What this calculator does
Find out how many deck boards, joists and screws to buy for a rectangular deck. A deck surface is a set of parallel rows of boards with a small gap between them, resting on joists set at a regular spacing. First you find how many rows fit across the deck and how many boards each row takes. The rest of the list follows from that: the joists underneath, and the fasteners that tie every board to every joist.
This calculator works on a rectangular deck with boards running along its length. It counts the rows from the board width and gap, and the boards per row from the stock length you buy. Then it adds a small allowance for defective boards. It also counts joists at 12, 16 or 24 in on center, and estimates face screws or hidden clips. On center means measured from the middle of one joist to the middle of the next. These are planning estimates only. Framing sizes, spans, footings and ledger attachment are governed by local building codes and must be checked before you build.
Where it's used
Why this math matters outside the classroom:
- Board rows. A 12 ft wide deck of 5.5 in boards with 1/8 in gaps needs 144.125 ÷ 5.625 ≈ 25.6, rounded up to 26 rows.
- Joist count. Joists at 16 in on center along a 16 ft deck need 192 ÷ 16 + 1 = 13 joists.
- Board lengths. Builders compare 12 ft and 16 ft boards to see which length covers a run with fewer pieces and offcuts.
- Hidden clips. Deck builders count one hidden clip or pair of screws for each place a board crosses a joist.
- Budgeting. Homeowners multiply the board count by the price per board to compare composite and wood decking for the same deck.
How to use it
- Enter the deck length (the direction the boards run) and width in feet.
- Enter the actual board width, the gap you will leave and the length of boards you plan to buy.
- Choose the joist spacing and the type of fastening, and set an allowance for defective boards.
- Read the number of boards, the linear feet, the joists and the fastener count.
What the inputs mean
- Board width (actual)
- Deck boards are sold by their label size, such as 2×6, but measured by their actual size. Wood 5/4×6 and 2×6 boards, and most composite boards, are about 5.5 in wide.
- Gap between boards
- Wet pressure-treated wood shrinks as it dries and is often laid tight or nearly so; kiln-dried wood and composites need the gap their maker specifies for drainage and expansion.
- Board length
- Choose a length that matches the deck, or divides it evenly, to cut waste. A 14 ft deck built from 16 ft boards wastes 2 ft per row; the calculator counts the whole board.
- Joist spacing (on center)
- 16 in on center is common for straight-laid wood decking. Diagonal layouts and many composite boards call for 12 in. Check the board maker's maximum span and your local code.
- Extra boards for defects
- Offcuts at row ends are already counted; this covers warped, split or rejected boards.
Formula and method
rows = (deck width in + gap) ÷ (board width + gap), rounded up boards per row = deck length ÷ board length, rounded up boards = rows × boards per row × (1 + extra % ÷ 100), rounded up joists = deck length in ÷ spacing, rounded up, + 1 screws = rows × joists × 2; hidden clips = rows × joists
A row of n boards has n − 1 gaps between them, so n boards span n × (board + gap) − gap. Adding one gap to the width before dividing accounts for that. The last row may need ripping to width.
Joists run across the boards. Dividing the deck length by the spacing gives the number of spaces, and one more joist closes the far end. Each board crosses each joist once and gets two face screws or one clip there.
Worked example
A 16 × 12 ft deck with 5.5 in boards, a 3/16 in (0.1875 in) gap, 16 ft boards, joists at 16 in and face screws, with 5% extra boards.
- Rows: (144 + 0.1875) ÷ (5.5 + 0.1875) = 25.35, rounded up to 26.
- Boards per row: 16 ÷ 16 = 1, so 26 boards; with 5%: 27.3, rounded up to 28.
- Linear feet on the deck: 26 × 16 = 416 ft.
- Joists: 192 ÷ 16 = 12 spaces, plus 1 = 13 joists, each about 12 ft.
- Screws: 26 × 13 × 2 = 676.
Result: Buy 28 boards of 16 ft, 13 joists and about 676 deck screws.
Understanding the result
The board count is what to order. The purchased linear feet will exceed the deck's linear feet by the row-end offcuts and the defect allowance.
Screws are usually sold by count or by box size, so round the fastener figure up to the next box and keep a handful spare.
Assumptions and limitations
- The deck is a rectangle and boards run parallel to its length.
- Joists run across the boards, the full width of the deck, at a uniform spacing.
- Offcuts are not reused in other rows; joints fall over joists.
- Beams, posts, footings, ledger boards, rim joists, blocking, stairs and railings are not counted.
- Joist size and span are not checked; consult span tables and your local building department.
- Picture-frame borders, diagonal layouts and breaker boards change the counts and need a layout plan.
Frequently asked questions
How many deck boards do I need per square foot?
With 5.5 in boards and a 3/16 in gap, each board covers about 0.474 ft of deck width, so one square foot of deck takes about 2.1 linear feet of board. The row method in this calculator is more accurate because it accounts for whole boards.
Should I use 12 or 16 inch joist spacing?
16 in on center is the usual choice for wood boards laid straight. Diagonal decking and many composite or thinner boards need 12 in. The board maker's instructions and your local code have the final word.
How many screws per deck board?
Two per joist crossing is the common pattern for face screws, one near each edge. A 16 ft board over joists at 16 in crosses 13 joists, so it takes 26 screws.
Do hidden fasteners need fewer pieces?
Typically one clip per crossing replaces the two screws, though starter and ending clips at the first and last rows often require face screws or a special clip. Check the system's instructions for its exact count.
How this calculator is checked
Method: Row count from board width plus gap, ceiling division by stock length, joists at a fixed on-center spacing plus one, and fasteners per board-joist crossing. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
Embed this calculator
Put this Decking Calculator on your own website or blog for free. Paste this code where you want it to appear: