Materials & Cost
What size should fence posts be?
For residential wood fence, 4x4 posts on the line and 6x6 at every corner, end and gate is the standard specification. Post length is fence height plus burial depth, so a six-foot fence generally needs a nine- or ten-foot post. The old one-third-in-the-ground rule of thumb is a sanity check, but in southeastern Pennsylvania the frost line governs — the footing has to reach below it. Chain link uses pipe sized by outside diameter and wall thickness, with terminal posts a size up from line posts.
A 4x4 line post only carries its share of the fence between two neighbouring posts, with equal tension pulling from both sides. A corner, end or gate post resists everything pulling on it from one direction with nothing pulling back, which is a completely different load. That is why terminal posts step up to 6x6 — more section to resist bending, more depth of timber for hinge lags to bite into, and more mass in the footing to resist rotation. On an eight-foot privacy fence, or on a long run exposed to wind, 6x6 on the line is worth the upgrade as well.
Buy the post long enough at the start, because length cannot be added later. The post has to reach from the top of the fence down to the bottom of the footing, which means finished height plus burial depth plus a little for trimming. A six-foot fence with a three-foot burial needs a nine-foot post, and stock is milled in eight, ten and twelve, so the practical answer is usually a ten-foot post cut to height on site. Gate posts routinely go a size larger and a step deeper than everything else on the run.
The one-third-in-the-ground rule comes out of agricultural fencing and it is a reasonable starting point, not a specification. What actually decides depth here is frost. Water in the soil freezes, expands, grips the sides of the post and lifts it, and each freeze-thaw cycle ratchets it a little higher until the line is visibly out. The footing has to sit below the depth frost reaches, and the frost depth enforced on your property comes from your municipality's adopted code rather than from a single statewide figure. Confirm it before digging.
Chester County ground makes this worse than the number alone suggests. It is clay-heavy rolling terrain draining toward the Brandywine and Red Clay watersheds, and clay holds water, which is the thing that freezes. A footing that traps water against the post also accelerates rot at grade on wood. Footings should be crowned at the top so water sheds away from the post rather than pooling against it, and belled slightly at the base so the mass sitting below frost depth resists uplift. Depth without shape is only half the job.
Chain link is specified differently. The posts are pipe, sized by outside diameter and wall thickness rather than by a nominal lumber dimension, and terminal posts always run a diameter larger than line posts. Heavier wall is the difference between a fence that holds its line under fabric tension and one that bows. Vinyl and aluminum are different again: posts are system components, so size and maximum spacing are set by the panel manufacturer, and vinyl gate posts normally take an internal steel or aluminum stiffener.
