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Post Hole Concrete Calculator

The concrete you need per post is the hole minus the post, which is why a chunky gate post in a big hole takes less than you would guess and a slim post in the same hole takes more. Enter the hole size, post size and how many you are setting.

PostHole diameterHole depth
Post set in concrete: hole size, depth and gravel base
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Sizing the hole

  • Diameter: three times the post width. A 100 mm (4 in) post wants a 300 mm (12 in) hole.
  • Depth: one third of the above-ground post height, and never less than 600 mm (2 ft) for fence panels.
  • Base: 50 to 100 mm of compacted gravel before any concrete goes in, so water draining down the post has somewhere to go.

Concrete per hole = π × (hole Ø ÷ 2)² × depth − post volume

Worked example. A 300 mm hole, 600 mm deep, with a 100 mm square post: π × 0.15² × 0.6 − 0.1 × 0.1 × 0.6 = 0.036 m³ per post, about two bags of postcrete. Ten posts brings you to 0.36 m³.

Postcrete or ordinary concrete?

Postcrete (post mix) goes into the hole dry after you have added water, and sets in five to ten minutes. For a run of fence panels on a Saturday it is unbeatable — no mixing, no bracing, post to post without waiting.

Ordinary concrete is cheaper per cubic metre and stronger. It is the better choice for gate posts, corner posts and anything taking real load, but you have to brace every post while it cures and come back the next day.

Whichever you use, finish the top of the concrete slightly above ground and slope it away from the post. A flat or dished top holds water against the timber and rots it from the collar up — the reason most fence posts fail at ground level rather than anywhere else.

Frequently asked questions

How many bags of postcrete per fence post?

One to two 20 kg bags for a standard 100 mm post in a 300 mm hole at 600 mm deep. Gate posts often take three. Round up per hole, not across the job.

How many bags of concrete per fence post?

Roughly two 50 lb bags, or one and a half 80 lb bags, for a typical 4×4 post set 2 ft deep in a 12 in hole.

How deep should fence posts go?

At least 600 mm for 1.8 m (6 ft) panels, or a third of the total post length as a general rule. Exposed, windy sites and gate posts want more.

Should concrete touch the bottom of the post?

Better not to. Sit the post on compacted gravel so water can drain away rather than pooling in a concrete cup under the end grain.

Can I set posts without concrete?

For light fencing in firm ground, well-compacted gravel around the post works and drains better than concrete. For anything carrying a panel in wind, concrete is the safer choice.

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