Fence Calculator
Posts = sections + 1 + 1 per gate, with sections = length ÷ post spacing, rounded up. A 100 ft fence with posts 8 ft apart and one gate is 13 sections: 15 posts. Enter each straight side as its own run, so every end and corner post is counted.
Fence posts
15 posts
13 sections + 1 end post + 1 gate post
- Pickets (incl. 10 % extra)
- 240
- Rails (3 per section)
- 39
- Concrete, all holes
- 18.99 ft³ (0.7 yd³)
- 50 lb fast-setting bags (0.375 ft³ each)
- 51
- 80 lb concrete mix bags (0.6 ft³ each)
- 32
- 60 lb concrete mix bags (0.45 ft³ each)
- 43
- Concrete per hole
- 1.27 ft³ = 3.38 bags of 50 lb
- Dig each hole
- 10 in wide, 42 in deep
- Gravel under the posts
- 4.09 ft³
Enter each straight side as its own run: where two sides meet, the corner post is counted with both, so you get one spare. Where the ground freezes, ask your building department how deep posts must go.
Worked examples
A 100 ft privacy fence, 6 ft tall, one gate
- 100 ÷ 8 = 12.5, so 13 sections; 13 + 1 end post + 1 gate post = 15 posts
- 1,200 in × 1.10 ÷ 5.5 = 240 pickets; 13 sections × 3 rails = 39 rails
Result: 15 posts, 240 pickets, 39 rails.
Checked by the automated test “fence — 100 ft at 8 ft spacing, 1 gate → 13 sections, 15 posts, 39 rails, 240 pickets of 5 1/2 in”.
Concrete for those 15 posts: 4x4 in 10 in holes, 36 in in the ground
- Hole 3.1416 × 10² ÷ 4 = 78.54 in², less 3.5 × 3.5 = 12.25 in² of post: 66.29 in²
- 66.29 × (36 − 3) = 2,188 in³ = 1.27 ft³ per hole
- × 15 = 18.99 ft³; ÷ 0.375 = 50.6, so 51 bags of 50 lb fast-setting (or 32 bags of 80 lb mix)
Result: 51 bags of 50 lb fast-setting concrete.
Checked by the automated test “fence — 15 posts 4x4 in 10 in holes 36 in deep → 18.99 ft³: 51 fast-setting bags of 50 lb, 32 of 80 lb”.
How it works
Sections of a run = run length ÷ post spacing, rounded up
Posts = Σ (sections of each run + 1) + 1 per gate
Pickets = run length (in) × (1 + extra %) ÷ (picket width + gap), rounded up run by run · Rails = sections × rails per section
Concrete per hole (ft³) = (π × hole diameter² ÷ 4 − post cross-section, in²) × (depth in the ground − 3 in) ÷ 1,728
Assumptions
- Counts follow Lowe's fence worksheet: one post per section plus one to end each run, one more per gate, pickets plus 10 %, 2 rails per section for a 4 ft fence and 3 for a 6 ft fence.
- Holes follow QUIKRETE: about 3 times the post width, posts set half as deep as the fence is tall (1/3 of the post length), on 6 in of gravel, concrete stopped 3 in below grade (QUIKRETE says 3 to 4 in).
- Bag yields: 0.375 ft³ per 50 lb bag of QUIKRETE Fast-Setting Concrete, 0.6 ft³ per 80 lb and 0.45 ft³ per 60 lb of concrete mix. A 4x4 post is 3 1/2 in square, a 6x6 5 1/2 in.
Reference table
| Fence length | Posts | Pickets, 5 1/2 in + 10 % | 50 lb fast-setting bags |
|---|---|---|---|
| 50 ft | 8 | 120 | 28 |
| 100 ft | 14 | 240 | 48 |
| 150 ft | 20 | 360 | 68 |
| 200 ft | 26 | 480 | 88 |
4x4 posts in 10 in holes, 36 in in the ground, as in the example. Add one post and its concrete per gate.
Before you dig
- Measure along a string. Stretch a line between the end posts and measure it: that is the length to enter, run by run.
- Pick the real post size. A 4x4 is 3 1/2 in square; the post itself is taken out of the concrete volume.
- Lumber by the board foot. To price the posts and rails as lumber, run their sizes through the board foot calculator.
Common mistakes
- Dividing the length by the spacing 100 ft ÷ 8 ft gives 12.5, but every section needs a post at both ends: 13 sections take 14 posts before any gate.
- Forgetting gate and corner posts Lowe's adds one post per gate and one to end each run. Two 12 ft sides meeting at a corner, posts 8 ft apart, need 5 posts; entered as one 24 ft run they get 4 and the corner is missed. Entered as two runs, they get 6: one spare.
- Setting posts too shallow QUIKRETE sets posts half as deep as the fence is tall: 36 in for a 6 ft fence, on 6 in of gravel. Where the ground freezes, ask your building department.
- Filling the hole to the top QUIKRETE stops the concrete 3 to 4 in below grade and fills the rest of the hole with the soil dug out of it.
Frequently asked questions
How many pickets do I need for a fence?
Multiply the fence length in feet by 12, divide by the picket width plus the gap, and add 10 %. For 100 ft of 5 1/2 in pickets set tight: 1,200 ÷ 5.5 = 218.2, so 240 with the extra.
How many fence pickets do I need for 100 feet?
240 pickets 5 1/2 in wide set tight, 10 % extra included. With a 2 in gap between them, 176.
How many fence posts do I need?
Divide the length of each run by the post spacing and round up, then add one post per run and one per gate. At 8 ft spacing, 150 ft of fence in one run with no gate takes 20 posts.
How much concrete do I need for fence posts?
QUIKRETE's own figure is two 50 lb bags of Fast-Setting Concrete for a 4 in post in a 10 in hole 2 ft deep (a 4x4 takes 2.15). Set 36 in deep for a 6 ft fence, a 4x4 takes 3.38 bags.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- Fence Calculator Info Sheet – Pickets (posts, rails and pickets per section) — Lowe's, accessed
- Project: Setting Posts (hole 3 times the post, 1/3 of the post length, 6 in of gravel) — QUIKRETE, accessed
- Fast-Setting Concrete #1004-50, specification data (0.375 ft³ per 50 lb bag; two bags per 4 in post) — QUIKRETE, accessed
- Concrete Mix #1101, data sheet (0.6 ft³ per 80 lb bag, 0.45 ft³ per 60 lb) — QUIKRETE, accessed