A fence is posts first and everything else second. Get the post count and spacing right and the rest follows; get it wrong and you are digging again.
Post spacing is set by the panel, not by preference
Six or eight feet on center covers almost all residential fence. Pre-built wood and vinyl panels are made for one or the other, so measure the panel before deciding — an eight-foot spacing with six-foot panels means cutting every one.
Chain link is more forgiving because the fabric is continuous, but ten feet is the practical maximum before the top rail starts to sag.
A straight run needs one more post than sections
Divide the length by the spacing to get sections, then add one. A 64-foot run at 8-foot spacing is 8 sections and 9 posts.
That formula is for one straight line. Every corner, every end and both sides of every gate is an additional post, and they are usually a heavier post than the line posts.
How deep and how much concrete
The rule is a third of the post's above-ground height, or below your frost line, whichever is deeper. A six-foot fence usually means a two-foot hole; in northern states frost depth pushes that to three or four feet.
Hole diameter should be about three times the post width — a 4x4 post gets a 10 to 12 inch hole. That works out to roughly two 50-pound bags of fast-setting mix per post.
Set the posts before anything else goes on
Set and brace every post, let the concrete cure overnight, and only then hang rails or panels. Hanging weight on green concrete pulls posts out of plumb and you will see it down the whole run.
Run a string line between the two end posts and set everything to the string. Eyeballing a long fence never works.
Following a slope
On sloping ground a fence is built one of two ways. Stepped, where each panel stays level and drops at each post, leaving triangular gaps at the bottom. Or racked, where the panel follows the grade and the pickets stay vertical.
Preassembled panels generally cannot rack more than a few degrees. A fence built stick by stick — posts, rails, then pickets — follows any slope you like.
Which you can do changes the material list, so decide before ordering. Stepped fences on a steep slope also need taller posts at the low end of each step.
Before you dig or build
Call the utility locate service before setting any post. It is free and required almost everywhere, and post holes are exactly the depth at which shallow gas, water and fibre lines live.
Confirm the property line, ideally from a survey rather than from where the last fence was. A fence is a permanent, visible marker and moving one later is expensive in more ways than money.
Check local height limits and any association rules before buying material. Front yard limits are commonly lower than back yard limits, and a fence built over the limit may have to come down.