Setting Fence Posts: How Deep, How Wide, and How Many Bags

Almost every leaning fence traces back to the holes: too shallow, too narrow, or drainage nobody thought about. The rules below are the ones the fence trade actually uses, and the post hole concrete calculator turns them into a bag count for your exact layout.

The two sizing rules

Depth: one third of the above-ground height, and below the frost line. A 6 ft privacy fence wants posts 2 ft in the ground, minimum. In cold climates the frost line governs instead: if the ground freezes 36 inches down, the bottom of the footing goes below 36 inches no matter what the one-third rule says, or frost heave will jack the post a little every winter until the gate does not close.

Diameter: three times the post width. A 4×4 post (actual width 3.5 inches) gets a 10 to 12 inch hole. The point is concrete cover on every side; a post jammed in a post-width hole with slurry poured around it has no real footing.

Add 6 inches of depth beyond the footing for a gravel base, which is the drainage detail that decides whether the bottom of the post spends its life in water.

Concrete, gravel, or dirt?

Concrete set is the default for gates, corners, ends, and any privacy fence that catches wind. A solid fence is a sail; the footing takes the load.
Gravel set (crushed stone compacted in lifts around the post) drains better than concrete, is easier to replace later, and is a legitimate choice for line posts on open (picket or rail) fences in firm soil. It is the wrong choice in sand or for solid panels.
Backfilled dirt is how farm fences with driven posts work, and how yard fences fail. If you are digging a hole, put something structural back in it.

Fast-setting versus standard mix

Fast-setting mix costs more per bag and earns it on fences. It is poured dry into the hole around the braced post, watered from the top, and holds the post firm in 20 to 40 minutes, which means you can set a run of posts in one pass without building braces for each one and waiting a day. Standard mix is cheaper and fine when you are not in a hurry, but it must be mixed wet and each post braced plumb until it sets. Whichever you use, slope the top of the concrete away from the post so water sheds rather than pooling against the wood, and stop the concrete an inch or two below grade if you want soil and grass to hide it.

How many bags

The volume per hole is the hole cylinder minus the post: for a 3.5 inch post in a 10 inch hole 24 inches deep, that is about 0.92 cubic feet, which is two 60 lb bags per hole. Multiply by your post count and the number gets large fast, which is why people who guess buy twice. The calculator does the cylinder math per hole and totals it in 80 lb, 60 lb, and 50 lb fast-setting bags. Buy one extra bag per ten holes; hand-dug holes are never the diameter you planned, and bell-shaped bottoms drink more than the cylinder predicts.

Worked example: a 40 ft privacy fence run at 8 ft spacing is 6 posts. Six holes at 10 inches × 24 inches around 4×4 posts need about 5.5 cubic feet of concrete, so 12 to 13 × 60 lb bags of fast-setting mix, call it 14 with the spare. At typical big-box pricing that is a two-digit material line, not a three-digit one, which is why skimping on hole size to save bags is a bad trade.

Layout, spacing, and the gate post

Spacing: 8 ft on center is the standard for wood fencing because rails and panels come in 8 ft lengths. 6 ft spacing stiffens a windy run and is standard for chain link.
Run a string line. Set the two end posts first, pull a mason's string between them at the top, and set every line post to the string. Eyeballing produces a fence you can see wander from the street.
Gate posts work harder than anything else in the fence. Go one size up on the post, 6 inches deeper on the hole, and always concrete, on both sides of the opening. Half of all sagging-gate complaints are actually a moving post.
Call before you dig. In the US, 811 marks buried utilities free of charge, and the two-day wait is considerably shorter than the consequences.

Before you hang rails

Fast-setting mix takes rail load in a few hours; standard mix wants a full day, ideally longer for a gate post. The fence will stand for twenty years, so the schedule can afford the wait. If a post sets slightly out of plumb, fix it now: cut it loose, re-dig the top of the footing, and reset. Every fix is easier before the panels go on.

Size the holes with the rules above, then let the post hole concrete calculator turn your post count into bags. If the same project includes a gravel path or a paver landing at the gate, the gravel calculator and paver base calculator use the same measure-then-order logic.