Deck Joist Span Calculator
Calculate bending and deflection span limits from entered lumber properties.
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What this tool does
Calculate bending and deflection span limits from entered lumber properties. Use professionally adjusted strength values; this is not a code-approved span selector.
How to use the Deck Joist Span Calculator
- Enter or select actual joist depth in in.
- Enter or select actual joist thickness in in.
- Enter or select joist spacing in in.
- Enter or select uniform design load (lb/sq ft).
- Enter or select adjusted modulus e (psi).
- Enter or select adjusted bending strength fb (psi).
- Enter or select deflection ratio l/.
- Read the calculated result; change any measurement to compare alternatives.
Formula
S=b×d²/6; I=b×d³/12; w=load×spacing/144 (lb/in); bending span=sqrt(8×Fb×S/w); deflection span=cbrt(384×E×I/(5×w×limit)); allowable=min(spans)/12
- depth
- Actual joist depth (in, before metric conversion)
- width
- Actual joist thickness (in, before metric conversion)
- spacing
- Joist spacing (in, before metric conversion)
- load
- Uniform design load (lb/sq ft)
- E
- Adjusted modulus E (psi)
- Fb
- Adjusted bending strength Fb (psi)
- limit
- Deflection ratio L/
Use professionally adjusted strength values; this is not a code-approved span selector.
Worked example
For deck joist span calculator, the following measurements illustrate the exact method: Actual joist depth: 9.25; Actual joist thickness: 1.5; Joist spacing: 16; Uniform design load (lb/sq ft): 50; Adjusted modulus E (psi): 1200000; Adjusted bending strength Fb (psi): 1000; Deflection ratio L/: 360.
Inputs
- Actual joist depth9.25 in
- Actual joist thickness1.5 in
- Joist spacing16 in
- Uniform design load (lb/sq ft)50
- Adjusted modulus E (psi)1200000
- Adjusted bending strength Fb (psi)1000
- Deflection ratio L/360
Result
- Calculated span limit13.82 ft
- Bending limit14.63 ft
- Deflection limit13.82 ft
Results explained
- Calculated span limit
- Calculated span limit from the formula above, expressed in ft before metric conversion. Use professionally adjusted strength values; this is not a code-approved span selector.
- Bending limit
- Bending limit from the formula above, expressed in ft before metric conversion. Use professionally adjusted strength values; this is not a code-approved span selector.
- Deflection limit
- Deflection limit from the formula above, expressed in ft before metric conversion. Use professionally adjusted strength values; this is not a code-approved span selector.
Frequently asked questions
The lower of the bending and L/deflection span is returned.
Actual width/depth, adjusted Fb and E, spacing and uniform load are needed.
Do not extend the result or treat a waste factor as a safety factor.
Split the outline into rectangles or triangles and calculate each section separately. Do not count overlapping sections twice.
An engineer must verify shear, bearing, connections, species, grade, treatment and local code.