Published August 11, 2026 · Updated August 23, 2026 · By Tupep
The off-by-one that wrecks the order
A single straight 100 foot run of 6 foot privacy fence is the simplest fence job there is, and it is where most people still order the wrong number of posts. The mistake is dividing 100 by the post spacing and buying that many posts. A fence is a chain of sections, and a chain of sections always needs one more post than it has sections. A single section has a post at each end. Two sections share a middle post and need three. Thirteen sections need fourteen. Come up one short and the whole line stops on the day the concrete is already mixed.
The formula
- Sections = run length divided by your maximum post spacing, rounded up
- Posts = sections + 1
- Actual spacing = run length divided by sections
Rounding up is not optional. If you round down you have quietly stretched every bay past the spacing you chose. Rounding up shortens every bay a little, which is always safe.
Worked example, 100 ft at 8 ft maximum spacing
100 divided by 8 is 12.5 sections. Round up to 13 sections. Posts equal 13 plus 1, so 14 posts. Actual spacing is 100 divided by 13, which is 7.692 ft, or 92-5/16 in on center. That is the number you write on your batter board tape, not 96 in.
Rails come next. A 6 ft privacy fence normally gets three rails per section, top, middle and bottom. That is 13 times 3, or 39 rails. Every rail is cut to 92-5/16 in, so 8 ft stock works with a 3-11/16 in cutoff each. We generally buy two or three extra rails because the last few cuts always find the crooked boards in the pile.
Picket math
Pickets are counted by module, not by board. A standard dog-ear fence picket is nominally 1×6 and measures 5.5 in of actual face. Add the gap you want between boards and you get the module width. A 1/4 in gap gives a 5.75 in module. Divide the run length in inches by the module and round up.
100 ft is 1200 in. At a 5.75 in module, 1200 divided by 5.75 is 208.7, so 209 pickets. Add about 5 percent for splits, crooks and cull boards and order 220. Here is how the gap changes the count on the same 1200 in run.
| Gap between pickets | Module width | Pickets for 1200 in | Notes |
|---|---|---|---|
| 0 in, butted tight | 5.50 in | 219 | For wet pressure treated stock that will shrink open |
| 1/8 in | 5.625 in | 214 | Reads as solid from the street |
| 1/4 in | 5.75 in | 209 | Most common for kiln dried or cedar |
| 3/8 in | 5.875 in | 205 | Gap becomes visible up close |
| 1/2 in | 6.00 in | 200 | No longer full privacy |
The gap decision is really a lumber moisture decision. Green pressure treated pickets are wet when you buy them and shrink across the width as they dry, so butting those tight still leaves you a gap later. Kiln dried cedar and dried treated stock barely move, so tight stays tight and the fence traps water at every seam.
Posts, holes and depth
Two rules of thumb do most of the work here. First, the hole diameter should be roughly three times the width of the post, so a 4×4 with a 3.5 in actual face wants a hole around 10 to 12 in across. A 12 in auger bit is the easy answer. Second, a common guideline is to bury about one third of the total post length. Both are guidelines, not law.
Verify embedment depth and frost depth with your local building department before you dig. Post depth is jurisdiction dependent and frost depth varies enormously across the country. In many jurisdictions a fence post must reach below the local frost line, and some places also regulate fence height, setback from the property line and which side the finished face must point. None of that is something a calculator can know.
There is an honest tension in the one third rule. An 8 ft post set 30 in deep leaves 66 in above grade, which is not a full 6 ft fence. A true 6 ft of fence plus a one third embedment means buying 9 or 10 ft posts.
Concrete per hole
A 12 in diameter hole 30 in deep holds about 1.96 cu ft. Subtract the volume of the post itself, roughly 0.21 cu ft, and you need about 1.75 cu ft per hole. An 80 lb bag of concrete mix yields about 0.6 cu ft, so that is three bags per hole. Fourteen posts times three bags is 42 bags, and we always throw two extra in the truck.
| Item | Quantity | Basis |
|---|---|---|
| Posts, 4×4 | 14 | 13 sections + 1 |
| Rails, 2×4 | 39 | 3 per section, cut to 92-5/16 in |
| Pickets, 1×6 | 220 | 209 net plus 5 percent |
| Concrete, 80 lb | 42 | 3 bags per 12 in x 30 in hole |
| Gravel for hole bottoms | about 3 cu ft | 3 in drainage layer per hole |
Two things that save the layout
Set the two end posts first, string a mason line between them at the picket top height, and measure every intermediate post off that string rather than off the previous post. Measuring post to post stacks your errors, and by post fourteen you are two inches off. Measuring from a single string keeps the error at zero.
Use a spacer block for pickets, never a tape. Rip a scrap to your gap width, hang it on the top rail, butt the next picket to it and drive the fastener. A tape measure across 209 pickets will drift and a block will not.
Related guides
- Post Spacing: 6 ft vs 8 ft
- Gate Framing: What the Calculator Does Not Count
- Fence Materials for a Quarter-Acre Backyard
- Deck Framing Lumber List for a 10×12 Deck
Sources and Further Reading
Run your own numbers with the Fence Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 23, 2026
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