Free post hole calculator. Concrete and aggregate per post for fences, decks, and pergolas.
This calculator works out how much concrete a batch of post holes takes, and how many 20kg bags that is. Give it the number of holes, the hole diameter, the depth, and the size of the post standing in each one.
A post standing in a hole displaces its own volume of concrete. Until 4 September 2026 this page assumed an open hole every time, so it over-ordered on every job where a post was actually going in — which is most of them.
The post size is still set to "None — open hole" by default, deliberately, so that nobody's saved answer changed underneath them. Pick your post size and the count drops to what the hole really takes.
It matters for a second reason: with no post size, the same 300mm by 600mm hole gave a different bag count here than on the fence calculator, which uses CostMate's shared footing module. Setting the post size makes the two agree.
Hole diameters offered are 200mm, 300mm and 400mm, and post sizes 100, 125 and 150mm square.
The volume comes from the same shared footing module the fence and deck estimates use. The bag count does not, and that is on purpose.
That module rounds up per hole, which is right for a single fence footing poured on its own. This page is for a batch, and a builder mixing a barrow carries part-bags from one hole to the next. Rounding up per hole would have taken a 12-hole job from 51 bags to 60 — an 18% over-order arriving as a side effect of a fix about post displacement.
So this page borrows the yield and keeps its own rounding on the total. Bags are worked out at 0.010 cubic metres per 20kg bag.
Hole depth and diameter are design decisions that follow from what the post carries. Check them against your plans, and confirm bag yields with your merchant, before you order or pour.