About Decking Board & Fastener Calculator
The decking quantity calculator turns a deck footprint into a shopping list: how many rows of boards it takes to cover the width, the total lineal footage including a waste allowance, the number of stock boards to buy at your chosen length, and the screws or hidden clips needed to fasten them down. Enter the dimension the boards run across (the row-stacking direction), the dimension along the board run, the actual board face width — 5.5 in for nominal 6-in and 3.5 in for nominal 4-in boards, or any custom width — and the side gap, typically 1/4 in per the AWC DCA 6 deck guide and composite manufacturer instructions.
Each row is one board pitch (width plus gap) of coverage, so the row count is the deck width divided by the pitch, rounded up. Straight layouts carry a 10% lineal waste allowance for end cuts and culls; a 45-degree diagonal layout wastes more at the angled cuts, so the trade allowance rises to 15%. Fasteners come from the joist crossings: a row crosses one joist per spacing interval — rounded up, because the board ends always land on a rim or end joist — plus the starting joist, with two face screws per crossing, or one hidden clip per crossing plus a starter clip per row. The tool reports the totals with whole-box counts for common 100, 350, and 1750-count fastener boxes, and charts the board count at every stock length so you can pick the least-cut option.
How It Works
- Enter the deck width (the direction the rows of boards stack up, perpendicular to the board length) and the deck length (the direction the boards run) in feet.
- Pick a board width preset — 5.5 in actual for nominal 6-in boards or 3.5 in for nominal 4-in — or enter a custom face width, and set the side gap (default 1/4 in, up to 1/2 in).
- Choose the stock board length you plan to buy (8, 12, 16, or 20 ft) and the layout: straight adds 10% lineal waste, a 45-degree diagonal adds 15%.
- Set the joist spacing (12, 16, or 24 in on center) and the fastening system: face screws count two per board-joist crossing; hidden clips count one per crossing plus one starter clip per row.
- The tool reports rows, lineal footage, boards to buy, deck area, and the fastener total with whole-box counts, plus a chart of the board count at every stock length at your current settings.
Worked Example
Take the 12 × 16 row from the table below: boards run the 16-ft direction, 5.5 in composite boards with a 1/4 in gap, straight layout, 16-ft stock, joists 16 in on center, face-screwed. The row pitch is 5.5 + 0.25 = 5.75 in, so covering 12 ft (144 in) of width takes ceil(144 / 5.75) = 26 rows. The lineal footage is 26 × 16 ft × 1.10 = 457.6 ft with the 10% straight-layout waste, and the buy decision follows: ceil(457.6 / 16) = 29 sixteen-foot boards. Each 16-ft row crosses ceil(192 / 16) + 1 = 13 joists, so screwing takes 26 × 13 × 2 = 676 screws — buy two 350-count boxes. Rotating the same deck to a 45° diagonal bumps the waste to 15%: 478.4 lineal ft and a 30th board, while hidden clips instead of screws would take 26 × (13 + 1) = 364 clips.
How many deck boards and screws do I need?
Board and fastener counts for the deck sizes people most often build, all with 5.5 in (nominal 6-in) boards at the default 1/4 in gap — a 5.75 in row pitch — bought as 16-ft stock over joists 16 in on center, face-screwed at two per crossing. Straight rows carry the 10% trade waste allowance on the lineal footage; the diagonal comparison row carries 15%. Board counts are the tool’s own round-up, so the 30th board on the diagonal deck is real.
| Deck | Board rows | 16-ft boards | Lineal ft (incl. waste) | Deck screws |
|---|---|---|---|---|
| 10 × 12 deck, straight boards | 21 | 18 | 277.2 | 420 |
| 12 × 12 deck, straight boards | 26 | 22 | 343.2 | 520 |
| 12 × 16 deck, straight boards | 26 | 29 | 457.6 | 676 |
| Same 12 × 16 deck laid diagonal (15% waste) | 26 | 30 | 478.4 | 676 |
| 16 × 20 deck, straight boards | 34 | 47 | 748.0 | 1088 |
| 20 × 20 deck, straight boards | 42 | 58 | 924.0 | 1344 |
Formulas
- Rows across the deck width
pitch = w_board + gap ; rows = ceil(W x 12 / pitch)- Lineal footage and boards
lineal = rows x L x (1 + waste) ; boards = ceil(lineal / l_board)- Joist crossings per row
crossings = ceil(L x 12 / s_joist) + 1- Fastener counts and boxes
screws = rows x crossings x 2 ; clips = rows x (crossings + 1) ; boxes = ceil(fasteners / box)
Standards & References
- AWC DCA 6 -- Prescriptive Residential Wood Deck Construction Guide (decking gapping and fastening guidance)
- Composite decking manufacturer installation instructions -- gap and hidden-clip spacing
- Trade practice -- 10% straight / 15% diagonal lineal waste allowances
Frequently Asked Questions
How many deck boards do I need for a 10 × 12 deck?
With 5.5 in boards, a 1/4 in gap, and the boards running the 12-ft direction, the 10 ft of width takes 21 rows at the 5.75 in pitch. That is 21 × 12 ft × 1.10 = 277.2 lineal ft with the 10% straight-layout waste — 18 sixteen-foot boards — and over joists 16 in on center each row hits 10 joists, so face-screwing takes 21 × 10 × 2 = 420 screws.
Do diagonal deck boards really take more material?
Yes, on the lineal footage: the trade waste allowance rises from 10% to 15% because every diagonal board ends in an angled cut. On a 12 × 16 deck that is 478.4 lineal ft instead of 457.6, which tips the purchase from 29 to 30 sixteen-foot boards. The screw count stays at 676 because the tool counts crossings along the board-run direction as a purchasing estimate — but check the decking span rating, since a 45° layout often requires dropping the joists to 12 in on center.
How many deck boards do I need for a 12 x 16 deck?
With 5.5 in (nominal 6-in) boards, a 1/4 in gap, and the boards running the 16 ft direction, the row pitch is 5.75 in, so the 12 ft width takes 26 rows. At 10% straight-layout waste that is 26 x 16 x 1.10 = 457.6 lineal ft — 29 sixteen-foot boards. Buy shorter stock and the count rises (58 eight-footers) but so does the number of butt joints; the chart in the tool compares every stock length at once.
What gap should I leave between deck boards?
For wood decking the common recommendation, echoed by AWC DCA 6 guidance, is about 1/8 to 1/4 in between board edges for drainage and airflow — kiln-dried boards get the full gap, while wet pressure-treated lumber is often laid tight because it shrinks as it dries. Composite boards follow the manufacturer instructions, typically 3/16 to 1/4 in edge-to-edge. The calculator accepts any gap from 0 (tight-laid) to 1/2 in.
How much waste should I add for decking?
Trade practice is 10% extra lineal footage for a straight layout, covering end trims, checked or damaged boards, and cut-offs too short to reuse. A 45-degree diagonal layout wastes more because every board ends in an angled cut whose offcut rarely fits elsewhere, so the allowance rises to 15%. Complicated shapes with angles, wrapped borders, or picture framing can run higher still — the percentages here are the standard rectangular-deck values.
How many screws do I need per deck board?
Two screws at every joist the board crosses. A row crosses ceil(run in inches / joist spacing) + 1 joists — rounded up because the board ends land on a rim or end joist even when the run is not an exact multiple of the spacing — so a 16 ft board over joists 16 in on center hits 13 joists and takes 26 screws. For a whole deck, multiply by the number of rows: the 12 x 16 example needs 26 rows x 13 crossings x 2 = 676 screws, which is two 350-count boxes. Screws should penetrate the joist by at least 1 in; use stainless or coated deck screws rated for treated lumber.
How are hidden fastener clips counted?
Hidden clip systems put one clip on each joist between adjacent boards — the single clip engages the grooved edges of both boards — so a row needs one clip per joist crossing, plus a starter clip (or starter screw) at the first board edge of each row. That gives rows x (crossings + 1): the 12 x 16 example takes 26 x (13 + 1) = 364 clips versus 676 face screws. Check the specific system, since some brands count starters differently at breaker boards and borders.
Does diagonal decking need closer joist spacing?
Often yes. Boards laid at 45 degrees span a longer diagonal distance between joists (the on-center spacing divided by cos 45, about 1.41x), so many wood and composite span ratings require dropping from 16 in to 12 in on-center joists for diagonal installation. This calculator counts crossings from the spacing you enter along the board-run direction as a purchasing estimate; confirm the structural spacing against the decking span rating and the joist sizing in our deck joist span calculator.