Floor Joist Calculator

Use this floor joist calculator to determine the number of floor joists you will need and how much they will cost. You can also use this calculator to determine the maximum allowable span a floor joist can be depending on its wood species, grade, size, and supported load.

Joists109 bays at 16 in centres, plus one

Joist length16 ft16 ft

Allowable span15.5 ftat 40 psf live load, 10 psf dead

Span checktoo farover by 0.5 ft — go deeper, closer, or add a beam

Span used103.2% of allowable

Linear feet of joist160 ft

Blocking9 pieces12 ft in total, 1 row

Rim joist56 ftaround the perimeter

Floor area192 sq ft

Subfloor sheets64 × 8 ft sheets, before waste

The method

joists = span ÷ spacing + 1, checked against the span table

Depth buys span, spacing buys a little

Stiffness goes with the cube of depth, so a 2×10 spans about 25% further than a 2×8 and nearly 70% further than a 2×6. Closing the spacing from 16 to 12 inches gains only around 8%. If a floor will not make the span, go deeper before going closer.

The spans here are for a residential floor at 40 psf live load and assume the joists are properly supported and blocked. They are a sanity check, not a design — for anything you will build from, use the span tables your building department accepts, and remember that a bouncy floor can be well inside code and still feel wrong.