A deck's decking is counted in rows, its lumber is bought in linear feet, and its frame is spaced in inches on-centre — three different languages for one platform. This calculator translates between them using one stated model, shown right on the page so there are no surprises: boards run parallel to the deck's length, laid side by side to cover the width, and joists run across the width, spaced 16 inches apart along the length.
Counting the boards
Each row of decking covers the board's width plus one gap — the gap is not wasted space, it is drainage and room for the wood to move, and it is part of every board's footprint. A 5.5 in board with a ⅛ in gap covers 5.625 in of width per row, so a 16 ft (192 in) wide deck needs 192 ÷ 5.625 = 34.1, rounded up to 35 rows. Each row is one board the length of the deck — 12 ft in the example — so the order is 35 boards, or 420 linear feet of decking. The ceiling rounding is why the last board is often ripped narrower on a real deck, or the gaps are breathed out slightly across the whole width to absorb the fraction.
Counting the joists
Joists are the frame the boards screw to, and 16 in on-centre is the standard spacing for residential decking (12 in for diagonal layouts and many composite boards, which flex more). Along a 12 ft length, joists sit at 0, 16, 32 … 144 in — that is 144 ÷ 16 + 1 = 10 joists, the “+1” being the joist at the very start that spacing arithmetic always forgets. Each joist spans the deck's width. Rim joists, beams, posts, footings and hardware are a structural conversation of their own — this calculator counts decking and joist repetitions, and your local code and span tables decide their sizes.
What the model leaves out, honestly
Picture-frame borders, diagonal inlays, stair treads and the boards a pattern consumes are not in this count — picture framing typically adds a perimeter's worth of boards plus the blocking beneath it. Waste is not added either: add 10% to the linear feet for cuts and defects (15% for diagonal decking), and remember that if the deck is longer than your yard's longest board, rows become two pieces with a butt joint over a joist — the linear feet do not change, but the piece count doubles for those rows and the waste rises.
Worked example
The defaults are a 12 × 16 ft deck with standard 5.5 in boards and a ⅛ in gap: rows = 192 ÷ 5.625 = 34.1, rounded up to 35 boards, each 12 ft long = 420 linear feet of decking for the 192 ft² surface, over 10 joists spaced 16 in apart along the length. Add 10% for cuts and you would order about 460 linear feet.
How to use this tool
- Enter the deck length — the direction the boards will run — and the width they must cover.
- Enter your board's actual width in inches (5.5 in for a standard 2 × 6 or most composite boards).
- Set the gap: ⅛ in (0.125) is typical for dry composite and PVC; fresh pressure-treated wood is often laid tighter because it shrinks.
- Read the board count and linear feet, then add 10–15% for waste before ordering.
- Note the joist count at 16 in on-centre for your framing list; check span tables and local code for joist sizes.
Screws, clips and the forgotten consumables
Decking counts boards; the fasteners that hold them are counted in hundreds. A rough planning figure for face-screwed decking is two screws at every board–joist crossing: the example deck has 35 boards crossing 10 joists, which is 700 screws before a single spare — buy the bulk tub. Hidden-clip systems are counted by coverage instead, typically sold in boxes rated by square feet at a stated joist spacing; the deck area and joist spacing at the top of this page are exactly the two numbers those box ratings ask for.