How many deck boards do I need?
Divide the deck's depth by one board plus one gap to get the number of rows. A standard deck board is 5 1/2" wide; with a 3/16" gap that's 5 11/16" per row. Then work out what each row is cut from. If a row is shorter than your longest board, it's one board. If it's longer, the boards have to meet over a joist.
Example: a 16' x 12' composite deck with 3/4" overhang needs 26 rows. Each row is 16' 1 1/2" long, so 20-foot boards (or 16s, if you trim the overhang) cover each row in one piece: 26 boards and no seams.
Why seams must land on joists
Deck boards can only end where they can be screwed down — over a joist. Where two board ends meet, most composite brands ask for a doubled joist or blocking so each end has its own support. Seams in neighboring rows should also land on different joists, or the deck shows a straight line of joints. The cut list here does both automatically and flags the joists that need doubling.
Joist spacing
| Decking | Boards at 90° | Diagonal (45°) |
|---|---|---|
| 5/4 wood, 1" composite | 16" on center | 12" on center |
| 2x6 wood | up to 24" on center | 16" on center |
Always check your product's span chart — some composites and all commercial decks need 12".
Gaps between boards
Composite boards typically need 3/16" between boards and about 1/8" end to end, set by the temperature on install day. Pressure- treated wood is often installed wet and tight, then shrinks to about a 1/8" gap; dry wood gets a 1/8" spacer.
Screws or hidden clips?
Face-screwing uses two screws per board at every joist — about 3 to 4 screws per square foot. Grooved composite boards use one hidden clip per board at every joist instead, plus face screws along the outside edges. The calculator adds 10% to screws and 5% to clips for drops and spares.
Footings for your deck posts? Use the concrete calculator for post holes. Painting the house wall behind it? Try the paint calculator.