How we worked it out
Updated as you change the numbers.
- Sections = 100 ft ÷ 8 ft, rounded up = 13. Posts = sections + 1 = 14.
- Rails = 13 sections × 3 = 39.
- Pickets = 1,200 in ÷ (5.5 + 0) in = 218.2; with 5% extra, rounded up: 230.
- Concrete per hole = (π × (10 in ÷ 2)² − post) × 24 in = 0.921 cu ft; × 14 posts = 12.89 cu ft.
- 80 lb bags = 12.89 ÷ 0.60 = 21.48, rounded up.
Method and assumptions
The fence line is split into sections of the post spacing. There is one more post than sections for a straight run; each corner and gate post that is already counted at a section end needs nothing extra. Rails are one per section per rail height.
Pickets are the fence length divided by one picket plus its gap. Concrete is the volume of each round hole less the part filled by the post, using the 0.60 cu ft yield of an 80 lb bag of standard concrete mix.
Tips
- Call 811 (in the US) a few days before digging so buried utility lines are marked.
- Check property lines, local fence height limits and any HOA rules before you buy.
- Put a few inches of gravel in the bottom of each hole for drainage, and crown the concrete so water runs away from the post.
Questions
How many posts do I need for a 100 ft fence?
With posts every 8 ft, 100 ft is 12.5 sections, so 13 sections and 14 posts. With a 6 ft spacing it is 17 sections and 18 posts.
Sources
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.