We just ordered our rails for our wood paddock fence (2x6 oak) and they will be ready to pick up this week. We have not decided on what to use for the posts yet and I was hoping you guys could help. Will 4x4 posts suffice or do I have to get 4x6's??? Some people have said to just use round pine posts (like they use in split rail fences, but there is more surface area to nail your rails to with square posts...not to mention the rounds are more expensive). AND, I know that the posts should be 4 feet in the ground (below frost level) and 5 feet above ground...well the posts only come in 8 or 10 foot in length, so is 3 feet in the ground good or do we need to buy the 10 footers and cut a foot off of the top once the rails are attached.

Thanks for your input.

8' treated 4X4s should work fine, with bigger rounds or squares at gates and corners.  Pouring about a fifth of a bag of premix concrete into each hole (you don't have to add water if you don't want to, the ground water will leach in) will fix any stability problems.  I've never had a problem with this method.

Terry

Forgot to mention...

You can call your electric company to see if they have any used smaller poles - ours gives them away; take your truck and a chainsaw.  Do be careful of foreign objects if you go that route  The ones from out in the country are usually no problem.  Our neighbor put these posts in the ground then used the full length as toprails and added horse fence underneath.. Actually makes a nice, SOLID, looking fence.

Also RR ties that have been replaced - call the owner of the tracks and see if you can get them before they sell them to the landscapers.

You DO know that round posts are stronger than 4x4s, don't you?

4x6s make a good sturdy fence. 

I prefer to see a wood fence that has the top of the post even with the top of the top rail, so it presents a smooth look.  To get that look, each post will have to be cut after the rail is attached.

Will 4x4s do?  Maybe.  If it's a small paddock, probably not.  Horses will tend to push against the fence if it a small paddock.  On a large pasture, they're less likely to touch the fence.

As another poster mentioned, anywhere near gates or corners need to be beefed up.  I use 6x6s at gates.