What a 100 foot fence takes
A straight run of 100 feet with posts no more than 8 feet apart is 12½ bays, so it is 13 bays of 7' 8 5/16" each, and 14 posts — always one more post than bays on a run with two free ends. A 6 foot privacy fence carries three 2×4 rails a bay, one for every 24 inches of height: 39 rails, each an 8 foot board. Its 1×6 pickets are 5½ inches wide, so 1,200 inches of fence takes 1,200 ÷ 5.5 = 218.2, rounded up to 219 pickets — 230 with 5% for the ones that split or warp.
Each post goes 36 inches into the ground, half the 72 inches that stand above it, on a 6 inch bed of gravel: a hole 42 inches deep and 10 inches across. The concrete fills it from the gravel to 3½ inches under the ground, around the post: (π ÷ 4 × 10" × 10" − 3½" × 3½") × 32½" is 1.247 cubic feet a post, 17.455 for fourteen, and at 0.375 cubic feet a 50 pound bag of fast-setting mix that is 47 bags. A post 72 + 36 = 108 inches long is bought as a 10 foot 4×4 and cut.
Runs, corners and gates
List your fence the way you walk it. Each run starts where the one before it ends, so the posts stand at the joints: where two runs meet is a corner post, either side of a gate is a gate post, and an open line has an end post at each end. Close the line — a garden or a pool all the way round — and the last run meets the first at a corner, with no ends at all. A back yard of 50, 100 and 50 feet is 27 bays and 28 posts: 24 inside the runs, two corners and two ends. Added up run by run it would come to 30.
A gate takes its own two posts, its hinges, a latch, and on a double gate a drop rod to hold the fixed leaf down. A wood gate is framed on site from the same 2×4s and pickets; chain link, vinyl and aluminum gates are bought made.
Why the holes are deeper than some calculators say
Quikrete's rule is a hole half as deep as the post stands — a third of the post's length — with 6 inches of gravel under it. Inch Calculator buries a 6 foot fence's posts 2 feet, a third of its height, which is a foot shallower. Where the ground freezes, a post set above the frost line heaves: your building department knows the depth, and typed into Your frost depth under Assumptions it sets every hole.
Bags are counted from the shape of the holes, not from a rule of so many a post. Quikrete's own example — two 50 pound bags for a 4 inch post in a 10 inch hole 2 feet deep — fills about four fifths of the hole: the geometry says 2.46 bags.
What this does not do yet
A fence down a slope, stepped or racked, is worked out as if the ground were level, and one height is asked for the whole line. Before you dig, call 811, and ask your town and any homeowners' association how tall a fence may be and where it may stand. We are not connected with any maker, store or supplier named here.