Fence post depth calculator

This is a local-code question, and there is no national answer

Two things about a fence are decided by your jurisdiction and by nobody else: how deep the posts must go, and where the fence is allowed to stand. Both are worth settling before you buy material, because both can change what you buy.

On depth, the surprise is that the model code does not contain a number. The residential code requires footings to extend below "the frost line specified in Table R301.2(1)" — and Table R301.2(1) is a blank the adopting jurisdiction fills in. New York's Department of State says so about as plainly as a code document can: "The local jurisdiction shall provide the minimum depth of footing below finish grade." It then lists what a jurisdiction should weigh — air freezing index, soil type, water table depth, local historical data and field experience. That is five local variables, which is precisely why no national map can answer it.

On position, setbacks, height limits and boundary rules are equally local, and they are the ones that cause disputes rather than failures. A fence built a foot inside the line is a different conversation from a fence built on it. Many jurisdictions also cap height differently for front and rear yards, and some require a permit above a threshold height.

So: call your building department, and call 811 before you dig anywhere near a service. This page does arithmetic on rules that publishers state; it does not know your code, and it cannot.

How this is calculated

1. Stability depth = fence height ÷ 2. The published rule is that a third of the whole post is buried, which is the same thing said from the other end: if a third is under the ground, two thirds are above it, so the buried part is half of what shows.
2. Required depth = the deeper of the stability depth and your frost depth. Never the sum — one keeps the post from leaning, the other keeps the ground from lifting it.
3. Post length = fence height + required depth, which is why a 6-foot fence with no frost requirement needs a 9-foot post.
4. Hole width = post width × 3, the published multiple.

Data used, with sources

Frost depth by state, and where it does not exist

Every state below has been read at its own source. The point of the table is the second column: a state that publishes no statewide frost depth is not a state without frost, and the two are shown as different things because merging them is how a gap becomes a fake number.

States that publish a statewide depth

States that publish none, and say so

Every other state is not yet verified here — which means we have not read its code, not that it has no requirement. We would rather show 5 states we have checked than fifty we have not. The remaining states are being worked through, and each will arrive with the source it came from.

One scope note that applies to the whole table, and that the pages ranking for this term tend to skip: these are footing depths for buildings. A fence is usually not a permitted structure, so the frost physics carries over but the legal requirement may not. Use these as the best published guide to how deep the ground freezes near you, then confirm what actually applies to a fence with your building department.

Why a third, and when a third is not enough

A fence post is a cantilever. Wind pushes on the fence, the fence pushes on the post, and the post pivots about a point below grade — so what resists it is the soil bearing against the buried length. The one-third rule is a proportion that has survived because it scales the buried length with the leverage above it, and it is what QUIKRETE's own data sheet states: "Hole depth should be 1/3 the overall post height."

It is a rule of thumb about geometry, and it knows nothing about your soil. Penn State Extension is direct that depth "varies considerably with soil conditions", and singles out sandy or chronically wet ground as wanting deeper posts, sometimes with a concrete collar. Solid resistant soil at the surface does more work than the same length of soft fill. No source publishes a soil multiplier to apply, so none is invented here — but if you are digging through loose fill, take the deeper end of what you are considering.

Two other cases beat the proportion. A gate post carries a swinging weight that levers it constantly, and a corner post is pulled sideways by every run of fence meeting it. The cited horse-fencing figures set those at 48 inches against 36 for line posts, and about a quarter larger in diameter. Solid-board and privacy fences also catch far more wind than the post-and-rail those figures describe, which is an argument for depth rather than against it.

Common mistakes

• Adding the frost depth to the stability depth. They are alternatives, not layers. Take the deeper and dig once.
• Buying posts the length of the fence. A 6-foot fence needs a 9-foot post before any frost requirement. This is the single most common trip to the lumber yard twice.
• Trusting a frost-line map you found online. Most assign confident numbers to states that publish none — New York among them. The figure that governs you is the one your building department has written down.
• Digging a hole that flares at the top. A cone of concrete wider at grade than below is exactly the shape frost needs to grip and lift. Keep the sides straight, or slightly wider at the bottom.
• Setting the post before checking the boundary. Concrete sets in under an hour; a survey does not.

Once the holes are dug

The hole dimensions on this page are what size everything that goes into it. The fence post concrete calculator takes the depth and width from here and returns the bag count in every size sold, subtracting both the post that displaces concrete and the few inches at the top the manufacturer says to backfill with soil. If you are setting posts in compacted gravel instead, the crushed stone calculator sizes that, and it applies the compaction allowance that makes loose stone arrive as more than its finished volume.

Anything you buy in bulk rather than by the bag arrives in cubic yards, and the cubic yard calculator converts between that and the cubic feet a hole is measured in. If your fence runs along a wall you are also building, the retaining wall calculator carries the other threshold on this site where a project stops being DIY and starts needing an engineer.

Frequently asked

Sources

• QUIKRETE Fast-Setting Concrete Mix (No. 1004-50) — product data sheet — The QUIKRETE Companies, accessed Aug 12, 2026.
• Fence Planning for Horses (Eileen E. Fabian and Jennifer Smith Zajaczkowski) — Penn State Extension, accessed Aug 12, 2026.
• Minnesota Rules, part 1303.1600 — Footing Depth for Frost Protection — Minnesota Office of the Revisor of Statutes, accessed Aug 12, 2026.
• Mich. Admin. Code R. 408.30522 — Minimum depth — Legal Information Institute, Cornell Law School, accessed Aug 12, 2026.
• Technical Bulletin TB-1009-RCNYS — Requirements for Completing Table R301.2(1) in the 2020 Residential Code of New York State — New York State Department of State, Division of Building Standards and Codes, accessed Aug 12, 2026.
• 675 IAC 14-4.4-43 — Section R403.1.4.1; frost protection — Legal Information Institute, Cornell Law School, accessed Aug 12, 2026.
• Ohio Admin. Code 4101:8-4-01 — Foundations — Legal Information Institute, Cornell Law School, accessed Aug 12, 2026.