How many deck boards do I need?
Count the rows across the depth, then the boards in each row; that is how this deck board calculator works:
- Rows: deck depth ÷ (board width + gap), rounded up. With 5-1/2 in boards and 1/8 in gaps, each row takes 5-5/8 in.
- Boards per row: deck width ÷ stock length, rounded up. A 16 ft deck uses one 16 ft board per row; a 20 ft deck needs two boards with a joint over a joist.
- Waste: add 10% for a square layout and more for diagonal or picture-frame decks.
The whole frame (joists, beam, posts, railing) comes from the deck calculator.
Joist spacing for deck boards
The decking decides the joist spacing. For wood boards that cross three or more joists, IRC Table R507.7 allows joists up to 16 in apart under 1-1/4 in (5/4) boards laid square to the joists and 12 in when laid diagonally (12 in and 8 in for a board on only two joists); 2x decking allows 24 in[1].
| Decking | Square to joists | Diagonal (up to 45°) |
|---|---|---|
| 1-1/4 in (5/4) boards | 16 in | 12 in |
| 2 in (2x) boards | 24 in | 24 in |
Fasten wood decking with two threaded nails or two No. 8 screws at every joist[1]. DCA 6 adds that each board should bear on at least three joists and gaps of about 1/8 in are typical[2]. Lay out the joists with the deck framing calculator.
Composite deck calculator: clips, gaps and border
Composite boards are counted the same way but usually fastened with hidden clips, one wherever a board crosses a joist; switch Fastening to hidden clips. The clip sets the gap. Composite makers publish their own maximum joist spacing, for square and diagonal layouts, in the product's installation guide and code evaluation report; follow it, as it can be tighter than for wood.
- Picture-frame border: boards around the three open sides, mitred at the corners, hide the cut ends of the field. They need blocking underneath.
- Fascia: covers the rim and end joists; the calculator gives its running length.
- Start and finish boards are face-screwed even on a clip system.
Diagonal and long decks
A diagonal layout covers the same area, so it needs the same length of board, but the cut ends waste more and the joists must be closer. Long decks need board joints: stagger them and land each end on a joist (or a doubled joist). For the steps down, see the deck stair calculator.
Frequently asked questions
Is this a composite decking calculator too?
Yes. Set Fastening to hidden clips and enter the clip gap; the count of boards, clips, border and fascia works the same for composite and wood.
How many deck boards do I need for a 12 × 16 deck?
With 5-1/2 in boards and 1/8 in gaps running the 16 ft way, 26 rows of 16 ft boards, 29 with 10% extra.
How many hidden clips do I need for a composite deck?
One for every place a board crosses a joist: about 1.6 per square foot with 5-1/2 in boards on 16 in joists.
How many screws per deck board?
Two at each joist. A 16 ft board over 16 in joists crosses 13 joists, so 26 screws.
What joist spacing do composite deck boards need?
Whatever the board maker lists for your layout; diagonal boards always need closer joists than square ones.