フェンス縦板間隔計算ツール

A fence picket spacing calculator solves a problem that eyeballing never quite solves: dividing a fixed bay width, the clear distance between two posts, into an exact number of pickets and perfectly even gaps, rather than a run of consistent-looking gaps that drift wider or narrower by the time they reach the far post. The inputs are simple: the clear interior width of the bay, the actual milled width of the picket board, not its nominal size, and a target gap. Nominal lumber sizing matters here more than almost anywhere else in a fence build, since a 1x4 picket actually measures 3.5 inches and a 1x6 measures 5.5 inches, and feeding the nominal number into a spacing calculation throws every gap off by the same margin across the whole bay. The calculator works out how many pickets fit the bay at roughly the target gap, then distributes the leftover width evenly across every gap, including the two end gaps against the posts, rather than concentrating the error in whichever gap happens to fall last.

A fence that visibly tapers from a wide gap on one end to a tight sliver against the post on the other almost always traces back to measuring gap from picket center rather than from the clear opening between adjacent board edges, or to guessing a round gap number and letting the error accumulate across a long bay instead of solving for the exact gap the bay actually divides into. On an 8 foot, 96 inch section using 3.5 inch pickets and a roughly 2 inch target gap, the exact math works out to 17 pickets at a gap of 2.03 inches each, a number that is close to but not exactly 2 inches, which is the whole point: solving for the exact value that divides the bay evenly, rather than rounding to a convenient fraction and letting the discrepancy land wherever it falls. Most builders also leave a half or full gap between the post and the first picket rather than butting the first board tight against the post, for symmetry across the finished run.

Getting the calculated gap to hold on site, rather than drifting board by board down the fence line, is a matter of cutting a single spacer block to the exact calculated dimension and using it as a jig between every picket as boards go up, rather than relying on a tape measure and a steady eye for 17 or more repetitions. Material matters for how stable that gap stays after installation: treated pine pickets installed wet continue to shrink across their width as they dry out over the following months, widening every gap by an eighth of an inch or more by the time the wood reaches equilibrium moisture, while cedar and vinyl pickets stay far more dimensionally stable and hold close to their installed spacing. That shrinkage is one reason it is worth being honest with a target gap up front rather than cutting it as tight as code and preference allow, since a gap that starts tight on wet pine can end up uncomfortably wide after a summer of drying.

Gap targets are not purely cosmetic. Two to 2.5 inches is the common gap for a front yard picket fence, close enough to block casual sightlines and slow pets without blocking airflow, but a fence around a swimming pool is a different case entirely: pool safety codes require clear picket spacing not to exceed 4 inches anywhere along the barrier, specifically so a 4 inch test sphere cannot pass through, and that constraint should be checked against local code before finalizing a gap on a pool enclosure. Shadowbox fences add another variable, alternating pickets on the front and back faces of the rails with a deliberate 1 to 2 inch overlap between them to block sightlines without needing a true solid gap on either face, which the shadowbox fence calculator handles directly. Once spacing is set, the fence picket calculator and the picket fence calculator extend the same bay-by-bay math into a full picket and rail order across the whole run, and the privacy fence calculator applies the same board-and-gap logic to a tighter, privacy-oriented spacing target. For the research behind common gap choices and code minimums, see how many pickets for 100 feet of fence, board spacing guide for wood picket fences and board on board vs shadowbox vs solid privacy fence, and for full installation sequencing see the wood fence installation guide and the fence materials guide. The rest of the measurement tools are collected on the measurement calculators hub. It is worth double-checking the clear bay width before cutting the spacer block, since a post that is slightly out of plumb or a bay measured at the face of one post and the center of the other will throw off the exact gap this calculator solves for, and that kind of small input error is far easier to catch on paper than after a dozen pickets are already nailed up with the wrong spacer.