Decking boards run along the entered deck length. Count those boards across the deck width, including gaps only between boards. Joists run perpendicular to the decking; count support positions along the deck length, including both ends.
The last spacing may be shorter.
This count does not size structural members or approve board spans. Use the decking product instructions for permitted spacing and gaps. Ledger, beams, posts, blocking, footings, guards and connections are separate quantities.
Decking boards needed from deck length
| Deck length | 16 |
|---|---|
| Deck width | 12 |
| Decking board width | 5.5 |
| Gap between boards | 0.25 |
| Joist spacing | 16 |
| Decking boards needed | 26 |
| Joists needed | 13 |
Common error: Counting the joists as the width divided by the spacing. There is one more joist than there are spaces, since both edges need one.
Deck Material: formula
American Wood Council: Span Options Calculator · AWC DCA 6-2015
Deck Material: changing Deck width
| Deck width | Decking boards needed | Joists needed |
|---|---|---|
| 10 | 21 | 13 |
| 11 | 23 | 13 |
| 12 (example) | 26 | 13 |
| 13 | 28 | 13 |
| 14 | 30 | 13 |
Deck Material: compare input ranges
| Input | Tested from | To | Decking boards needed: low input | Decking boards needed: high input | Swing |
|---|---|---|---|---|---|
| Deck width | 11 | 13 | 23 | 28 | 5 (19%) |
| Decking board width | 5 | 6.1 | 28 | 23 | 5 (19%) |
| Gap between boards | 0.225 | 0.275 | 26 | 25 | 1 (3.8%) |
| Deck length | 14 | 18 | 26 | 26 | 0 |
| Joist spacing | 14 | 18 | 26 | 26 | 0 |
Deck Material: individual input comparisons
| Input | Value | Decking boards needed | Joists needed |
|---|---|---|---|
| Deck length | 13 | 26 | 11 |
| 14 | 26 | 12 | |
| 16 | 26 | 13 | |
| 18 | 26 | 15 | |
| 19 | 26 | 16 | |
| Deck width | 10 | 21 | 13 |
| 11 | 23 | 13 | |
| 12 | 26 | 13 | |
| 13 | 28 | 13 | |
| 14 | 30 | 13 | |
| Decking board width | 4.4 | 32 | 13 |
| 5 | 28 | 13 | |
| 5.5 | 26 | 13 | |
| 6.1 | 23 | 13 | |
| 6.6 | 22 | 13 | |
| Gap between boards | 0.2 | 26 | 13 |
| 0.225 | 26 | 13 | |
| 0.25 | 26 | 13 | |
| 0.275 | 25 | 13 | |
| 0.3 | 25 | 13 | |
| Joist spacing | 13 | 26 | 16 |
| 14 | 26 | 15 | |
| 16 | 26 | 13 | |
| 18 | 26 | 12 | |
| 19 | 26 | 12 |
Decking boards needed: Deck width against Decking board width
| Deck width / Decking board width | 4.4 | 5.5 | 6.6 |
|---|---|---|---|
| 10 | 26 | 21 | 18 |
| 11 | 29 | 23 | 20 |
| 12 | 32 | 26 | 22 |
| 13 | 34 | 28 | 23 |
| 14 | 37 | 30 | 25 |
Deck Material: values and definitions
| Quantity and definition | Value used |
|---|---|
| Deck length — Deck dimension parallel to the decking boards, in feet; joists are counted along this dimension. | 16 |
| Deck width — Width of the deck to be covered by decking boards, in feet. | 12 |
| Decking board width — Face width of one decking board, in inches. | 5.5 |
| Gap between boards — Spacing left between adjacent boards, in inches. | 0.25 |
| Joist spacing — On-center joist spacing, in inches. | 16 |
| Decking boards needed | 26 |
| Joists needed | 13 |
Deck width: half and double
| Deck width | Decking boards needed |
|---|---|
| 6 | 13 |
| 12 | 26 |
| 24 | 51 |
A deck needs decking boards and joists, and each is counted from the width and the spacing. A deck 16 feet long and 12 feet wide, with 5.5 inch boards and a quarter inch gap, needs about 26 boards across the 12 foot width, and joists at 16 inch centers number 10 across it. Each board is a full length or a length cut from a longer board, and the boards run across the joists.
Extra boards cover the cuts and any that are warped.
Decide which way the boards run and whether they will be laid at an angle, since a diagonal layout wastes more. Check the board manufacturer's gap and joist spacing.
See the framing on the joist span reference and board feet calculators, footing on the footing size page and concrete for posts on the concrete volume calculator. The stair stringer and rectangle area pages help with steps and areas, and the fence and paint calculators cover related projects.
Is a gap counted after the last board?
No. N boards cover N times board width plus N minus 1 gaps.
How far apart should deck joists be?
Use the decking manufacturer’s span and orientation instructions together with the adopted structural requirements. Species, grade, joist dimensions, spacing and design loads matter. A materials count is not a span approval, and diagonal decking can have different support requirements from perpendicular boards.
How much gap should I leave between deck boards?
Follow the selected product’s installation instructions for board moisture, temperature and expansion. End gaps, side gaps and clearances from structures can differ. A generic wood or composite spacing figure should not replace the product-specific requirements.
Do I need a permit to build a deck?
Ask the authority having jurisdiction about the proposed attachment, height, area and location. Exemptions and required drawings depend on the adopted rules. Even a permit-exempt project can have structural, setback and utility constraints; the calculator does not determine compliance.