Fence Post Length Rule of ThumbIn general, 1/3 - 1/2 the height of your fence should be below ground, with a minimum of 2 ft. What this means in practice is that if you have a 4' high fence, your posts should be 6' with 2' below ground. If your fence is 6', then you should use 8 1/2' - 9' posts with 2 1/2' - 3' below ground.
If you follow the guideline above - for most standard fence systems - your posts should be deep enough to provide strength and stability for your fence. However, if you live in an area of high wind or extreme cold, you may want to opt for a longer post, because it will make your fence less vulnerable to the elements. On the other hand, if you are installing an light ornamental system in a low wind area, then you may be able to get away with shorter posts; although longer posts won't hurt.