Key takeaways
- This is a shopping count, not a structural design.
- Joists = length in inches ÷ spacing, rounded up, plus one.
- Hot tubs, multi-level decks, unknown soil and uncertain ledger attachment are reasons to stop and get a design.
Enter your deck's size and spacing for a shopping count. This tool doesn't check whether joists, beams, posts or footings are big enough. Those need your local code's tables or an engineer.
How the calculator works
Boards run along the deck's length, with joists running across it:
- Joists: length in inches ÷ spacing, rounded up, plus one
- Board rows: width in inches ÷ (board width + gap), rounded up
- Linear feet: rows × length × (1 + waste %)
- Fasteners: 2 × rows × joists
The two per support comes from 2021 IRC Table R507.7, which calls for two 8d threaded nails or two No. 8 screws at each support for wood decking. Composite decking follows its own label.
Worked example
A 16 × 12 foot deck with joists at 16 inches, 5 1/2-inch boards, a 1/8-inch gap and 10% waste:
- Joists: 192 ÷ 16 = 12, plus 1 = 13 joists
- Rows: 144 ÷ 5.625 = 25.6, so 26 rows
- Boards: 26 × 16 × 1.10 = about 458 linear feet
- Fasteners: 2 × 26 × 13 = about 676
Measure your actual board width, because a board sold as 6 inches wide is usually narrower.
Choosing joist spacing
For wood decking perpendicular to joists, Table R507.7 allows these maximum spans:
| Decking | Single span | Multiple spans |
|---|---|---|
| 1 1/4 in wood, perpendicular | 12 in | 16 in |
| 1 1/4 in wood, diagonal | 8 in | 12 in |
| 2 in wood, perpendicular | 24 in | 24 in |
| 2 in wood, diagonal | 18 in | 24 in |
So 5/4 boards laid on a diagonal may need closer joists, which raises your count. See how spacing changes it on a 16-foot deck:
| Joist spacing | Joists |
|---|---|
| 12 in | 17 |
| 16 in | 13 |
| 24 in | 9 |
Stop and get a design if
- The deck will carry a hot tub, a roof, or any other heavy load
- It has several levels or isn't a simple rectangle
- You can't verify the ledger's attachment to a solid band joist, or it would attach through brick or stone veneer. The IRC doesn't allow ledgers on stone or masonry veneer, and says a deck must be self-supporting if a positive connection can't be verified
- You don't know your soil or frost depth
- Your size falls outside the local code's tables, which can't be extended
The American Wood Council's DCA 6 prescriptive guide is a useful read for how decks work. It's based on the 2015 IRC, and it's stricter than the 2021 IRC in places, so ask your building department which they accept.
Next steps
- For concrete footings, use the Sonotube calculator or footing calculator.
- For railings, use the baluster calculator.
- For steps, use the stair calculator.
- Before digging, contact 811.
Frequently asked questions
Can this tell me what size joists I need?
No. That depends on species, span, spacing and load, using your local code's tables or an engineer's design.
Do I need a permit for a deck?
Often, but it varies. The 2021 IRC's Section R105.2 exempts some small detached decks, and that doesn't excuse other laws. Ask your building department.
Why is there one more joist than the math says?
Dividing the length by the spacing counts the spaces between joists. You need one more joist than spaces.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 5, Section R507
- American Wood Council: DCA 6 deck guide
- 811: What is 811?