How deep should fence posts be?

The published figures cluster around 600mm and they broadly agree: a domestic fence post goes 600mm into the ground almost regardless of fence height, and the hole is dug about three times the post's width so there is a proper collar of concrete around it rather than a skin. A 100mm post therefore wants a hole nearer 300mm across. That collar is what resists the leverage of a 1.8m sail in a February gale.

Ground changes the answer, which is why we look before we quote. On the free-draining limestone brash up towards Leckhampton Hill, Cleeve Hill and Winchcombe, the hole is hard to cut but what you cut holds its shape and grips. On the Charmouth Mudstone clay through the Severn Vale it digs easily in winter and then holds water. On alluvium and made ground near the rivers, and on the gravel spit under old Tewkesbury, the sides slump as you dig and there is less holding the post, so we go deeper.

Two details that matter as much as the number. Never bed a post on a layer of loose gravel in the hope of drainage; on clay it turns the hole into a sump that keeps water permanently against the timber. And finish the concrete collar proud of the ground and sloped away from the post so rain runs off rather than pooling at the joint. Both are free at the time and decide whether the post rots in seven years or twenty.

This is also the honest answer to why quotes differ. Post depth and hole diameter are invisible the day the fence goes up and decisive five years later, so they are the easiest place for a cheap quote to find its margin. Ask any quote what depth the posts go to and how much concrete goes in each hole. A contractor who is proud of the answer will tell you without hesitating.

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