How Deep ?

[woodchopper]

Four small boundary posts that divide the front gardens of neighbours and mine are rotten.
Solution is to dig them out and replace with same (3 x 3 treated timber).
They are only 15" above ground level - how deep in post-crete do they need to be below the surface to make them stable ?

[spathiphyllum]

I would do 6".

[spathiphyllum]

Just over 1/3. ^

[johnk]

Concrete posts are better, they don't rot.