Free post hole concrete calculator. Concrete volume and 20kg bags for a batch of holes from hole diameter, depth and post size. 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, so a hole with a post in it takes less than an open one.
The field is set to "None: open hole" by default, which assumes nothing is standing in the hole. Pick your post size and the count drops to what the hole really takes.
It matters for a second reason: with no post size set, the same 300mm by 600mm hole gives a different bag count here than on the fence calculator, which uses CostMate's shared footing module.
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 take a 12-hole job of open 300 by 600mm holes from 64 bags to 72: an over-order on a job you are mixing in one go.
So this page borrows the yield and keeps its own rounding on the total. Bags are worked out at 0.008 cubic metres per 20kg bag: the maker's figure for Dricon's 20kg High Strength RapidSet post-hole concrete.
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.