 cubic foot of concrete — under two 80-lb bags. That is exactly why fence work is a bagged-concrete job: you carry a few bags per hole instead of paying a truck minimum. Fast-setting (no-mix) bags are popular here: pour dry, add water, set the post level in minutes.

Crown the top

Finish each pour with the concrete sloped slightly away from the post so rain runs off rather than pooling around the wood. It is a two-second trowel move that adds years to the post.

FAQ

How many bags of concrete per fence post hole?

A 12-inch hole, 3 feet deep, takes about 1 cu ft — roughly 2 bags of 80-lb mix (or 3 of the 50-lb fast-set bags). Narrow 10-inch holes at 2 feet take about one 80-lb bag.

Should fence posts be set in concrete or gravel?

Concrete for wood and steel fence posts in almost all cases. Gravel-only setting works for some farm fencing and lets posts drain, but it is looser in wind and freeze-thaw.

How deep is the frost line?

It ranges from 0 inches in the deep South to 42+ inches in northern states. Your local building department publishes the number — posts set above it will heave.