Deck board calculator

Face screws are counted at two per joist, and the calculator warns you when the boards come up short once they overhang the frame. New to this? Words explained.

How the count works

Each row covers one board width plus one gap. Dividing the deck width by that and rounding up gives the rows. Each row is the deck length plus the overhang at both ends — boards usually run about an inch past the rim or fascia — and rows × row length is the linear footage, which is then split into the board length you buy. Joists are counted from one end of the deck to the other at your spacing, plus one — the spacing is “on center”, measured from the middle of one joist to the middle of the next.

rows = (deck width + gap) ÷ (board width + gap), rounded up
row length = deck length + 2 × end overhang
linear length = rows × row length
boards = rows ÷ (rows per board) × (1 + waste)
joists = deck length ÷ joist spacing, rounded up, + 1
screws = rows × joists × 2

Example: a 16 × 12 ft deck with 5½ in boards and a 3/16 in gap needs 26 rows. With a 1 in overhang at each end every row is 16 ft 2 in, so 16 ft boards are too short — buy 18 ft boards (26, or 29 with 10% waste) or plan butt joints over doubled joists. At 16 in joist spacing there are 13 joists, so about 676 face screws — the numbers behind How many deck boards for a 12x16 deck. A square deck works the same way: see How many deck boards for a 12x12 deck.

Picture-frame border

A picture-frame border is one board run around all four edges, mitered at the corners, with the field boards running between the two end border boards. Pick it under More options. On the 16 × 12 ft deck above, the field rows shrink to 15 ft 2-5/8 in, so 16 ft boards fit them — but the two long border boards are still 16 ft 2 in and need 18 ft stock or a joint. The end border boards sit along the end joists, so plan blocking or a doubled rim under them to fasten into.

Choosing board length

• Boards that run the full deck length in one piece look cleanest and avoid butt joints. Remember the overhang: a deck framed at exactly 12 or 16 ft needs boards a little longer than that.
• If the deck is longer than your longest board, plan the joints so they land on joists and stagger them across rows — or use a picture-frame border with a breaker board.
• Many lumberyards sell even lengths (8 to 20 ft). A 12 ft deck from 16 ft boards wastes nearly 4 ft per row; buy the shortest length that still covers a row with its overhangs.
• Metric lengths such as 3.6 m and 4.8 m are a few inches shorter than 12 and 16 ft, so they cannot run a 12 or 16 ft deck in one piece. The board list includes 3.66 m (12 ft) and 4.88 m (16 ft) for imperial stock.

Gaps, joist spacing and fasteners

Wood decking is often installed tight when wet (pressure-treated) and opens up as it dries; kiln-dried and composite boards are gapped at install, typically ⅛–¼ in. Follow the board manufacturer’s spacing — composite makers also specify end gaps for thermal expansion.

Joist spacing is limited by the decking. Under IRC Table R507.7, 5/4 (1-1/4 in) wood boards laid square to the joists need joists 16 in on center or closer when each board crosses three or more joists (12 in if a board only spans two), and 2 in thick boards allow 24 in. Diagonal decking needs closer spacing; composite decking follows its label. The code calls for at least two 8d threaded nails or two No. 8 wood screws at each joist — this calculator counts two face screws per joist crossing, set about ¾ in from each edge; pre-drill near board ends to avoid splitting.

Screening the deck from the neighbours? Privacy-fence pickets count the same way as deck rows — width plus gap into the run — as in How many pickets for a 100 ft privacy fence.