Post setting is the part of a fence project that is hardest to fix after the fact: a post set too shallow, in the wrong soil, or without enough time to cure before load is applied, tends to fail slowly, months or years later, rather than immediately.
Depth: the one-third rule versus frost line
A common starting point is to set a post about one third of its total length into the ground. In a climate with real ground freeze, that rule can be overridden by your local frost line: if frost depth is deeper than one third of the post length, standard practice is to set the post at least as deep as the frost line instead, since a shallower post risks frost heave pushing it out of level over successive winters.
Frost depth varies meaningfully by region and even by specific site, so check with a local building department or an experienced local contractor rather than guessing; see the fence post depth calculator and how deep should a 6 foot fence post be for the fuller version of this rule.
Hole diameter
Hole diameter is typically matched to post size: roughly 10 inches for a 4x4 post, 12 inches for a 6x6. A wider hole gives more concrete volume around the post for the same depth, which matters more than people expect since concrete volume scales with the square of the radius.
Concrete versus gravel
Concrete is the standard choice for a permanent post, since it fully encases the post and resists movement in most soil types. Some installers use a gravel base at the bottom of the hole beneath the concrete to improve drainage away from the post’s end grain, which can slow rot in wood posts over the fence’s lifetime. Check what your specific post material and local practice call for; this is a detail worth getting right once rather than redoing later.
Computing concrete volume
Post hole concrete is a cylinder: volume equals pi times radius squared times depth. The fence post concrete calculator and how many bags of concrete per fence post both walk through this formula and convert the result into 50, 60 or 80 lb bag counts.
Bracing while concrete cures
A post needs to stay plumb while concrete sets, which typically means temporary bracing (stakes and scrap lumber angled against the post) checked and adjusted until the concrete has enough initial strength to hold the post on its own. Skipping this step, or checking plumb only once, is a common way a post ends up slightly out of level after the concrete has already cured.
What depth and diameter affect downstream
Post depth feeds directly into total post length (fence height plus hole depth) and concrete volume. Getting these numbers right before ordering material avoids being short on lumber length or concrete on installation day.
Plan your posts
The fence post calculator computes total post count broken into line, corner, end and gate posts; the fence post depth calculator and fence post concrete calculator cover depth and concrete for whichever material you are installing, per the wood fence installation guide or chain link fence installation guide.