Guides
Fence posts, depth and layout questions answered with the same arithmetic the calculators use.
Set posts at least a third as deep as the fence is tall, never less than 24 in, and below the frost line where the ground freezes. For a 6 ft fence that is 24 in in mild climates, so an 8 ft post; in frost country 36 to 48 in, which takes a 10 ft post.
No more than 10 ft apart for line posts, spaced evenly along each straight run, with a heavier terminal post at every end, corner and gate. A straight 100 ft run takes 11 posts: 2 terminal and 9 line posts at 10 ft.
Yes, for line posts on a fence up to about 6 ft. Fill the hole with crushed stone that has fines in it, tamping every few inches: a 4x4 in a 9 in hole 30 in deep takes about 0.89 cu ft, roughly 2 bags of 50 lb. Keep concrete for corner, end and gate posts.
Measure each straight run of the fence line separately, in feet, and add them up. Take gate openings out of the runs, count each corner post once, and work out posts run by run. A back yard fenced on three sides, 60 × 80 × 60 ft with a 4 ft gate, is 200 ft of fence and about 27 posts at 8 ft spacing.