Span Calculator For Joists
Calculate the maximum span for wood joists based on species, grade, size, spacing, and load conditions.
Published by:
isCalculator Editorial Team
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How to use this tool?
How to Use the Span Calculator for Joists
- Select Species: Choose the wood species (e.g., SPF, Douglas Fir-Larch).
- Select Grade: Pick the lumber grade (e.g., No.2).
- Choose Joist Size: Pick the nominal size (e.g., 2x8).
- Set Spacing: Select the center-to-center spacing in inches (12, 16, or 24).
- Enter Load: Input the total load in pounds per square foot (psf) — default is 40 psf. Must be between 10 and 100.
- Calculate: Click the "Calculate Span" button.
Output: The maximum allowable span in feet is displayed, along with design values (Fb and E). The formula uses simple beam bending: Span = √((8 × Fb × S) / (w × 12)), where S is section modulus and w is load per linear inch.
Note: All inputs are in imperial units (inches, psf). Ensure load includes both live and dead loads.
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Frequently Asked Questions
A joist span is the distance between supports that a joist can safely cover without exceeding bending or deflection limits. It depends on the species, grade, size, spacing, and load.
The span is calculated using the bending formula: Span = sqrt((8 * Fb * S) / (w * 12)), where Fb is the allowable bending stress, S is the section modulus, and w is the uniform load per linear inch.
Typical residential loads are 40 psf (30 live + 10 dead) for floors. This calculator uses a combined load input; default is 40 psf.