Post-Hole Concrete Calculator

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.

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.