How many floor joists do I need?
The fetched OmniCalculator floor joist page gives the joist count formula in terms of floor length, joist thickness, and on-center spacing. Interpreting the rendered formula gives:
n = 1 + (L - w) / s
Here, L is the floor length measured perpendicular to the joists, w is actual joist thickness, and s is on-center spacing. As Omni notes, round up to the next whole joist.
How to calculate floor joist span
The fetched source combines the allowable deflection rule δ = L / 240 with the simply supported uniform-load deflection equation δ = 5uL^4 / (384EI). Solving those together gives an allowable span estimate for a joist under uniform floor loading.
Species and modulus values referenced by Omni
This rebuild includes several source-backed wood stiffness presets taken from the fetched American Wood Council table on the Omni page, including Douglas Fir-Larch, Southern Pine, Hem-Fir, and Spruce-Pine-Fir grades.