Estimate a conservative floor joist span from its depth.
Floor joist span scales roughly with joist depth and lumber grade. Wider spacing and lower grades reduce the safe span, and this estimate flags those adjustments. Under-sizing joists causes bouncy floors and sag, and this approximate check prompts a proper engineering table lookup.
Joist Span Estimate
Est. span = joist depth x grade ratio (reduced at wide spacing)
Est. span = joist depth x grade ratio (reduced at wide spacing) Floor joist span scales roughly with joist depth and lumber grade. Wider spacing and lower grades reduce the safe span, and this estimate flags those adjustments.
Under-sizing joists causes bouncy floors and sag, and this approximate check prompts a proper engineering table lookup.
This calculator takes 3 inputs: Joist depth, Joist spacing, Lumber grade. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.