Joist Spacing Calculator
Calculate how many floor or deck joists you need from the run length and your joist spacing on center.
How this calculator works
Floor and deck joists run at a set spacing along the length of a run, measured center to center. To find the total count, convert the run length to inches, divide by the spacing in inches, and add one joist for the starting end — since a joist sits at both ends of the run, not just in the gaps between them.
Formula: Joist count = floor(run length in inches ÷ spacing in inches) + 1 (one joist at each end plus the ones in between).
Worked example
A 16 ft run, joists at 16 in on center:
- 16 ft × 12 = 192 in
- 192 ÷ 16 = 12
- 12 + 1 = 13 joists
Notes
16 in on center is the most common joist spacing for both floors and decks, though 12 in on center is used for tile floors or heavier loads, and some engineered joists allow 19.2 in or 24 in.
Check your joist product’s allowable spacing for your span and load before finalizing the count.
How to use
Enter the total run length and the joist spacing you’re planning to use, commonly 16 in on center. The calculator returns the number of joists needed along that run, which you can use as a starting point for material ordering.
Counts joists from spacing you provide. Maximum allowable joist spacing depends on your joist size, species, and load — confirm with a span table or your joist manufacturer.
Frequently asked questions
Why does the count add 1 to the division result?
A joist sits at both ends of the run, not just spaced out in between. Dividing the run length by the spacing alone only counts the gaps between joists, so adding 1 accounts for the starting joist at the first end, matching how framers actually lay out joists.
What's the most common joist spacing?
16 in on center is the most common spacing for both floors and decks. Some situations call for tighter spacing, like 12 in on center for tile floors or heavier loads, while some engineered joists allow wider spacing, like 19.2 in or 24 in on center.
How do I know if my spacing is allowed for my span?
Maximum allowable joist spacing depends on your joist size, species, and the load it needs to carry, and it typically gets tighter as the span gets longer. Check a span table for your specific joist product, or confirm with your joist manufacturer, before finalizing your spacing.
Does this count work for both floor joists and deck joists?
Yes. The formula is the same for both — it's just counting evenly spaced members along a straight run length. Enter the run length and the on-center spacing you're using, whether it's an interior floor or an outdoor deck.
Estimates only. Verify quantities with your supplier before purchasing.