Steel Joist Selection Calculator

Total Uniform Load -- the figure you match against SJI capacity tables -- comes from a single multiplication: Total Load times Tributary Width. That means Joist Span and Live Load never move this output at all; span only enters the deflection side of the calculation (Allowable Deflection at L/360, and the Required Moment of Inertia that follows from it), and live load only feeds the live-load deflection check, not the uniform load a joist has to carry. Widening tributary spacing from 6 ft to 6.6 ft (a 10% increase) raises Total Uniform Load from 600 to 660 plf, exactly proportional -- the same proportional bump you'd get from a 10% increase in Total Load instead.

Required Moment of Inertia grows much faster with span than with load, since the allowable-deflection denominator (L/360) also scales with span, leaving deflection stiffness requirements to scale with roughly the cube of span length once that cancellation is worked through -- pushing span from the 30 ft default to the 144 ft maximum multiplies the required moment of inertia by roughly 110x. This tool estimates loads and stiffness requirements and surfaces a Preliminary Joist Designation from a simplified nearest-match table lookup keyed to real designations (10K1 through 30K12) drawn from the Steel Joist Institute's own Standard Load Tables, but it does not select a final joist designation for you (the actual SJI K-Series and LH-Series load tables must still be cross-referenced by hand) and does not check bearing, bridging, or connection design.