# Fence Post Concrete Calculator

Hole and post size to bags per hole.

- Page: https://jianzhujisuan.com/en/fence-post-concrete
- Category: Concrete (https://jianzhujisuan.com/en/c/concrete)
- Chinese version: https://jianzhujisuan.com/zh/fence-post-concrete.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Hole diameter 10 in, Hole depth 30 in, Post size (square) 4 in, Number of posts 10 pcs, Waste 10%, Bag size 50 lb fast-set (0.375 ft³)): **Bags per post 4 bags**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Hole diameter | number | 10 | in |  |
| Hole depth | number | 30 | in |  |
| Post size (square) | number | 4 | in |  |
| Number of posts | number | 10 | pcs |  |
| Waste | number | 10 | % |  |
| Bag size | select | 50 lb fast-set (0.375 ft³) | — |  |

## Formula

- **Hole = π × (D/24)² × depth/12**: D in inches
- **Post = (side/12)² × depth/12**: the buried length only
- **Net = (hole − post) × (1 + waste)**: 
- **Bags = net ÷ bag yield**: 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 ft³

## Worked example

Ten 4x4 posts in 10 in holes 30 in deep with 10% waste and 50 lb bags: the hole holds 1.36 ft³ and the post takes 0.28 ft³, so net with waste is 1.20 ft³, which is 4 bags per post, 40 bags total, 0.44 cubic yards or about 1730 lb.

## Questions

### Why does this say 4 bags when two per post is the usual rule?

It comes down to hole size. A 10 in hole at 30 in deep holds 1.36 ft³, less the 0.28 ft³ that the 4x4 post occupies, which with 10% waste is 1.20 ft³ - exactly 4 bags of 50 lb at 0.375 ft³. The common 8 in by 24 in hole holds only 0.70 ft³, so after the post about 0.52 ft³ is needed and 2 bags do it. That is the 'two bags a post' figure people quote. Measure the holes you are actually digging.

### How deep should the post hole be?

The rule is one third of the exposed height, with 24 in as an absolute minimum. In cold regions the hole must go below frost depth or heave lifts the post and the fence leans within a season. In exposed or high-wind sites add a gravel base and dome the concrete away from the post so water runs off.

### Fast-setting or regular concrete?

For fence posts fast-setting is far easier: pour it dry into the hole, add water, and it stiffens enough to hold the post in 20-40 minutes, so you can brace and carry on. It costs more per bag and comes out a bit weaker, so load-bearing members - deck and porch posts - want regular mix with reinforcement. Fast-setting cannot be re-wetted: once water is in, that bag has to be used.

## Related tools

- [Concrete Volume Calculator](https://jianzhujisuan.com/en/concrete-volume): Enter length, width and thickness to get the concrete volume you need and how many truck loads that is.
- [Formwork Area Calculator](https://jianzhujisuan.com/en/formwork-area): Pick a member type, enter section and length, and get the formwork contact area plus sheet and batten estimates.
- [Concrete Mix Design Calculator](https://jianzhujisuan.com/en/concrete-mix-ratio): Work out cement, sand, gravel and water per cubic metre from the target grade and water-cement ratio.
- [Cement Quantity Calculator](https://jianzhujisuan.com/en/cement-quantity): Get total cement, bag count and the cement/fly-ash split for a given volume and grade.
- [Concrete Pour Sequencing Calculator](https://jianzhujisuan.com/en/concrete-pour-plan): Divide a slab into strips and get the volume, truck count and duration for each pour.
- [Concrete Strength Converter](https://jianzhujisuan.com/en/concrete-strength-convert): Convert between grade, MPa, psi and kgf/cm², with cube-to-cylinder correction.

---

Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.