How this calculator works
The number of board rows is the deck width divided by one board plus its gap. Each row needs enough boards to cover the deck length. Joists run perpendicular to the boards at the chosen spacing, and each board gets two screws at every joist it crosses.
Rows = round up( Width×12 ÷ (Board width + Gap) )
Boards = round up( Rows × round up(Length ÷ Board length) × (1 + Waste%/100) )
Joists = floor( Length×12 ÷ Spacing ) + 1
Screws = Rows × Joists × 2
Assumptions and tips
- Assumes boards run parallel to the deck length and joists across it.
- Beams, posts, footings, railings and stairs are separate — use the Concrete Column Calculator for footings.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Deck length (board direction) | 16 ft |
| Deck width | 12 ft |
| Board width (actual) | 5.5 in |
| Gap between boards | 0.1875 in |
| Board length | 16 ft |
| Joist spacing | 16 in on center |
| Waste allowance | 10 % |
| Result | Value |
|---|---|
| Deck boards (16 ft) | 29 boards |
| Board rows | 26 rows |
| Joists | 13 × 12 ft |
| Deck screws (2 per joist crossing) | 676 screws |
| Deck area | 192 sq ft |
Step by step:
- Rows = 144 in ÷ (5.5 + 0.188) → 26
- Boards per row = 16 ÷ 16 → 1
- Boards = 26 × 1 × 1.1 → 29
- Joists = floor(192 ÷ 16) + 1 = 13; screws = 26 × 13 × 2 = 676
Deck boards for common deck sizes
5.5 in boards, 3/16 in gap, 16 ft boards, joists 16 in on center, 10% waste.
| Deck | Deck boards | Joists | Deck screws |
|---|---|---|---|
| 10 × 10 ft | 25 boards | 8 × 10 ft | 352 screws |
| 12 × 12 ft | 29 boards | 10 × 12 ft | 520 screws |
| 12 × 16 ft | 29 boards | 13 × 12 ft | 676 screws |
| 16 × 20 ft | 75 boards | 16 × 16 ft | 1,088 screws |
Common mistakes to avoid
- Using nominal width (6 in) instead of actual (5.5 in) — you end up a row short.
- Ignoring the joist spacing composite decking requires.
Frequently asked questions
How many deck boards for a 12×16 deck?
With 5.5 in boards and 3/16 in gaps, about 26 rows of 16 ft boards — roughly 29 boards with 10% waste.
What gap should deck boards have?
Follow the board maker’s spec; wood is often gapped about 1/8–1/4 in depending on moisture content.