How Many Bags of Concrete per Fence Post?

A post hole is not the same volume as a full cylinder of concrete when the post sits in it. The post occupies part of the hole. For a round hole around a square post, calculate the cylinder volume and subtract the square post area multiplied by the filled depth. Multiply the result by the number of posts, then by any allowance you choose.

Measure actual diameter and depth rather than relying on a generic chart. Ground conditions, fence height, wind exposure, corner loads, gate loads and the manufacturer’s installation method all influence the hole specification. The DIYSums concrete tool accepts hole dimensions for volume estimation, while the Australian fence tool uses one CMI WA system and soil example. Neither designs a footing.

The Cement Australia sheet estimates two 20 kg bags for one post hole, but does not state the hole dimensions or a cubic-metre yield for that post example. Therefore it cannot safely be translated into a general rule for every fence. For a measured 300 mm diameter hole, 600 mm deep, around a 100 mm square post, the volume calculation gives about 0.0364 m³. Dividing by the cited concrete mix yield of 0.0084375 m³ per bag gives 4.32 bags, rounded up to five.

That example is intentionally distinct from the maker’s approximate “two bags per post hole” statement because the sheet does not define the hole size behind it. This is a source limitation, not a recommendation to use five bags. Confirm your exact hole and product with the fence supplier. If a product sheet gives a specific allowance for the same hole and system, follow its instructions and understand how it relates to the material yield.

A straight 10 m run with 2.37 m Australian CMI steel panels takes five whole panel kits in the calculator example. Five bays need six standard support positions. On the rocky-soil setting in that named guide, the calculator applies one concrete bag per standard position, for six bags. That figure is specific to the guide’s example hole and soil selection, and should be checked against current instructions before ordering.

A gate is entered separately and adds two gate posts. The cited guide calls for gate-post holes to be 10 mm deeper, but does not provide a distinct gate-hole concrete quantity, so the calculator excludes gate-post concrete rather than silently reusing the standard post allowance. Corners, slope changes and the final cut bay also affect the layout.

Products with similar bag weights can have different yields, mixing methods and setting instructions. “Rapid set” describes a product feature, not a substitute for confirming the manufacturer’s stated use with the post material and conditions. Read the current label and safety data sheet, and do not mix product-specific figures between brands.

Some concrete mixes require mixing before placement while certain post-setting products have different installation sequences. Follow the product instructions exactly, including water quantity and curing or bracing requirements. Wind can put load on a newly installed fence, so keep posts supported for the stated period.

Get a qualified fencer or engineer to check the design when the fence is high, exposed to strong wind, retaining soil, supporting a heavy gate, near services or on uncertain ground. Local boundaries, easements and approval rules can also affect where and how a fence may be installed.

This material estimate does not establish a safe post size, footing size, concrete strength or legal boundary. Treat the calculator as a shopping starting point only after the system supplier confirms the design.

When checking a post-hole estimate, distinguish the excavated hole volume from the amount of concrete placed. A sloped hole, bell-shaped base, gravel layer or a post that is not centred changes the simple cylinder assumption. Measure a representative excavation and ask the fence supplier how its footing detail handles those conditions. Do not reduce the specified foundation simply because a smaller bag count appears cheaper.

For a fence with varied ground, record each soil condition and hole depth separately rather than applying one setting to the entire boundary. Keep gate posts separate from standard line posts. A gate can put different forces on its supports, and the generic line-post allowance may not apply.

A practical site schedule can list each post position, its measured hole diameter, depth, post section and selected product. That makes it easier to spot when one section crosses from firm soil into sand or when a corner needs a different detail. Do not rely on a single sample hole if the ground changes along the run. The supplier or installer can review that schedule before material is ordered.