Joist Span Calculator
The question is never "how strong is 2×8" but "how far can this specific joist at this specific spacing carry the load." This calculator looks it up and tells you plainly whether your plan works, and what to change if it does not.
Results
Enter your measurements above and press Calculate.
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Questions people ask
How far can a 2x6 joist span?
At 16 inches on center a 2×6 spans about 9 feet 10 inches for typical residential loads. At 24 inches on center that drops to about 8 feet 5 inches. Over 12 feet you need a 2×10 or a beam.
Does the joist span table change with wood species?
Yes. Southern pine and Douglas fir span longer than SPF spruce-pine-fir at the same size and spacing. The table here is conservative SPF values. If you are using a premium framing lumber, check the manufacturer grade stamp — it lists the actual allowable span.
What does clear span mean?
Clear span is the distance between the supporting members, not the total length of the joist. A joist bearing on a wall and on a beam spanning 10 feet has a clear span of 10 feet, but the board itself is cut longer to cover both bearings.
Do I need blocking between joists?
Blocking is required over bearing walls to stop racking, and at mid-span when joists exceed 10 feet. Solid blocking also provides extra nailing surface for attaching a wall later.
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