支柱のコンクリート計算ツール

The fence post concrete calculator exists because a post hole is not an empty cylinder once the post goes in. A 4 by 4 post sitting in a 10 inch hole takes up real volume, and treating the whole hole as fillable concrete overstates the bag count by roughly the space the post occupies below grade. This tool starts from the hole diameter and depth, works out the cylinder volume, subtracts the post's own footprint over its embedded length, and converts what is left into bag counts for 50, 60 and 80 pound mixes side by side. The result is reported per post, which is the useful unit when post sizes or hole sizes vary across a run, for instance where corner posts sit in a wider hole than the line posts between them. For the embedment depth that feeds this calculation in the first place, the fence post depth calculator turns your frost line and post height into the single number this tool expects as an input.

Bag size choice is a real cost decision, not just a packaging preference. An 80 pound bag yields about 0.6 cubic feet of cured concrete, a 60 pound bag about 0.45, and a 50 pound bag about 0.375, so the per-bag price only tells half the story: what matters is cost per cubic foot delivered, and that favors the larger bags more often than not once the trip to the truck and the mixing effort are factored in. Quickset or fast-setting mixes carry the same yield figures as standard mix, so the bag math here does not change if you choose speed over cure time, only the number of hours before a post can safely carry load changes. The bags per post guide walks through a few common hole and post combinations if you want ballpark numbers before running your own exact hole dimensions through this calculator.

Not every post footing needs concrete at all. In free-draining sandy or rocky ground, a gravel backfill can hold a post adequately while draining better than a solid concrete collar, which matters in regions where standing water around the base causes more problems than it solves. The dirt versus concrete comparison lays out when each approach makes sense, and if gravel is the right call for your soil, the gravel calculator converts the same hole dimensions into cubic yards and tons of stone instead of bags of mix. That distinction, empty hole versus post-displaced hole, is the single most common source of over-ordering concrete on a DIY fence project. The full reasoning behind depth, hole sizing and the concrete-versus-gravel decision together is in the post depth and setting guide, which is worth reading once rather than re-deriving the rules of thumb project by project.

Once the per-post bag count is settled, most projects need to multiply it out across every hole in the run rather than buy footing material one post at a time. The concrete bags calculator handles that multiplication directly from hole dimensions and a hole count, which is the faster path once you already know your hole size from this page. For the post count that determines how many holes you are filling in the first place, run the fence post calculator on your total run length and spacing, and once posts and concrete are both accounted for, the fence cost calculator rolls everything, materials and footing alike, into one project total. Every other foundation-stage tool, from gravel to retaining wall blocks, sits together under the foundation calculators category for side by side comparison.

Weather at the time of the pour is a practical factor the bag math itself does not capture but affects how the concrete behaves once mixed. Hot, dry conditions accelerate curing and can cause a standard mix to set faster than expected, which is one more reason some installers default to fast-setting bags regardless of ambient temperature, while cold weather slows strength gain and can require extra curing time before a post safely carries load. None of this changes the bag count from this calculator, since yield per bag is a volume figure independent of cure speed, but it does affect project scheduling around when rails and pickets can go on. Mixing consistency matters as much as bag count: a mix that is too wet weakens the cured footer and a mix that is too dry leaves voids that reduce strength and let water in around the post, so following the bag manufacturer's water ratio is as important as getting the quantity right in the first place. For a project with many identical holes, it is also worth batching the total bag order from the per-post figure here rather than buying bag by bag at each hole, since a single bulk purchase is simpler to plan around than repeated partial loads.