Fence Post Calculator
How many fence posts and sections a fence run needs, and how many bags of concrete set the posts, from the length and spacing.
How this is calculated
A straight run of fence needs one more post than it has sections. The sections are the length divided by the post spacing, rounded up, so a 100 foot run at 8 foot spacing has 13 sections and 14 posts. Spacing of 6 to 8 feet is usual; prefabricated panels set it for you.
Each hole is a cylinder: pi times the radius squared times the depth. The post takes up part of it, so the post's cross-section times the depth is subtracted. Square wood posts are smaller than their name: a 4x4 is 3-1/2 by 3-1/2 inches under the American Softwood Lumber Standard. Bag counts use the manufacturer's published yields. QUIKRETE Concrete Mix (No. 1101) data sheet: a 40 lb bag yields about 0.30 cubic feet, 50 lb 0.375, 60 lb 0.45 and 80 lb 0.60.
Hole size depends on your soil, frost depth and fence height, so the defaults here are only a starting point: use the hole size your local building code or the fence or post maker gives for your area.
Limits
One straight run. For a fence with corners, work out each side and remember a corner post is shared between two sides. Gates need their own posts, often larger. Gravel at the bottom of each hole for drainage is not included.