Deck Board Calculator
Count the deck boards a deck needs, in rows, linear feet and boards.
Last reviewed September 19, 2026
Result
Buy 29 boardsEstimate
Ten per cent is the decking default. It covers cuts at the ends of each run, the odd board rejected for a bad knot or a bow, and picture-frame borders, which waste more than a plain run.
- Rows of boards
- 26
- Total board length
- 416 ft
- Deck area
- 192 ft²
https://calcgallery.com/decking-fencing/deck-board-calculator/
Results are estimates for planning only. Actual needs vary with site conditions, products and workmanship. Always confirm quantities with your supplier and follow local building codes and manufacturer instructions. Read the full disclaimer
How to use it
- Enter the deck length, meaning the direction the boards will run.
- Enter the deck width, measured across the boards.
- Set the actual board width and the gap you want between boards.
- Enter the stock length you can buy.
- Read the number of boards, with the waste allowance already included.
How it is calculated
Deck boards are counted in rows across the deck, then converted to whole stock boards:
rows = ceil(width ÷ (board width + gap))
linear = rows × length
boards = ceil(linear × (1 + waste) ÷ stock length)The pitch is the board width plus one gap. At 5.5 in wide with a 1/8 in gap each row takes 5.625 in, so a 12 ft width needs 26 rows rather than the 27 you would get from the width alone.
Boards are then counted by total linear length rather than row by row. That assumes joins land on a joist and offcuts get used, which is true on a well-framed deck and is why the waste allowance is there.
Practical tips
Decide the board direction before you frame. Boards run across the joists. If you want the boards running the long way, the joists have to run the short way. Changing your mind after the frame is up means reframing.
Dry-lay the last few rows. The final row almost never lands full width. Check it early, and if it is going to finish as a 1 in sliver, open the gaps very slightly across the last several rows to absorb it.
Buy the longest stock that fits the run. A 16 ft deck built from 16 ft boards has no joins at all. Built from 12 ft boards it has a join in every row, each of which needs a joist under it and each of which is somewhere water can sit.
Sight every board before you fix it. Look down the length for bow and crown. Put the crowned edge up and fix the worst boards where they will be least seen, or cut them into the short pieces you need anyway.
Composite is less forgiving. Spacing, fixing method and support spacing are all set by the manufacturer, and getting them wrong voids the warranty. Read the instructions before ordering, not after.
With your numbers
- Rows = 12 ft across ÷ (5.5 in board + 0.125 in gap) = 26
- Board length = 26 rows × 16 ft = 416 ft
- Boards = that length plus 10% waste, ÷ 16 ft stock, rounded up to 29
Quick reference
Ten per cent is the decking default. It covers cuts at the ends of each run, the odd board rejected for a bad knot or a bow, and picture-frame borders, which waste more than a plain run.
| Job | Boards to buy |
|---|---|
| 12 × 12 ft deck | 29 |
| 16 × 12 ft deck | 29 |
| 20 × 12 ft deck | 29 |
| 16 × 16 ft deck | 39 |
| 10 × 10 ft deck | 25 |
| 5 × 4 m deck | 33 |
| 6 × 4 m deck | 31 |
Common questions
- How many deck boards do I need for a 16 by 12 foot deck?
- Twenty-nine 16 ft boards, if the boards run the 16 ft direction. That is 26 rows across the 12 ft width, 416 linear feet of board, plus a 10 per cent waste allowance.
- Does it matter which way the boards run?
- Yes, for the count and for the look. Running boards the long way needs fewer, longer rows. It also changes where joins fall and which way water runs off. The framing underneath has to suit: boards run across the joists, not along them.
- What gap should I leave between deck boards?
- About 1/8 in, or 3 mm, for kiln-dried timber, which swells when wet. Wet or green treated boards can be laid nearly tight because they shrink as they dry. Composite boards follow the manufacturer's spacing, which is usually wider and non-negotiable for the warranty.
- Why is the board width 5.5 inches and not 6?
- Nominal sizes are not actual sizes. A 5/4 by 6 deck board measures 5.5 in across. Using 6 in undercounts the rows by about one in twelve, which is two rows short on a 12 ft width.
- How much waste should I allow for decking?
- Ten per cent is the usual figure and it is the default here. Allow more for a diagonal layout, which wastes a lot at the ends of each run, or for a picture-frame border. Allow less only if every run is a single full-length board with no cuts.
- Does this count the joists and framing?
- No, only the deck boards. Framing depends on joist spacing, span and local code, and it is not something to size off a general calculator.