A fence looks professional when the gaps between pickets are identical from one end to the other. The fastest way to ruin that is to start nailing boards at one post with a spacer block and hope it works out, because whatever space is left at the far end shows up as one obviously wrong gap. Solving the spacing on paper first tells you exactly how many pickets to buy and what gap to actually cut your spacer to.
How to Calculate Fence Picket Spacing (step by step)
Step One: Convert the fence run to inches
Picket widths are measured in inches, so the run has to be in inches too before anything divides cleanly.
run = fence length × unit factor (ft = 12, in = 1, m = 39.3701)
run = 8 × 12 = 96 in
Step Two: Estimate how many pickets fit
Each picket carries one gap with it, except the last one. Adding a single gap to the run before dividing accounts for that, and rounding down guarantees you never overrun the post.
pickets = floor((run + desired gap) ÷ (picket width + desired gap))
pickets = floor((96 + 0.5) ÷ (5.5 + 0.5))
pickets = floor(96.5 ÷ 6) = floor(16.083) = 16 pickets
Step Three: Count the gaps
Gaps sit only between pickets, not at the posts.
gaps = pickets − 1
gaps = 16 − 1 = 15 gaps
Step Four: Solve for the true gap width
Subtract all the solid picket material from the run. Whatever is left is the total air, split evenly.
leftover = run − (pickets × picket width)
actual gap = leftover ÷ gaps
leftover = 96 − (16 × 5.5) = 96 − 88 = 8 in
actual gap = 8 ÷ 15 = 0.53 in
Step Five: Convert to on-center spacing
On-center spacing is what you mark on the rails if you prefer layout marks over a spacer block.
on-center = picket width + actual gap
on-center = 5.5 + 0.53 = 6.03 in
What your results mean
With the defaults, an 8-foot section takes 16 pickets with 15 gaps of 0.53 inch each, on 6.03-inch centers. That gap is a hair wider than the 0.5 inch requested, which is the correct outcome: forcing exactly 0.5 inch would leave a half-inch sliver of extra space somewhere, and the eye catches that instantly. The spacing style label tells you where you have landed, from near-solid privacy under a quarter inch to open ranch-style spacing above 1.5 inches.
Practical tips
Cut a scrap spacer to the actual gap value rather than trusting a nail or a tape mark on every board. If the calculated gap drifts far from your target, change the picket count by one and rerun it: dropping a picket widens the gaps, adding one tightens them. Also check the result against your local code, since pool and child-safety enclosures often cap the gap at a fixed maximum regardless of how good it looks.
When to rip a picket instead
Sometimes no whole-picket count gives a gap you like. In that case, keep the gap you want and rip the two end pickets to equal narrower widths. Symmetric end boards read as intentional; a single odd gap does not.