Floor Joist Calculator

Joists span the length, spaced across the width: ceil(12 / (16/12)) + 1 = ceil(9) + 1 = 10 joists, each 16 ft long.

One row of mid-span blocking needs joist_count minus 1 pieces. Each piece is cut to fit between joists (roughly spacing minus 1.5 inches).

16 inches on center is the residential standard. 12 OC is used for heavy loads or long spans; 24 OC is used with deeper joists in some engineered systems.

For a 40 psf live load at 16 OC, 2x10 lumber (Douglas fir or southern pine #2) is typically adequate for a 14-foot span. Always confirm with local span tables.

ceil(room_width / spacing) gives the number of full spaces. Adding 1 gives the number of joists, since joists define the boundaries of each space. A 10-foot span at 16 OC needs ceil(10 / 1.333) + 1 = 9 joists.

Yes, one continuous row of blocking at mid-span covers all joist bays. The pieces are cut to fit snugly between each pair of joists and are typically face-nailed through the adjacent joists.

Yes. Joists can span either dimension. Choose the shorter span when possible to reduce the required lumber size and cost. The calculator works regardless of which dimension you assign as length versus width.

Rim joists (band joists) close the open ends of the floor system. They are the same size lumber as the floor joists. Two rim joists equal 2 x room_width linear feet; add these to your order separately.