Decking calculator: boards, joists and fasteners to buy
Twenty-six rows of 5 1/2 inch boards with 1/8 inch gaps cover a 16 by 12 ft deck, 416 linear feet. That is 29 sixteen-foot boards with 10% extra. They rest on 13 joists at 16 inches on center and are held down with about 700 screws.
Rows, boards and the gap between them
Boards run along the deck's length and sit side by side across its width, so the first number is rows. That is the width in inches plus one gap, divided by the board face width plus the gap, rounded up. The last row rarely lands exactly, and the calculator tells you how wide the final board rips to. If that is a sliver, widen the gaps a hair or shift the first board out over the rim joist. Rows times length gives linear feet. The waste percentage is added, and the total is divided by the stock length. The rule is that every row needs at least enough whole boards to reach the end. Joists run across the boards, so they are counted along the length: spans of 16 inches plus one. Fasteners are rows times joists times two.
Rows = (width in + gap) ÷ (board width + gap), rounded up. Linear feet = rows × length. Boards = linear feet × (1 + waste) ÷ stock length. Joists = length in ÷ spacing, rounded up, + 1. Screws = rows × joists × 2.
Boards per 100 square feet by board width and length
| Board face width | Linear feet per 100 sq ft (1/8 in gap) | 12 ft boards | 16 ft boards | 20 ft boards |
|---|---|---|---|---|
| 3 1/2 in (2x4) | 331 | 28 | 21 | 17 |
| 5 1/2 in (5/4x6, 2x6, composite 1x6) | 213 | 18 | 14 | 11 |
| 7 1/4 in (2x8) | 163 | 14 | 11 | 9 |
Board counts are before waste. Add 10% for a straight layout and 15% for a diagonal or a picture-frame border.
A 20 by 14 ft composite deck, board by board
Boards will run the 20 ft way, so the 14 ft width is across the boards. In 5 1/2 in composite with the clip's 3/16 in gap, that is 168.19 ÷ 5.6875 = 29.6, so 30 rows. The last board rips to 3 in. A clip gap is fixed, so plan the rip for the house side where it is least seen, or trim the deck width by an inch. Thirty rows at 20 ft is 600 linear feet, or 660 with 10% extra, which is 33 boards of 20 ft.
The joists are counted the 20 ft way: 240 ÷ 16 = 15 spans, so 16 joists. They are each 14 ft long, 224 linear feet of 2x8 or whatever the span table calls for. Hidden clips: 30 rows times 16 joists is 480 clips. Clip boxes are sold by the square foot of deck. Buy the box size above the deck's square feet, plus the starter set the maker lists.
Measuring and planning the layout
Measure the framed deck, not the drawing, and measure the width at both ends. Say a deck is 12 ft 0 in at the house and 12 ft 1 in at the rim. It needs the extra inch in the board count. Decide which way the boards run before counting: running them parallel to the house is usual, and the joists then run away from the house. Count any picture-frame border as its own run of boards around the perimeter and shrink the field by two board widths. Stairs and a railing cap are separate lists. The stair treads are short boards cut from the same stock, two per tread, and the linear feet calculator handles the cap rail.
Composite and wood spacing rules
Composite boards are rated by their maker for a maximum joist spacing. Trex and TimberTech both put their standard boards at 16 inches on center for straight runs. Trex requires 12 inches for diagonals and allows 24 inches only for its 2x6 board; TimberTech allows 24 only for its thicker MAX boards. Wood decking follows the span table for its thickness in your code. The American Wood Council's guide has 2 inch nominal boards fastened to every joist with two nails or screws and spaced about 1/8 inch. The starter row and a picture frame take their own fasteners. None of this replaces the span table for the joists themselves, which comes from the deck construction guide or your local code.
Questions about deck boards and framing
How many deck boards do I need?
Divide the deck width (across the boards) by the board width plus the gap, and round up for the rows. Then multiply by the deck length for linear feet, and divide by the board length you will buy. A 12 ft wide deck in 5 1/2 in boards with 1/8 in gaps is 144 ÷ 5.625 = 25.6, so 26 rows. At 16 ft long that is 416 linear feet, 26 boards of 16 ft before waste and 29 with 10% extra.
How far apart should deck joists be?
Trex says composite decking joists should be no more than 16 inches on center for a straight layout and 12 inches for a diagonal one. TimberTech says never more than 16 inches except for its MAX boards. For wood decking the spacing comes from the span table for the board's thickness in your code. The American Wood Council's deck guide covers 2 inch nominal boards fastened to every joist. The calculator's 16 inch default matches the composite makers' maximum. Joist size and span themselves are a structural matter for the deck guide or your inspector, not the board count.
What gap should I leave between deck boards?
The American Wood Council's deck guide says to space wood decking about 1/8 inch apart. TimberTech notes that freshly treated wood is wet and is often laid tight to shrink into its own gap. Trex requires at least 3/16 inch between composite boards and never more than 1/2 inch, with its hidden fasteners setting the gap for you.
How many screws do I need for a deck?
Two fasteners per board at every joist, which is the American Wood Council's rule (two 8d threaded nails or two #8 screws). Multiply rows by joists by two: 26 rows on 13 joists is 676 screws, and the list adds 5% for the ones that strip. Hidden fasteners take one clip per board per joist, sold in boxes sized by square feet of deck.
How much decking do I get from a 5/4x6 board?
A 5/4x6 is 5 1/2 inches wide. So each linear foot covers 0.46 square feet before the gap, or about 0.47 with a 1/8 inch gap. A 16 ft board covers about 7.5 square feet. So 100 square feet of deck takes about 13.3 boards of 16 ft, 14 bought, plus waste.
Should I buy 12, 16 or 20 ft boards?
Buy the length that runs the full deck without a joint when you can. A butt joint has to land on a joist and wastes the offcut. For a 14 ft deck, 16 ft boards lose 2 ft each. For two 12 ft runs, a 24 ft board cut in half has no waste, if the yard stocks it. Another choice is to run 12 ft boards across a 12 ft deck. The calculator counts whole boards per row, so change the stock length and compare.
How much does a deck cost?
The 2025 Cost vs Value report from Zonda gives national averages. It puts a professionally built deck addition at $18,263 to $25,096, wood to composite, recouping 94.9% and 88.5% at resale. Material for the boards alone, the part this calculator prices from the per-board or per-square-foot price you type in, is a minority of that. Framing, footings, railing, stairs and labor make up the rest. Prices vary by region and by whether the deck is on the ground or a second story, so get local quotes.
6 source documents
- American Wood Council, DCA 6, Prescriptive Residential Wood Deck Construction Guide, based on the 2015 IRC (decking spaced about 1/8 in, two fasteners per board at each joist). Read 2 November 2026.
- NIST and the American Lumber Standard Committee, Voluntary Product Standard PS 20-20, American Softwood Lumber Standard, Table 3 (5/4x6 and 2x6 dressed to 5 1/2 in). Read 2 November 2026.
- Trex Company, 2024 Decking Installation Guide (board sizes, 3/16 in minimum gap, 1/2 in maximum). Read 2 November 2026.
- Trex Company, How to Space and Install Deck Joists (16 in on center, 12 in for diagonal decking). Read 2 November 2026.
- TimberTech (AZEK), Deck Joist Spacing and Blocking; Deck Board Spacing (16 in maximum; wood laid about 1/8 in apart). Read 2 November 2026.
- Zonda (Remodeling magazine), 2025 Cost vs. Value Report, wood and composite deck addition, national job cost and resale value, posted 18 September 2025. Read 2 November 2026.