Floor Joist Calculator

ft
ft
in
12, 16, or 24 in on-center spacing.
Construction

Calculate the number of floor joists needed for a room based on spacing and span.

Formula

Joists = room width / spacing + 1; LF = joists × span length
  • Add one joist for each end (rim joist pairs).
  • Common joist sizes: 2×8 for up to 12 ft; 2×10 for up to 15 ft; 2×12 for up to 18 ft.
  • Verify with span tables per species and grade.

16 ft room width, 14 ft span, 16 in OC

Inputs
  • Room Span (joist length): 14 ft
  • Room Width (direction of joists): 16 ft
  • Joist Spacing: 16 in

16/(16/12) + 1 = 12+1 = 13 joists. LF = 13×14 = 182 lf.

Frequently asked questions

What size joist for a 14-foot span?
Typically 2×10 at 16 in OC for standard residential loads. Verify with IRC span tables.