Fence post calculator

Post count is the run divided by the spacing, rounded up, plus one closing post. Each hole should be a third of the post height deep and three times the post width across, which for a standard 100 mm post at 1.8 m high is about 0.03 cubic metres of concrete.

The rule of thumb — bury a third — exists because a fence post is a lever. Wind pushes at the top and the buried section resists in two directions at once, so shallow posts do not fail by pulling out, they fail by rocking until the hole widens. That is also why the hole should be a straight-sided cylinder rather than a cone: a cone-shaped plug of concrete works its way loose. A gravel base under the post lets water out of the end grain, which is the difference between a post that rots in six years and one that lasts fifteen.