How this instrument works
The most widely cited rule of thumb for how deep to set a wood fence post is to bury roughly one-third of its total length below grade — a 6ft-tall fence post, for instance, typically gets about 2ft of that buried underground, leaving 6ft standing above grade. It's a simple, generally reliable starting point for stability against wind and leverage on an ordinary fence, though it's a widely repeated guideline rather than a single universal building-code figure.
In climates where the ground freezes, the one-third rule alone isn't always enough. Water in the soil expands as it freezes, and a post footing that sits above the frost line can be gradually pushed upward over repeated freeze-thaw cycles — a problem called frost heave. Cold-climate building codes commonly require footings to be set below the local frost line specifically to avoid this, so a post that would only need 2ft by the one-third rule might need 3ft or more if the local frost line runs that deep.
This instrument computes both figures — the one-third rule's bury depth and the frost line depth you enter, converted to feet — and uses whichever is greater as the recommended bury depth, then adds that to the above-ground post height for the total post length to buy. Frost line depth is genuinely local: it can range from under a foot in the Deep South to four feet or more in the northern US, so check with your local building department for the figure that applies to your specific site rather than guessing.
- Enter Post height above ground (ft) — how tall the finished fence needs to stand.
- Enter Local frost line depth (in, 0 if unknown/not applicable) — check your local building department; leave at 0 in climates where frost heave isn't a concern.
- Read Recommended bury depth (ft) — the greater of the one-third rule and the entered frost line depth.
- Read Total post length to buy (ft) — the above-ground height plus the recommended bury depth, the full post length to purchase.
Worked example — a 6 ft post in a cold climate
A fence needs to stand 6ft above ground. With no frost line entered (0in), the one-third rule alone governs: bury depth = 6 ÷ 3 = 2.0ft, so Total post length to buy = 6 + 2 = 8ft — an 8ft post buried 2ft deep leaves exactly 6ft standing.
Now suppose the same 6ft-tall fence is being built in a cold-climate area with a 36in (3ft) frost line. The one-third rule still suggests 2.0ft, but the frost line at 3.0ft is deeper, so it governs instead: bury depth = max(2.0, 3.0) = 3.0ft, and Total post length to buy rises to 6 + 3 = 9ft — a full foot more post than the one-third rule alone would have suggested, needed to keep the footing safely below where the ground freezes.
Questions
Do I always need to bury a post below the frost line?
Only if you're in a climate where the ground actually freezes deep enough for frost heave to be a real risk — in mild climates with little or no ground freeze, the one-third rule alone is typically sufficient and this calculator's frost line field can be left at 0. In colder regions, check your local building department for the frost line depth that applies to your specific location before finalizing post depth.
Where do I find my local frost line depth?
Your local building department is the most reliable source, since frost line depth is set locally and varies significantly even within a single state or region — it can run under a foot in parts of the Deep South and four feet or more in the northern US and much of Canada. Some regional NRCS or extension office frost-depth maps are also published online, but a local building department's figure for your specific jurisdiction takes precedence.
Why is the one-third rule not always enough on its own?
The one-third rule is a general stability guideline based on a post's above-ground height and typical wind/leverage loads on an ordinary fence — it has nothing to do with soil freezing. In a cold climate, a post footing set only by the one-third rule can sit entirely above the frost line, leaving it exposed to the seasonal expansion and contraction of freezing, water-saturated soil, which can gradually heave the post upward and knock it out of plumb over a few winters.
Does this apply to vinyl or metal fence posts too?
The underlying frost-heave physics applies to any post material set in the ground, but exact bury-depth guidance often varies by post material and manufacturer, since vinyl and metal post systems sometimes use specific footing designs or minimum depths from the manufacturer. Treat this calculator's output as a general planning estimate and check your specific fencing system's installation instructions for any depth requirement that differs from the general one-third-and-frost-line approach.
Should I check with my local building department before digging?
Yes — this calculator applies a widely used general rule of thumb, not a specific building code, and some jurisdictions have their own minimum post-depth or footing requirements that can differ from (and legally supersede) the one-third rule or a generic frost-line figure. A quick check with your local building department before digging avoids having to redo post holes that don't meet a local code requirement.