Deck Board Calculator

How to use the deck board calculator

1. Type the deck’s size: its length along the house and how far it reaches out. Measure the surface the boards will cover.
2. Pick your boards: 27 composite and PVC lines from Trex, TimberTech and Fiberon, each with the width and the lengths its maker prints, or wood 5/4×6, 2×6 or 2×4.
3. Which way they run: along the house (the usual way, with the joists running out from the house) or out from it.
4. Hidden clips or face screws, the joist spacing (the calculator starts at the most your board’s maker allows) and, if you want one, a picture-frame border on the three open sides.
5. You get how many boards of each length to buy, how every row is cut, the clips or screws, the joists and the fascia. Copy link keeps your deck; the green button opens the composite decking spacing calculator with your board, for the gaps and the end-to-end spacing on the day you lay them.

How the boards are counted

Most decking calculators divide the deck’s area by the area of one board. That gives a number of boards, but not which lengths, and it hides where the joints fall. This calculator lays the deck out row by row:

1. Rows: the distance across the boards, divided by one board’s width plus the side gap, rounded up. A 12 ft deck with 5.5-inch boards and 3/16-inch gaps is 144 ÷ 5.69 = 25.3, so 26 rows, and the last row is ripped to fit.
2. Each row’s lengths: a row longer than one board needs a joint, and the joint has to land on a joist. TimberTech’s guide says it plainly: “The ends of the boards must fall on a joist. Always double joists at butt seams.” So a board that ends mid-row counts only up to its last whole joist spacing. A 16 ft board over joists 16 inches apart reaches exactly 12 spacings; a 10 ft wood board over 16-inch joists reaches 7 spacings (112 inches) and loses 8 inches. The calculator tries every mix of the lengths your board is sold in and keeps the one that buys the fewest feet, then the fewest pieces.
3. Staggered joints: when a row is made of two different lengths, every other row lays them the other way round, so the joints do not line up across the deck. When all the joints fall on one joist line (two 12 ft boards on a 24 ft deck), the calculator says so: double that joist, or lay a breaker board across the deck there.

Board widths and lengths, maker by maker

A board sold as a “1×6” is about 5.5 inches wide. The width matters more than it looks: over a 12 ft deck, TimberTech’s 5.36-inch boards need one row more than Trex’s 5.5-inch ones.

How many hidden clips and screws

A hidden clip sits in the grooves between two boards, one at each joist. TimberTech’s guide: “Install one fastener and screw at each joist location.” So the clips are the joints between rows times the joists: a 16 × 12 ft deck with 26 rows and 13 joists takes 25 × 13 = 325 clips, plus a starter clip at each joist under the first row. At a butt joint each board end gets its own clip (TimberTech: “Each board end needs it’s own conceal clip. Two Boards at the butt joints cannot share the same clip.”), so every joint adds two. The last row, against the house, is face-screwed.

Face screws are two per joist in every board. TimberTech: “Fasten no closer than 3/4″ from the outside edge of each board,using two screws per joist including end joints.” For wood, the code asks the same: “Wood decking shall be attached to each supporting member with not less than two 8d threaded nails or two No. 8 wood screws.” A 16 × 12 ft deck with 26 rows over 13 joists is 26 × 13 × 2 = 676 screws.

Joist spacing for wood deck boards

For composite and PVC boards, the joist spacing is the board maker’s (the composite decking spacing calculator has every line). For wood, the IRC 2021’s Table R507.7 sets it: boards 1¼ inches thick (5/4 decking) over joists at most 16 inches apart, square to the joists and across three or more of them; 2-inch-thick boards (2×6, 2×4) at most 24 inches. Pick a closer spacing above and the joist count follows; pick a wider one than your board allows and the line turns red.

Worked examples

• 16 × 12 ft, Trex Enhance, along the house, hidden clips: 26 boards of 16 ft (one per row, no joints), 325 clips and 13 starter clips (4 boxes of 90), 13 joists at 16″, 4 fascia boards of 12 ft.
• 28 × 12 ft, the same boards: 26 × 16 ft + 26 × 12 ft. Each row is 16 + 12 ft, the joint on the 13th joist; every other row is 12 + 16 ft, so the joints alternate between two joists.
• 20 × 16 ft, TimberTech PRO Terrain: 36 rows of 20 ft boards (5.36″ wide), 560 clips, 4 boxes of 100 sq ft. TimberTech’s own quick rule (square feet ÷ 9.1 for 20 ft boards) gives the same 36.
• 22 × 12 ft, wood 5/4×6, face screws: 26 × 12 ft + 26 × 10 ft, 18 joists at 16″ and 988 screws. A 12 ft board reaches nine 16-inch spacings exactly; the 10 ft board finishes the row.

Common mistakes

• Counting by area only. 192 sq ft ÷ the area of a 16 ft board gives a number, but a 24 ft deck cannot be laid with 16 ft boards alone without a joint off the joists. Count rows, then lengths.
• A board exactly as long as the deck. The factory ends are trimmed square before the board goes down (TimberTech: “Always be sure to check and cut the factory cut ends of all boards to ensure they are square.”). If the deck is exactly 16 ft and the boards finish flush, measure again before you order 16 ft boards.
• Forgetting the last row. The deck is rarely a whole number of boards across. Plan the rip: a strip under 2 inches is fragile, so split the difference between the first and the last row.
• Single joists at the joints. Two board ends need room to be fastened. Double the joist under a butt joint, or use the maker’s butt-joint clip.
• No spares. TimberTech’s guide adds roughly 10% for scrap. The count here already includes this layout’s offcuts; a spare or two still saves a trip.

Deck boards: questions people ask

How many deck boards do I need?

Divide the distance across the boards, in inches, by one board’s width plus its gap, and round up: that is the number of rows. Then cut each row from the lengths the board is sold in, with every joint on a joist. A 16 × 12 ft deck in 1×6 composite boards running along the house is 26 rows of 16 ft boards.

How do I calculate decking for a 12 × 16 deck?

With the boards running along the 16 ft side, 26 boards of 16 ft for 5.5-inch boards with 3/16-inch gaps. With them running along the 12 ft side, 34 boards of 12 ft. The joists are 13 or 10 at 16 inches.

How many hidden clips per square foot?

About 1.75 to 2 at 16-inch joists: Trex prints 90 clips for 50 sq ft, Fiberon 175 for 100 sq ft. At 12-inch joists it is more (Fiberon: 210 for 100 sq ft). The calculator counts them from your rows and joists.

How many screws per deck board?

Two at each joist the board crosses: a 16 ft board over 16-inch joists crosses 13 joists, so 26 screws. TimberTech and the IRC both ask for two per joist.

What length deck boards should I buy?

The one that covers a row with the least left over, with any joint on a joist. Composite boards come in 12, 16 and 20 ft; 12, 16 and 20 ft boards all end on a joist at 12, 16 or 24-inch spacing. For a deck longer than 20 ft, a mix of two lengths with the joints alternating looks best.

How do I estimate Trex decking?

Pick your Trex line above: every 1×6 Trex board is 5.5 inches wide and comes in 12, 16 and 20 ft, with a 3/16-inch gap between boards. The calculator gives the boards, the Hideaway clips in boxes of 90 and the fascia.

Read next

• Low deck ideas at ground level
• Patio deck designs
• Deck railing designs
• Pool deck ideas
• Deck lighting ideas

Choosing the boards’ color? The free composite decking color picker shows them against your house. With the boards counted, the deck span calculator checks how far the joists and the beam can reach, the deck footing calculator sizes the footings, and the 2D planner places the furniture on the deck.

Sources, read October 16, 2026: Trex’s product pages for Enhance, Transcend, Select and Lineage and its Hideaway hidden fasteners; the TimberTech composite and Advanced PVC installation guides; Fiberon’s Concordia, Good Life, ArmorGuard and Promenade specifications and its hidden fastener FAQ; the IRC 2021, Section R507.7. Trex’s Signature page prints no size, so the calculator gives it Trex’s other 1×6 boards’ 5.5 inches. Makers change their lines: the label on your boards has the last word, and this page is re-checked every six months.