# Post Hole / Footing Calculator

Calculate how much concrete you need for post holes and footings. Enter diameter, depth, and number of holes. Get bags of concrete and cost.

## What this calculates

Setting posts in concrete requires the right amount of mix for a solid, lasting foundation. This calculator determines the concrete volume needed for round post holes, accounting for the post itself displacing some of the volume. Enter the hole diameter, depth, and number of holes to get the exact number of concrete bags needed.

## Inputs

- **Hole Diameter** (in) — min 4, max 36 — Typically 3x the post width. 4x4 post = 10-12" hole
- **Hole Depth** (in) — min 12, max 72 — Below frost line. Typically 36-48" in cold climates
- **Number of Holes** — min 1, max 100
- **Post Width** (in) — min 0, max 12 — 4x4 actual = 3.5", 6x6 actual = 5.5". Set to 0 if no post.
- **Fill Entire Hole** — Fill entire depth or leave 4" for dirt cap
- **Price Per 80-lb Bag** ($) — min 0

## Outputs

- **Volume Per Hole** (ft³) — Concrete volume (minus post displacement)
- **Total Volume** (ft³)
- **Total Volume** (yd³)
- **60-lb Bags** — 0.45 ft³ per bag
- **80-lb Bags** — 0.60 ft³ per bag
- **Estimated Cost (80-lb)** — formatted as currency

## Frequently Asked Questions

**Q: How deep should a fence post hole be?**

A: The general rule is 1/3 of the total post length should be buried. For a 6-foot fence using 8-foot posts, dig 24-30 inches deep. In frost-prone areas, holes should extend below the frost line (typically 36-48 inches) to prevent heaving.

**Q: How wide should a post hole be?**

A: Post holes should be 3 times the width of the post. For a 4x4 post (3.5" actual), dig a 10-12 inch diameter hole. For a 6x6 post (5.5" actual), dig a 16-18 inch diameter hole. This provides adequate concrete around the post.

**Q: How many bags of concrete per fence post?**

A: A typical 4x4 fence post in a 10-inch diameter, 36-inch deep hole needs about 1 bag of 80-lb concrete (0.6 ft³) or 1-2 bags of 60-lb concrete (0.45 ft³ each). Deeper or wider holes require more.

**Q: Can I use quick-setting concrete for post holes?**

A: Yes, quick-setting concrete (like Quikrete Fast-Setting) is ideal for fence posts. Pour the dry mix into the hole around the post and add water. It sets in 20-40 minutes, so you can continue building the same day. Regular concrete sets in 24-48 hours.

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Source: https://vastcalc.com/calculators/construction/post-hole
Category: Construction
Last updated: 2026-04-21