Sizu Home › Calculators › Pickets, Gaps and Boards
Opens on the book's own fence. Change any number to your own and press Calculate.
One bay, a gap at both ends
The gate: flush edges, no gap at the ends
The order and the cutting list
Butted boards on a long run
Enter the bay and press Calculate.
The formula
- Clear span = post spacing − post width
- Equal gaps with a gap at both ends: gap = (span − n × w) ÷ (n + 1)
- Count for a target gap: n = (span − gap) ÷ (w + gap), rounded up so the gap is no bigger than you wanted
- Flush-edge gate: gap = (W − n × w) ÷ (n − 1). Butted boards: count = run ÷ board width
The book's worked example, Chapter 5. Route A's bay is 2,400 − 89 = 2,311 mm clear. 15 pickets of 89 mm fill 1,335 mm, leaving 976 mm for 16 gaps: 61 mm each. The gate's 7 pickets have flush edges, so 6 gaps of 59.5 mm. 52 pickets, 55 with 5 %, two from each 8 ft board: 28 boards. Route B butts 52 boards of 140 mm along 7,289 mm.
Wet treated boards shrink as they dry, about 6 mm across a 140 mm board, so butted boards open small gaps on their own. Mark the gaps on a story pole and step it along the rail. Prices are illustrative US home-center prices from October 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.