Beam & Joist Span Calculator

Pick a species, grade, size, spacing and load, and this tool estimates how far a floor joist, ceiling joist, rafter or beam can span, running the same bending, shear and deflection checks that sit behind the IRC span tables, and telling you which one governs. Use it to test schemes and set out framing early; the numbers are a starting point for an engineer, never a replacement.
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How the Span Is Estimated

The tool converts your area load (psf) into a line load on one member: live load times the tributary width, which for joists and rafters is simply the spacing. Then it finds the longest span that passes all three limit states and reports the shortest of them:

Bending. The maximum span where the applied moment stays under the adjusted bending value Fb′ = Fb × C_D × C_F × C_r: load-duration, size and repetitive-member factors applied to the NDS reference value.

Shear. The span where horizontal shear stays under Fv′, using the 1.5·V/A rectangular-section stress.

Deflection. The span meeting L/360 under live load and L/240 under total load, from the 5wL⁴/384EI simple-beam formula. For everyday floors this is almost always the controlling check: stiffness, not strength, is what keeps a floor from feeling springy.

Joists, rafters and ceiling members get the 1.15 repetitive-member bonus on bending; a single or built-up beam does not. All spans are clear simple spans with a uniform load and a laterally braced compression edge.

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Reference Design Values Used

Nominal NDS reference values for visually graded dimension lumber, dry service. Bending (Fb) is shown before the size factor C_F and repetitive factor C_r; Southern Pine values are already size-adjusted.
*Before the size factor C_F (1.3 for 2×6 down to 1.0 for 2×12). Confirm the exact grade-stamped values with your lumber supplier; region, mill and moisture content all shift them.

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