How deep should the posts go?

Deep enough to sit below the local frost line, which is set by your building department rather than by any rule that could be printed here. A third of the post length in the ground is a common starting point for a standard fence.

How much concrete does each post take?

Use the post hole option on the concrete calculator — it subtracts the post itself from the hole, which most calculators do not, and a 4×4 takes up about a sixth of a 10 in hole.

Does the picket count allow for the ends?

It divides the run by one picket plus one gap, which slightly overstates the count because the final picket needs no gap after it. That errs towards one spare picket rather than one short.

Home & construction

Fence Calculator

Enter the run and your spacing to see the posts, sections, rails and pickets.

The formula

sections = ⌈ run ÷ spacing ⌉

posts = sections + 1

rails = sections × rails per section

pickets = ⌈ run × 12 ÷ (picket width + gap) ⌉

Worked example

A 100 ft run at 8 ft spacing, two rails, 5½ in solid pickets

100 ÷ 8 = 12.5 → 13 sections
13 + 1 = 14 posts
13 × 2 = 26 rails
100 × 12 ÷ 5.5 = 218.2 → 219 pickets

Where this goes wrong

Buying one post per section

A fence needs one more post than it has sections, because the last section still has to end somewhere. A 100 ft run at 8 ft spacing is 13 sections but 14 posts. It is a single post, yet it is the one that stops the job on the last afternoon — and on a fence with gates and corners the error repeats at every run.

Questions

How deep should the posts go?: Deep enough to sit below the local frost line, which is set by your building department rather than by any rule that could be printed here. A third of the post length in the ground is a common starting point for a standard fence.
How much concrete does each post take?: Use the post hole option on the concrete calculator — it subtracts the post itself from the hole, which most calculators do not, and a 4×4 takes up about a sixth of a 10 in hole.
Does the picket count allow for the ends?: It divides the run by one picket plus one gap, which slightly overstates the count because the final picket needs no gap after it. That errs towards one spare picket rather than one short.