# Concrete for Post Holes: Volume Per Hole & Bags Needed

Calculate round post-hole concrete volume from diameter, depth and count. Account for post displacement and use the stated bag yield.

- Canonical HTML: https://www.chippy.tools/building-codes/concrete/concrete-post-holes/
- Publisher: Chippy Tools
- Updated: 2026-09-06
- Market: global
- Scope: Geometry and quantity estimates only; use the adopted code and approved design for requirements.
- Worked arithmetic checked: 2026-09-06


## Round-hole volume

**Gross volume = π × (diameter ÷ 2)² × depth × hole count.** Use metres for all lengths to get cubic metres. Measure the hole itself. A hole wider than the auger needs more concrete.

## Worked example: six holes

Take six holes, each 300 mm across and 600 mm deep. These are example sizes, not a footing design.

- Diameter: 0.30 m. Radius: 0.15 m. Depth: 0.60 m.
- One hole: π × 0.15² × 0.60 = 0.04241 m³.
- Six holes: π × 0.15² × 0.60 × 6 ≈ 0.25447 m³, or 254.47 L gross. Keep full precision until the final result.

## Allow for the post

A post takes up some of the space in the hole. Say each square post is 100 mm × 100 mm and sits 600 mm deep. Each post takes up 0.10 × 0.10 × 0.60 = 0.006 m³. Six posts take up 0.036 m³. Subtract that from the gross hole volume. This leaves about **0.21847 m³** for concrete.

An example 10% allowance brings it to 0.24032 m³, or 240.32 L. For a product with a stated mixed yield of 10 L per bag, buy **25 bags**. Round up once at the end of the sum.

Use the real post shape and the length below ground. The [web tool](https://www.chippy.tools/building-codes/tools/#concrete) gives gross hole volume. Note the space taken by posts on the [Chippy Tools concrete sheet](https://www.chippy.tools/building-codes/printables/).

## Choose the hole size from the design

Soil, loads, wind and the structure all affect the footing design. A volume sum does not tell you how deep to dig. [Find footing guides for your country](https://www.chippy.tools/building-codes/topics/#footings).

## Basis and product source

The cylinder formula gives the full hole volume. For bag counts, use the mixed yield on the exact product’s label or data sheet. Follow the specified mix, placing and curing steps. Confirm the final order with your supplier.


This page is general information for tradespeople and is not compliance, engineering or legal advice. Requirements are subject to state and territory variations and adoption timing. Always confirm requirements with your building certifier and relevant state authority.


## Frequently asked questions

### How do I calculate concrete for round post holes?

Use π × (diameter ÷ 2)² × depth × hole count, with dimensions in metres for cubic metres. This is gross hole volume; subtract the submerged post volume if appropriate.

### How many bags do I need?

Convert the required concrete volume to litres, add your chosen allowance, divide by the product's stated mixed yield per bag, and round the final count up.

### Does this tell me how deep to dig?

No. Hole depth and diameter depend on the approved design, soil, loads and local requirements. This calculation estimates quantity from dimensions already selected.