Craft & Design
Joist Spacing & Count
Joist count for a deck or floor at 12, 16, 19.2, or 24 in on center — plus decking board count.
Joist Spacing & Count
Inputs
Direction joists run
Direction joists cross
Results
Joist count
16 in on center
9
Total linear feet of joist
108.0 ft
Decking boards (5/4×6)
Effective 5.625 in (5.5 + 1/8 gap)
26
How to use this
Joists run parallel to the span-length direction and are spaced out across the span-width. For a 12 × 10 ft deck with joists running the 12 ft direction at 16 in on center, you need 9 joists (8 bays + 1) spanning 12 ft each.
Decking boards run perpendicular to the joists. A nominal 5/4×6 deck board is 5.5 in wide; leave a 1/8 in gap for drainage, so each board covers 5.625 in of span-length. Round up — you can’t buy half a board.
Formula
bays = ceil(span_width_in ÷ spacing_in)
joists = bays + 1 (includes both end joists)
linear_ft = joists × span_length_ft
deck_boards = ceil(span_length_in ÷ 5.625)
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Formula
joists = ceil(span_width_in ÷ spacing_in) + 1; deck_boards = ceil(span_length_in ÷ 5.625)