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Floor Joist Calculator

Enter the span length and joist spacing to count the joists you need.

How it is calculated

The tool counts joists along the length you enter: span (in) = span (ft) × 12 and joists = ⌊span (in) ÷ spacing (in)⌋ + 1. The span is in feet and is the distance measured across the joists, from the first joist to the far edge. Spacing is the on-center distance in inches, such as 12, 16 or 24. The division is rounded down and one joist is added for the starting joist at the edge. When the span is not an exact multiple of the spacing, the last bay is shorter and the tool does not add a joist for the far edge, so you may need one more there. The spacing shown is rounded to a whole inch, although the exact value you typed is used. Both values must be above zero. This counts pieces only and does not check joist size or allowable span.

Worked example

A floor 13 ft across with joists 16 inches on center. Convert the span: 13 × 12 = 156 in. Divide by the spacing: 156 ÷ 16 = 9.75, rounded down to 9. Add the starting joist: 9 + 1 = 10 joists. The 10th joist sits at 144 in, leaving a 12 in gap to the far edge, so a framer would add an end joist or rim there.

Try this example →

Common mistakes

  • Entering the span in inches. The span field is feet, so 156 would be read as 156 ft and give a count about twelve times too high.
  • Mistaking the span for the length of a joist. The span here is the width the joists are laid across, not the length of each board.
  • Forgetting the far edge. For a 13 ft floor at 16 in the tool returns 10, the same as for 12 ft, because the last 12 in has no joist counted; add one for the end or rim joist.
  • Ignoring doubled joists under walls, around openings and for rim boards. The tool counts regular spacing only.
  • Treating the count as proof of strength. Joist size, species and span tables decide whether the floor is adequate, not the number of joists.

About this calculator

Floor joists are spaced at a fixed on-center distance (commonly 12, 16, or 24 inches) to distribute the floor's load evenly across the framing, so the number of joists needed for a given span is found by dividing the span length by that spacing and adding one for the starting joist at the edge. This calculator applies that standard framing formula to your entered span and spacing to estimate the joist count for a floor section.

It's a routine planning step for carpenters, framers, and DIY builders ordering lumber for a deck, subfloor, or addition before construction begins, giving a quick material estimate that can then be checked against local building code span tables, which also factor in joist size and load requirements.

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Comments (1)

  • Editorial team

    Treat the count as the evenly spaced run only. Lumber orders usually need extra pieces that the formula cannot see: doubled joists under bearing walls, around stairwell openings and below heavy fixtures, plus a spare or two for boards that arrive crowned or twisted. The count also says nothing about whether the joist size can carry the span, which comes from your local span tables. What are you framing, and what does your local table say for your spacing?

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