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Construction Calculators
Floor Joist Calculator
Calculate the number of floor joists needed to frame a floor of a given length at a chosen on-center spacing.
Joists needed
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About the Floor Joist Calculator
This calculator finds how many floor joists you need to frame a floor of a given length. Like fence posts, joists mark off equal spaces along a run, so the count is one more than the number of spacing intervals.
Formula: Joists Needed = (Floor Length ÷ Spacing in Feet) + 1
16in o.c. is the most common residential spacing, but 12in o.c. is used for heavier loads like tile floors, and 19.2in or 24in o.c. is common with engineered I-joists. Always add extra joists for doubled openings around stairs and mechanical chases.
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