Fence post depth

[Kipper at sea]

18" or 500mm deep and square fill with a stiff mix of concrete 3 ballast to 1 cement or use post create. Fit panels as you erect post and prop fence panels.

[zaax]

For a little more cost do it only once, using concrete spurs. The wooden posts will last longer and only need unbolting when they do need replacing.

[Dave Liquorice]

The 1/3 rule still applies and as little ground disturbance as possible. But good reinforced concrete spurs are the thing for the job.

"Only need unbolting" stainless steel bolts washers and nuts might not be a bad idea. I don't use any steel fixings outisde, steel just doesn't last or is so rustied up you have to cut the things off. No I tell a lie I do use hot dip galvanised clouts...