Gravel is a common backfilling for fence posts and comes in a variety of types. Some gravel comes mixed with sand to make it finer, while other gravel is made of crushed rock. For most projects, size will make a difference. There are a couple of major factors to keep in mind when choosing gravel for fence post holes.
When a fence post hole is dug, it creates a place for water to collect. This isn't a problem for the ground, but when you put a wooden fence post there, the collected water can cause the wood to rot over time. For metal fence posts, it will cause rust. Larger sized gravel will allow the water to drain through to the ground below so that the post isn't sitting in stale water for long periods of time. Overly large gravel will not sit in the hole as well, and the fence post will lack support.
Finer gravel will provide a more secure bottom for the fence post to set on. If the gravel is too fine, however, water will not drain through as well and may end up collecting around the fence post. For this reason, gravel mixed with sand, and extra fine gravel should be avoided.
What to Use
Pea gravel is commonly used for fence post holes. Gravel that is roughly three eighths of an inch in size is generally a good medium between drainage and support, though gravel close to that size also may be used. If you want to use the best gravel for your area, you may want to consult a contractor. They will know what kind of support is needed based on ground and weather conditions.