Sections, posts, and the off-by-one that costs money
A hundred foot run at eight foot spacing is twelve and a half sections, so thirteen sections, so fourteen posts — not twelve or thirteen. The rule is posts equals sections plus one, because a fence has a post at both ends of every bay and the last bay needs its own closing post. That plus-one repeats every time the fence is interrupted. Each corner is a break in the run and takes a post of its own. Each gate splits the run in two and, once you count the post either side of the opening, adds one more post than the uninterrupted run needed.
Those additions are why the count here comes out higher than dividing length by spacing. On a typical back garden with two corners and a gate that is three or four posts, which is not a rounding error when a post is a heavy pressure-treated timber and a bag of concrete.
The other consequence of the arithmetic is that the actual spacing is rarely the spacing you typed. Thirteen sections across 96 fenced feet is 7.38 foot centres, not 8. Spread the difference evenly across all the bays rather than running full bays and leaving a stub at the end — the eye picks up one odd bay immediately and does not notice a uniform 7.4.
How much concrete a hole actually takes
The volume is the hole minus the post. A 10 inch hole 30 inches deep is 1.36 cubic feet of void; a 4x4 post is genuinely 3.5 inches square, and over that depth it occupies 0.21 cubic feet, so the concrete is about 1.15 cubic feet — around three 50 lb bags. Put four inches of gravel in the bottom for drainage and the concrete depth drops to 26 inches, and so does the quantity.
| Hole | Post | Concrete per hole | 50 lb bags |
|---|---|---|---|
| 8" x 24" | 4x4 | about 0.53 cu ft | about 1.4 |
| 10" x 30" | 4x4 | about 1.15 cu ft | about 3.1 |
| 12" x 36" | 6x6 | about 1.73 cu ft | about 4.6 |
Those are geometric figures for clean cylindrical holes. Real holes are not cylinders. An auger in soft or sandy ground bells out at the bottom and sloughs at the sides, and a hole dug with a clamshell digger is wider at the top than the tool. Buy a bag or two beyond the count, because being one bag short with eleven posts already set is a trip to the store in the middle of the job.
Depth, frost and why nobody can give you the number
A post held in concrete resists two things: the fence being pushed sideways by wind, and the ground trying to lift it. The second is the one that surprises people. Where soil freezes, the water in it expands, the frozen layer grips the post or its footing and jacks it upward, and each thaw lets a little soil fall in underneath. Over a few winters the post walks out of the ground and the fence goes wavy. The defence is to get the bottom of the footing below the depth at which the ground freezes, so that the frozen layer has nothing to lift against.
How deep that is depends entirely on where you are, and it is not a number this page can supply. Frost depth requirements are set locally, published by the building department that has jurisdiction over your property, and they differ substantially between neighbouring regions, let alone between climates. Call them or check their published requirement. Where the ground does not freeze the governing factor is instead lateral load — the taller the fence and the more solid the infill, the more embedment it wants, because a six foot board fence is a sail.
Before you dig anything
Two calls and one measurement, in that order. Call the utility locating service, which in most places is free and legally expected, and wait for the marks — a fence post hole is exactly deep enough to find a gas service or a buried cable. Call or check with the authority that issues permits, because fences commonly need one and are commonly subject to height limits, setback rules from the property line and street, corner visibility triangles, and sometimes rules about which face points outward. And find the actual property line, from the survey rather than from the old fence, hedge or a neighbour's recollection. A fence built a foot over the line is a legal problem that outlives the fence, and none of the three checks costs anything next to moving finished work.
Questions people ask
How many posts do I need for 100 feet of fence?
For a single straight run at 8 foot spacing, 14 — thirteen sections plus one closing post. Add one for each corner and one for each gate, so a 100 foot fence around two sides of a garden with a 4 foot gate is 16. At 6 foot spacing the same straight run is 18 posts. The number people arrive at by dividing 100 by 8 is 12 or 13, and that is the single most common shortfall in a fence material list.
How deep should fence posts go?
Deep enough that the footing sits below the local frost depth, and deep enough for the height and solidity of the fence — and only your local building department can give you the first of those figures. Frost depth requirements vary enormously by climate and jurisdiction and are published locally; a depth that is correct in one region is inadequate in another and unnecessary in a third. Ask them. Separately, a general principle that holds everywhere is that taller and more solid fences need more embedment, because wind load on a solid six foot fence is considerable and the post is a lever.
How much concrete per fence post?
For the common case of a 10 inch hole 30 inches deep around a 4x4 post, about 1.15 cubic feet, which is roughly three 50 lb bags or two 60 lb bags. Bigger posts in bigger holes climb quickly: a 6x6 in a 12 inch hole 36 inches deep is about 1.73 cubic feet, five 50 lb bags. Put gravel in the bottom of the hole and subtract that depth. Then add a couple of bags over the total, because real holes are wider than nominal ones.
Do fence posts have to be set in concrete?
Not always, and the alternatives have real advocates. Compacted gravel around the post drains rather than holding water against the timber, and a post set in gravel can be pulled and replaced without excavating a lump of concrete. Concrete gives more lateral rigidity, which matters most for tall solid fences, gate posts and corner posts carrying tension. A common compromise is concrete on the posts that take load — corners, gates, ends — and compacted gravel on the line posts between them. What matters more than the choice is that water can get away from the bottom of the post rather than sitting in a concrete cup around it.