How Many Deck Boards Do I Need?
Rows = deck depth in inches ÷ (board width + gap)
Then work out boards per row. If the deck is longer than a board, each row needs more than one board; if it’s shorter, one board covers more than one row.
Worked example. A 16 × 12 ft deck with 5.5 in boards and 3/16 in gaps has 144 ÷ 5.6875 = 25.3, so 26 rows. With 16 ft boards that’s one board per row: 26 boards, or 28 with 5% extra. Joists at 16 in on center across 16 ft is 13 joists, each 12 ft long. At 2 screws per joist crossing that’s 676 screws, about 710 with extra.
Joist Spacing
Trex’s guidance for composite decking:
- 16 in on center for boards running straight across the joists.
- 12 in on center for diagonal decking.
- 12 in or less for stairs, heavier loads, and intricate patterns.
Wood decking makers publish their own spacing; follow the instructions for your boards.
Board Gaps
Trex recommends 3/16 in between composite boards side to side (up to 3/8 in under trees where debris collects), and 1/8 in end to end on days above 40°F, or 3/16 in on colder days. Leave 1/4–1/2 in where boards meet the house.
What This Calculator Doesn’t Cover
The decking and joists are the easy part. Footings, posts, beams, the ledger board, and railings depend on your deck’s height, your soil, frost depth, and local building code. Most decks need a permit and inspection. Use the counts here for the surface, and get the structure from a span table, a deck plan, or a pro.
What It Costs
HomeGuide puts deck materials at $14–$28 a sq ft and labor at $11–$22 a sq ft for a new deck. The deck cost calculator adds labor and permit.
Frequently Asked Questions
How Many Deck Boards Do I Need?
Divide the deck’s depth by the board width plus the gap to get rows of boards, then work out how many boards each row takes. A 16 × 12 ft deck of 5.5 in boards with 3/16 in gaps has 26 rows, so it needs 26 sixteen-foot boards, or 28 with 5% extra.How Far Apart Should Deck Joists Be?
Trex calls for 16 in on center for straight decking and 12 in when boards run diagonally. Heavier loads and some patterns need closer spacing; check your decking maker’s instructions and local code.How Much Gap Between Deck Boards?
Trex recommends 3/16 in between composite boards side to side, 1/8 in end to end on warm days (3/16 in when it’s colder than 40°F), and 1/4–1/2 in where boards meet the house.How Much Does a Deck Cost?
HomeGuide puts new decks at $25–$50 a sq ft installed for pressure-treated wood, $30–$47 for cedar, $40–$80 for composite, and $100–$125 for PVC.Sources
- How to space deck boards (Trex)
- How to space and install deck joists (Trex)
- Cost to build a deck (HomeGuide, updated Feb 2026)
Prices are national ballparks checked October 2026. Local prices vary.