支柱の深さ計算ツール

The fence post depth calculator settles one question that two separate rules of thumb both claim to answer: the one-third rule, which ties embedment to the post's above-ground height, and the frost line rule, which ties embedment to how deep the ground freezes in your climate. A post that only satisfies one of the two can still fail. Set a short post deep enough for frost but shallow relative to its height, and it can rack under wind load; set a tall post at a depth that ignores frost, and the ground itself can heave it out of plumb over a hard winter. This calculator takes both inputs, your post height and your local frost depth, and returns whichever of the two embedment requirements is larger, so the number on screen already accounts for both failure modes rather than just one. Depth also interacts with hole diameter indirectly: a deeper hole with the same diameter holds proportionally more concrete, so the depth number from this page is the first input the footing volume calculators downstream actually need.

Frost depth is the input most people get wrong, mainly because it is a local number rather than a universal constant. It increases with latitude and elevation in general terms, but the only reliable source for your specific address is a published county or state frost-depth table or a call to the local building department, since the same state can span a meaningful range of frost depths from one county to the next. Soil type changes the practical picture too: sandy or well-drained soil is more forgiving of a slightly shallow footing than dense clay that holds water against the post base through a freeze-thaw cycle. Before you dig at all, the 811 and utility locate guide covers the separate, mandatory step of having underground utilities marked, which has nothing to do with frost depth but matters just as much before the first hole goes in.

Post height is the more predictable half of the equation, since it is a measurement you already have rather than a regional lookup. The one-third rule means a 6 foot post wants roughly 24 inches of embedment from height alone, and an 8 foot post wants roughly 32 inches, before frost depth is even considered. Taller posts used for privacy screens or pool enclosures push embedment further still, and the calculator scales the recommendation linearly with whatever height you enter rather than capping it at a residential default. The 6 foot post depth guide and the post spacing guide both work from the same underlying assumptions this calculator uses, so reading them alongside the calculator output is a reasonable way to sanity check the number before you commit to a dig depth, especially at gate and corner posts where the rule is treated as a floor rather than a target.

Depth is only one half of a footing; diameter and volume are the other half, and they come from a different tool. Once you have the depth from this page, the fence post concrete calculator turns depth plus hole diameter into a per-post bag count, or the concrete bags calculator does the same across a full set of holes at once. Where soil conditions favor drainage over rigidity, the gravel calculator gives the equivalent volume in stone instead of mix. All three steps, hole count from the fence post calculator, depth from this page, and fill material from whichever of the concrete or gravel tools fits your soil, are covered together in the post depth and setting guide, and the rest of the foundation tools live under the foundation calculators category.

Regional frost maps are a starting point, not a substitute for a local check, because frost depth can vary within a single county depending on elevation, soil moisture and exposure. Many building departments publish a minimum footing depth specifically for structures in their jurisdiction, and that local number should be treated as the floor when it is available rather than a national average pulled from a general map. Sloped yards add a wrinkle to depth planning as well: on a hillside the embedment depth itself does not change, but digging straight down from a sloped grade means the uphill side of a hole can look shallower than the downhill side if the excavation is not leveled off consistently before measuring. Frost heave risk is also not uniform across a single fence line: a shaded, consistently damp corner of a yard can hold more moisture and freeze more severely than a sunny, well-drained stretch a few yards away, which is one reason some installers deliberately over-embed posts in the wettest part of a lot even when the calculated minimum would technically be enough everywhere else. None of these local variables can be built into a single calculator input, which is exactly why the frost depth field here is meant to be filled from a real local source rather than a guess.