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Free deck board spacing calculator for uniform drainage gaps. Enter your clear inside span, measured deck board face width, and gap preferences to auto-pick a board count—or lock a count and read the resulting gap before you chalk lines or set hidden fasteners.
Last updated: April 17, 2026
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Inside frame-to-frame along the joists (where board faces span). Equivalent: 12 ft clear.
Auto mode picks an integer board count whose uniform gap is closest to your target while respecting min and max gap.
Uniform gap between boards
0.271 in
≈ 9/32 in (nearest 1/32 in)
Boards across span: 25 (auto)
Max boards still honoring min gap: 25
Uses clear inside-to-inside span along the joist run and the actual face width of dry boards (not nominal lumber labels). Wet boards shrink—field-verify gaps after acclimation and follow local codes plus manufacturer guidance for hidden fasteners.
Joist run
Inside line
Per board
Wood or composite
Min / target / max
Search & fit
Integer boards
Across the deck
Layout math
g = (W − N·B)/(N−1)
Reality
Tape wins theory
12 ft clear span, 5.5 in face, ⅛–½ in window, ¼ in target (defaults):
0.271 in
25 boards · 9/32 in
For N deck boards with a uniform gap g between neighbors, the clear span W satisfies W = N·B + (N−1)·g where B is actual face width. Solving for gap gives g = (W − N·B)/(N−1) whenever N ≥ 2. The auto mode scans every feasible integer N that keeps g inside your minimum and maximum gap, then chooses the layout whose g is closest to your target gap—ideal for drainage without unsightly wide slots. When you already own N boards, the same formula reports the gap you will see on site.
W = N·B + (N−1)·gg = (W − N·B) / (N−1)N_max = ⌊(W + g_min) / (B + g_min)⌋N_max estimates how many boards can fit if every gap were at least your minimum—helpful sanity-check before you optimize toward the target gap.
After boards are set, estimate lumber volume with our woodworking board foot calculator.
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