Divide the deck’s width (in inches) by the board width plus the gap to get the number of rows, then work out how many boards each row needs. A 14 × 12 ft deck with 5.5 in boards and 3/16 in gaps has 26 rows; each 14 ft row is one 16 ft board, so you need 26 boards, 28 with 5% extra.
Why count rows instead of dividing the area?
Dividing deck area by board area gives a number, but it ignores how boards are actually cut. Every row must be covered end to end, and a 2 ft offcut from a 16 ft board is often too short to use anywhere. Counting rows and planning each row’s boards gives a number you can actually build with.
Step by step
- Rows. Width in inches ÷ (board width + gap), rounded up. For a 12 ft (144 in) deck: 144 ÷ (5.5 + 0.1875) = 25.3, so 26 rows.
- Boards per row. If a board covers the full length, it’s one board per row. If not, the row needs a joint, and the joint must land on a joist.
- Reuse offcuts. A long offcut from one row can finish another. A 16 ft board cut to 12 ft leaves 4 ft, which can be the short piece in a row that needs one.
- Add extra. About 5% for warped or split boards and a miscut. More for diagonal decking.
The deck calculator does this row-by-row planning for you, reusing offcuts of 2 ft or longer, and adds joists and screws.
Rows by board width
Board width and gap change the row count. For a 12 ft wide deck:
| Board and gap | Rows | Linear feet on a 12 × 12 ft deck |
|---|---|---|
| 5.5 in ("6 in" board), 3/16 in gap | 26 | 312 |
| 5.5 in, 1/8 in gap | 26 | 312 |
| 3.5 in ("4 in" board), 1/8 in gap | 40 | 480 |
Which board length wastes least?
For a deck 12 ft wide (26 rows of 5.5 in boards), here is how many boards each length takes before the extra allowance, and how much material is wasted. The best choice for each deck length is in bold.
| Deck length | 8 ft boards | 10 ft boards | 12 ft boards | 16 ft boards | 20 ft boards |
|---|---|---|---|---|---|
| 10 ft | 33 (2%) | 26 (0%) | 26 (20%) | 26 (60%) | 13 (0%) |
| 12 ft | 39 (0%) | 32 (3%) | 26 (0%) | 26 (33%) | 26 (67%) |
| 14 ft | 52 (14%) | 39 (7%) | 31 (2%) | 26 (14%) | 26 (43%) |
| 16 ft | 52 (0%) | 52 (25%) | 35 (1%) | 26 (0%) | 26 (25%) |
| 18 ft | 59 (1%) | 52 (11%) | 39 (0%) | 30 (3%) | 26 (11%) |
| 20 ft | 65 (0%) | 52 (0%) | 52 (20%) | 33 (2%) | 26 (0%) |
| 24 ft | 78 (0%) | 65 (4%) | 52 (0%) | 39 (0%) | 32 (3%) |
Board lengths that divide evenly into the deck length waste almost nothing: 20 ft boards cut in half for a 10 ft deck, 8, 12 or 16 ft boards for a 24 ft deck. A 14 ft deck is the awkward one. 16 ft boards give one clean, joint-free board per row but cut 2 ft off every board, about 14% waste. 12 ft boards waste only about 2%, but every row then has a joint with a short 2 ft piece, which looks busy and needs extra blocking. Many builders choose the 16 footers anyway. Lowest waste isn’t always the best-looking deck.
Plan the joints
Butt joints where two boards meet must fall on a joist, and many builders add a second joist or blocking there so each board end gets full bearing. Staggering joints across rows looks better than lining them up. Some decks use a “picture frame” border or a divider board instead, so boards run full length between borders.
Don’t forget
- Fascia and stair treads. Rim joist fascia and stair treads are extra boards on top of the decking count.
- Fasteners. Two screws per board at every joist. A 14 × 12 ft deck with joists 16 in apart has 11 joists and 26 rows: about 572 screws.
- Hidden fastener clips for grooved composite boards replace face screws; count clips per the maker’s instructions.
- Gap. Wood decking shrinks as it dries, so many builders install pressure-treated boards tight and let gaps open. Composite boards need the gap the manufacturer specifies.