支柱の長さ計算ツール

Post length is a different question from embedment depth, even though the two are closely related: depth tells you how far into the ground a post needs to go, while length tells you what size of lumber or pipe stock actually gets you there plus the above-ground height you want. This calculator adds the above-ground fence height to the required embedment, the larger of one third of that height or your local frost line depth, plus a gravel drainage allowance at the bottom of the hole, and then rounds the result up to the nearest commercial stock length, since lumber yards sell posts in even increments like 8, 10, 12, 14 and 16 feet rather than custom lengths.

Frost line is the input that varies most by region and most changes the final stock length. In frost-free or mild climates, an 8 foot post is routinely enough for a 6 foot fence, with roughly 2 feet going into the ground. Where frost depths reach 24 to 36 inches, the same 6 foot fence needs a 9 or 10 foot post to keep the embedment below frost while still leaving the full 6 feet standing above grade. The post depth guide for a 6 foot fence walks through that exact comparison, and the embedment number itself, independent of stock length, comes from the fence post depth calculator.

The gravel allowance at the bottom of the hole is a small number, typically 4 to 6 inches, but it is easy to forget when estimating total dig depth and stock length by hand. A post should rest on top of that compacted gravel pad rather than sitting directly on bare soil, since the gravel lets water drain away from the end grain instead of holding it against the wood. Gate posts are the one case where this calculator's baseline numbers should be treated as a minimum rather than a final answer: a gate post carries cantilevered, dynamic load every time the gate swings, and is commonly set 6 to 12 inches deeper than a standard line post, which often pushes the stock length up to the next standard size.

Once stock length is settled, several follow-on numbers typically need checking next. The fence post hole calculator sizes the hole diameter and volume that this length of post needs to sit in, the concrete bags calculator turns that hole volume into a bag count using the same frost and depth assumptions, and the bags per post guide gives ballpark figures if you want a sanity check before placing a material order. Post spacing is a separate decision again, handled by the fence post spacing calculator, and a side by side comparison of wood, vinyl and metal post stock is in the fence materials guide, alongside the full depth and setting reasoning in the post depth and setting guide. Every related footing tool sits together under the foundation calculators category.

Material choice changes which stock length table actually applies, even though the depth and height math behind this calculator is the same for wood, vinyl and metal posts. Wood posts are sold in the familiar 8, 10, 12, 14 and 16 foot lumberyard lengths this calculator rounds up to, but vinyl fence systems are more often sold as a sleeve over an internal metal or wood insert post, where the insert, not the visible vinyl sleeve, is what actually needs to satisfy the embedment length this calculator returns. Metal posts, whether steel pipe for chain link or aluminum for an ornamental fence, are typically sold in stock lengths matched to the fence height the manufacturer offers rather than the broader range of wood lengths, so the rounding step for metal often lands on a narrower set of available options than it does for lumber. Corner and end posts in some systems use a heavier gauge or larger diameter than line posts at the same length, which does not change the length this calculator returns but does mean the length number should be checked against both line post and corner post stock availability before finalizing an order. Buying slightly long and trimming the top after the concrete has cured is a common practice specifically because it lets installers set every post to a consistent, level finished height across an uneven starting grade, rather than trying to cut every post to an exact pre-measured length before it goes in the ground, where small depth variations from one hole to the next would otherwise show up as an uneven fence line along the top. Checking stock availability at the lumberyard before finalizing a post count matters too, since a yard that stocks 8 and 12 foot posts but skips 10 foot lengths effectively forces a jump to the next size up, which is worth confirming before the length from this calculator becomes a firm order.