Post-Hole Concrete Calculator NZ 2026

This calculator works out how much concrete you need to set posts, and how many bags of premix to buy, for a fence, deck, pergola, carport, gate or letterbox in New Zealand. Setting posts in concrete is one of the most common backyard jobs, but it is easy to under-buy because each hole holds more than people expect once you account for the round shape and the depth needed to hold a post firmly. A good rule is that the hole should be about three times the width of the post and deep enough to bury at least a third of the post above ground, which for a standard fence often means a 300mm wide hole around 600mm deep. Multiply that across a run of fence posts and the concrete adds up quickly. You enter the number of holes, the hole diameter and depth, the width of the post that sits in the hole, and a waste allowance, plus the yield of the bag you are buying. The calculator subtracts the space the post itself takes up and returns the total volume of concrete in cubic metres, the volume per hole, and the number of 20kg premix bags to buy. This helps you turn up to the job with the right number of bags rather than making a second trip to the store, or over-ordering and storing leftover bags that absorb moisture and harden. For a long fence the totals can be large enough that ready-mix or a mixer becomes worthwhile. This is an estimate; dig a test hole to confirm your real hole size.

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19 bags
20kg premix bags to buy
Total concrete0.2 m³
Per hole0.036 m³
Holes5

Assumes a round hole and a square post. Holes in soft ground may need to be larger. An estimate only; check your real hole size.

How it works

Each hole is treated as a cylinder: pi times the radius squared times the depth. The space taken by the square post is subtracted. That per-hole volume is multiplied by the number of holes and increased by your waste allowance to give the total concrete, which is divided by the bag yield and rounded up for the bag count.

Worked example

Five holes at 300mm diameter and 600mm deep are about 0.0424 cubic metres each as a cylinder. A 100mm post takes up 0.006 cubic metres, leaving 0.0364 per hole. Across five holes that is 0.182 cubic metres, or 0.20 once a 10 percent waste allowance is added. At 0.011 cubic metres a bag, that is about 19 bags of 20kg premix.

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Frequently asked questions

How much concrete do I need per fence post?

A typical 300mm wide, 600mm deep hole holds about 0.036 cubic metres of concrete once the post is allowed for, which is roughly three to four 20kg bags. Bigger or deeper holes for gate and corner posts need more, so size each hole for its job.

How deep should a fence post hole be?

As a guide, bury at least a third of the post, so a 1.8 metre high fence post often sits in a hole around 600mm deep. Corner, gate and strainer posts carry more load and are usually set deeper and in wider holes.

Do I need to allow for the post in the hole?

Yes. The post occupies part of the hole, so the concrete only fills the space around it. This calculator subtracts the volume of the post so you do not over-buy, though a small waste allowance is still wise for spillage.