Deck calculator: how many deck boards and joists?
Enter the deck size and your board width, gap and length. The calculator counts board rows across the deck, adds waste, rounds up to whole boards and counts joists at your spacing.
Price this job as an estimate in Quotesprig (free)
Common sizes, worked out
5.5 in boards with a 3/16 in gap, 16 ft boards, joists at 16 in on centre, 10% waste. Every number below comes from the same calculator code as the tool above.
| Job | Deck area | Board rows | Deck boards | Joists (10 ft long) | Labour |
|---|---|---|---|---|---|
| 10 × 10 ft deck | 100 sq ft | 22 rows | 16 boards | 9 joists | 12.5 hr |
| 12 × 12 ft deck | 144 sq ft | 26 rows | 22 boards | – | 18 hr |
| 16 × 12 ft deck | 192 sq ft | 26 rows | 29 boards | – | 24 hr |
| 20 × 12 ft deck | 240 sq ft | 26 rows | 36 boards | – | 30 hr |
| 20 × 16 ft deck | 320 sq ft | 34 rows | 47 boards | – | 40 hr |
Assumptions you can change
Starting values: board width 5.5 in, gap between boards 0.1875 in, board length 16 ft, joist spacing 16 in, waste allowance 10 %, your production rate 8 sq ft/hr. They are typical starting points, not rules: use the numbers on your product labels and your own production rate.
FAQ
How many deck boards for a 12x12 deck?
For a 12 × 12 ft deck (5.5 in boards with a 3/16 in gap, 16 ft boards, joists at 16 in on centre, 10% waste): deck area 144 sq ft; board rows 26 rows; deck boards 22 boards; joists (12 ft long) 10 joists. Change any input above for your own job.
What joist spacing does the calculator use?
16 inches on centre to start, a common spacing for wood decking. Composite boards and diagonal layouts often need 12 inches; check the board maker's instructions and change it.
Does it include waste?
Yes, 10% by default on the board length, editable.
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