Fence post and picket calculator
Fence quantities are simple arithmetic with one persistent error: the post count. A 100 ft run at 8 ft spacing is not 12.5 posts, it is 13 — one more bay than the division suggests, because each bay needs a post at each end.
Your fence
How this is calculated
- Bays = ⌈run ÷ spacing⌉. Posts = bays + 1.
- Actual bay length = run ÷ bays, which is usually shorter than the spacing you entered — that is where the last post lands.
- Rails = bays × rails per bay × actual bay length.
- Pickets = ⌊run in inches ÷ (picket width + gap)⌋.
- Concrete per hole = π × (diameter ÷ 2)² × depth, minus the post, converted at 0.60 ft³ per 80 lb bag.
Worked example
100 ft run, 8 ft spacing, 6 ft high, 3 rails, 5.5″ pickets with a ⅛″ gap, 10″ holes 30″ deep:
- Bays = ⌈100 ÷ 8⌉ = 13, so 14 posts and an actual bay of 7.69 ft
- Rails = 13 × 3 × 7.69 = 300 linear feet
- Pickets = ⌊1,200 ÷ 5.625⌋ = 213
- Concrete per hole = π × 25 × 30 = 2,356 in³ = 1.36 ft³, less a 3½″ post ≈ 1.15 ft³ → 2 bags of 80 lb
- All holes = 14 × 2 = 28 bags
The 14 posts is the number to check against a quote. A contractor who prices 13 posts has priced a fence that is one bay short.
Height and spacing are a wind decision
A 6 ft solid fence is a sail. Wind pressure on a solid panel at 6 ft is roughly three times that on a 4 ft fence, and it acts on a lever twice as long, so the post load goes up by a factor of six. That is why tall fences use deeper holes, longer posts and often 6 ft rather than 8 ft spacing — not because the rails need it, but because the posts do.
If a fence has failed in wind, it is almost always at the post, at the soil line, in a hole that was too shallow or too wide with loose backfill.
Where post arithmetic stops being the question
- It does not check fence law. Height limits, setback from the property line, and which face goes outward are local ordinances, and they vary street by street in some places.
- It does not place gates. A gate bay needs heavier posts and a different detail, and it changes the run arithmetic.
- It does not include caps, trim, or a top rail. Add them as extra linear feet.
- It does not size posts. Post size and embedment follow from height and wind exposure, and are usually set by local code or the manufacturer's table.
Questions people actually ask
How many posts for a 100 foot fence?
At 8 ft spacing, 14 posts: 13 bays, one post at each end. The common error is 12 or 13, which leaves the run one bay short.
How deep should fence posts be?
A third of the above-ground height as a rule of thumb — 2 ft for a 6 ft fence — and below frost depth wherever the ground freezes. Local code or the manufacturer governs.
How many bags of concrete per fence post?
About 2 bags of 80 lb for a 10″ hole at 30″ deep with a 4×4 post displacing some volume. Setting posts in gravel instead is common and drains better, but resists less.
What gap between fence pickets?
⅛″ for a privacy fence that will shrink as it dries, ¼–½″ where you want airflow and a less solid look. Treated pickets shrink, so a tight fence becomes a gappy one.
Is 8 feet too far apart for fence posts?
For a 4 ft fence, no. For a 6 ft solid fence in an exposed site, 6 ft is safer, because the post carries the wind load and the lever arm is longer. Spacing is a wind decision at that height.