Deck Joist Spacing Calculator
Enter the deck length (the direction the joists are spaced along), the deck width (the direction each joist runs), and the on-center spacing you plan to use. The calculator returns how many joists to buy and how much framing lumber that adds up to.
This tool counts joists. It does not check how far a given joist size can span; that comes from span tables for your lumber species and grade, plus your local code.
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Worked example
Inputs: deck length 14 ft; deck width 8 ft; joist spacing, on center 16 in.
- Length in inches: 14 ft × 12 = 168 in.
- Whole spaces that fit: 168 ÷ 16 = 10.5, so 10 spaces.
- Joists = spaces + 1 (one at each end) = 11.
- Leftover at the end: 168 − (10 × 16) = 8 in. Many builders add one more joist or shift the layout so the last gap is not awkward.
- Total lumber: 11 joists × 8 ft = 88 linear ft.
Result: 11 joists needed.
Where the formula comes from
Layout counting uses the standard on-center method. Maximum spacing for your decking is set by the decking manufacturer; framing requirements are in IRC Section R507 and your local amendments.
Estimates only. Results are planning figures, not engineering or code advice. Check your local building code and confirm quantities with your supplier.
Frequently asked questions
What joist spacing should I use?
16 in on center is common for wood and many composite boards. Some products allow 24 in, and diagonal board patterns often call for 12 in. The decking manufacturer sets the maximum, so check the installation sheet for your product.
Does the count include the rim joists?
The count includes a joist at each end of the length, which you can treat as the end joists. Add the two boards for the other sides (rim or band boards) separately.
How far can a joist span?
That depends on lumber size, species, grade, spacing and load. This calculator does not decide it. Use the span tables referenced by your local code, or ask your building department.
Why is there a leftover gap?
If the length is not an exact multiple of the spacing, the last space is shorter than the rest. Adjust the layout or add a joist so the end of the decking boards is fully supported.