# Post Hole Concrete Calculator: How Many Bags?

> Calculate how many bags of concrete you need to set fence posts, deck posts, a mailbox or a flagpole — after subtracting the volume of the post itself.

**Quick answer:** A 4×4 post in a 10-inch hole 30 inches deep, with a 4-inch gravel base, takes about 1 cubic foot of concrete, or roughly 1.7 bags of 80-lb mix, so plan on 2 bags per post. Ten posts need about 19 bags including 10% extra.

_Last updated: October 3, 2026. Estimates only; confirm quantities with your supplier._

## How much post hole concrete do you need?

Post holes are cylinders: π × radius² × depth. Subtract the volume of the post that sits in the concrete, and what remains is the concrete you need.

```
hole volume = π × (diameter ÷ 2)² × concrete depth
concrete = hole volume − post width² × concrete depth
```

The 80-lb bag yields about 0.60 cubic feet, the 60-lb bag 0.45 and the 50-lb bag 0.375. Divide the total volume by the yield and round up.

### Bags per post (4×4 post, 4 in gravel base, 80-lb bags)

| Hole size | Concrete per post (ft³) | 80-lb bags per post |
| --- | --- | --- |
| 8 in dia. × 24 in deep | 0.44 | 0.7 |
| 10 in dia. × 24 in deep | 0.77 | 1.3 |
| 10 in dia. × 30 in deep | 1.00 | 1.7 |
| 12 in dia. × 30 in deep | 1.52 | 2.5 |
| 12 in dia. × 36 in deep | 1.87 | 3.1 |

### Hole size rules of thumb

- Diameter: about three times the post's width (10 in for a 4×4, 12–16 in for a 6×6).
- Depth: at least one-third of the post's total length, and below your local frost line for decks and anything that must not heave.
- Place 4–6 inches of gravel at the bottom for drainage, then slope the concrete away from the post at the surface.

### Ready-mix for many posts

For more than about ten posts, ready-mix delivered by the cubic yard may be cheaper than bags — use the [cubic yard calculator](../cubic-yard-calculator/) to compare. For the fence itself see the [fence calculator](../fence-calculator/), and for deck footings the [deck calculator](../deck-calculator/). Larger pours (slabs, walls) are covered by the [concrete calculator](../concrete-calculator/).

## Frequently asked questions

### How many bags of concrete for a 4×4 fence post?

In a 10-inch diameter, 30-inch deep hole with a 4-inch gravel base, a 4×4 post needs about 1.7 80-lb bags, so plan on 2 bags per post.

### How deep should a fence post hole be?

Typically one-third of the post's total length — roughly 24–30 inches for a 6-foot fence — and below the frost line where required.

### Do I need gravel in the hole?

Yes. Gravel at the bottom helps water drain away from the post and extends its life.

### Can I use fast-setting concrete for posts?

Yes. Fast-setting mix is designed for posts: pour it dry around a plumb post and add water on top, per the bag instructions.

### How many bags of concrete for 10 fence posts?

About 17–20 bags of 80-lb mix for ten 4×4 posts in 10×30 in holes.

### How many bags for a 6×6 deck post?

A 12-inch hole 36 inches deep with a 6×6 post takes roughly 2.6 80-lb bags per footing.

### How long before I can attach the fence rails?

Fast-setting mixes allow work after about 20–40 minutes; regular mixes need 24–48 hours.

### Should concrete go to the top of the hole?

Fill to a few inches below grade for soil cover, or finish just above grade and slope it away so water runs off the post.

## Sources and further reading

- [Call 811 before you dig](https://call811.com/) — Common Ground Alliance
- [Prescriptive Residential Wood Deck Construction Guide (DCA 6)](https://web-media.awc.org/wp-content/uploads/2022/02/17210514/AWC-DCA62015-DeckGuide-1804.pdf) — American Wood Council (post and footing guidance)