Fence post depth calculator
Two rules govern post depth and they do not agree. The one-third rule is about standing up to wind; the frost rule is about not being lifted out over winter. You dig to whichever is deeper — and this works out which that is, then gives you the hole size and the post length to order.
The deeper of the two rules governs, plus a margin on corner and gate posts. Confirm your frost depth with the building department — no calculator can know it.
What it counts
The one-third rule says roughly a third of the post's total length goes in the ground — which is the same as burying half the height that shows. A 6-foot fence therefore wants about 3 feet buried and a 9-foot post, which is why tall fences get expensive faster than the extra picket suggests.
The frost rule says the bottom of the hole must be below the depth at which the ground freezes locally. That number is published by your building department and this tool takes it as an input, because it cannot be derived or guessed.
Hole diameter is about three times the post's actual width, giving a concrete collar thick enough to spread the bending load rather than crack. Note the word actual: a 4×4 measures 3½ inches.
Worked example
A 6-foot privacy fence, frost depth 24 inches. The one-third rule wants 36 inches buried; frost wants 24. The deeper governs, so 36 inches, and you buy 9-foot posts. Same fence in a jurisdiction with a 48-inch frost depth: frost governs, 48 inches buried, and you are buying 10-foot posts and considerably more concrete.
That is the whole reason to run both numbers rather than repeat one rule. Which one wins changes the material order, not just the digging.
Where it stops
Rock, running sand and steeply sloping ground all change the job and none of them are in here. Neither is a fence acting as a pool barrier, which has its own requirements — see fence permit rules.
Whatever depth you land on, keep it consistent along the run. Differential movement between posts is what makes a fence go wavy, and one shallow hole on a Friday afternoon is the one you see from the kitchen window for the next decade.
Code requirement
Frost depth and any local footing requirement are set by your jurisdiction. Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.
Sources
Nothing here is asserted as a current figure. The tool is arithmetic on what you type, and every default is a round placeholder for your yard's quote.