Counting deck boards is a two-step calculation: how many rows of boards cover the deck, and how many boards each row uses. Area-based estimates (square feet ÷ coverage per board) are close, but they ignore the gap between boards and the way board lengths divide into the deck width, so they can be off by several boards on a small deck. Here is the exact method the calculator uses.
Step 1: count the rows
Boards usually run parallel to the house, across the joists, so the rows stack up along the depth of the deck. Each row takes up one board width, and every row except the last also needs one gap:
rows = (depth + gap) ÷ (board width + gap), rounded up.
A standard 5/4×6 or 2×6 deck board is 5 1/2″ wide. With a 3/16″ gap the pitch is 5 11/16″ (5.6875″). For a 12 ft (144″) deep deck: (144 + 0.1875) ÷ 5.6875 = 25.35, so 26 rows. The last row will be ripped narrower or left to overhang the rim slightly. If you plan a 1″ overhang at the front, add it to the depth first.
Narrow 3 1/2″ boards (2×4 decking) need about 55% more rows: (144 + 0.1875) ÷ 3.6875 = 39.1, so 40 rows.
Step 2: boards per row
If the deck width is no longer than a board, each row is one board — and if the deck is less than half a board long, one board makes two rows. A 16 ft board covers two rows of an 8 ft deck. If the deck is wider than one board, each row needs two or more boards joined end to end, and every butt joint must land on a joist (some builders double the joist there). Stagger joints from row to row so they don't line up.
That makes the board length you buy important. On a 12 ft wide deck, 12 ft boards use exactly one board per row with no offcut, while 16 ft boards also need one per row but waste 4 ft each. On a 20 ft wide deck, two 10 ft boards or one 20 ft board per row both work, but 16 ft boards need two per row with 12 ft of offcut. The calculator lists every stock length and highlights the one that uses the least material.
| Deck | Rows | Best board length | Boards | Buy | Screws |
|---|---|---|---|---|---|
| 8 × 8 ft | 17 | 8′ | 17 | 19 | 250 |
| 8 × 10 ft | 17 | 10′ | 17 | 19 | 322 |
| 10 × 10 ft | 22 | 20′ | 11 | 12 | 416 |
| 10 × 12 ft | 22 | 12′ | 22 | 24 | 462 |
| 12 × 12 ft | 26 | 12′ | 26 | 29 | 546 |
| 12 × 16 ft | 26 | 16′ | 26 | 29 | 710 |
| 16 × 16 ft | 34 | 16′ | 34 | 37 | 929 |
| 10 × 20 ft | 22 | 20′ | 22 | 24 | 740 |
Step 3: gap size
The gap lets water drain and air circulate. Typical gaps:
- Kiln-dried wood and most composites: 1/8″ to 1/4″, or exactly what the manufacturer specifies — composite boards also need end gaps that depend on temperature.
- Wet pressure-treated lumber: often installed tight, because it shrinks as it dries. One IRC-based deck detail notes: if wet, place decking with no gap so that a 1/8-inch gap is created after drying (City of Alexandria, Typical Deck Details).
Over 26 rows, the difference between a 1/8″ and a 1/4″ gap is more than 3 inches — enough to change the count by a row.
Step 4: screws and clips
Face-screwed boards get two screws at every joist they cross. Each butt joint adds two more. For a 16 ft × 12 ft deck at 16″ on center there are 13 joists and 26 rows: 26 × 13 × 2 = 676 screws, plus 5% for drops and stripped heads. As a rule of thumb that's about 3.2 screws per square foot at 16″ o.c., or 2.1 at 24″ o.c. Hidden fastener systems use one clip per board per joist — half the number of screws — and come in boxes rated for a stated square footage.
Step 5: waste
Add about 10% for boards laid square to the joists, to cover defects, end trimming and offcuts that are too short to use. Diagonal (45°) decking produces much more offcut — allow around 15% and check the joist spacing: most decking needs closer joists when laid diagonally. Picture-frame borders and inlays add more again. If the board-length choice already wastes a lot (16 ft boards on a 12 ft deck), you may be able to reuse offcuts for stair treads or the border and lower the allowance.
A worked example
A 16 × 12 ft deck (16 ft along the house, 12 ft out) with 5 1/2″ boards and 3/16″ gaps: 26 rows. 16 ft boards cover a row each, so 26 boards; with 10% waste, 29 boards. Screws: 26 × 13 × 2 = 676, plus 5% = 710. If you used 12 ft boards instead you would need two per row (52 boards and 8 ft of offcut per row), so 16 ft is the clear choice.
Board count is only part of the deck. Joists, beams, posts and railings come next — see deck joist spacing and span, or run the full deck materials calculator.
Planning information only — framing and deck requirements vary by jurisdiction. Verify with your local building code or a licensed professional.