Timber Grade Selection Calculator
Compare 9 species/grade combinations for a beam application with bending and deflection checks.
About this calculator
This calculator runs the same beam against all 9 combinations of 3 species (Douglas Fir-Larch, Southern Pine, Eastern White Pine) and 3 grades (Select Structural, No.1, No.2), checking bending and deflection for each so you can see which combinations are strong enough -- and which are needlessly expensive -- for one span, load, and cross-section. Max Bending Moment and Section Modulus Provided are computed once from Beam Span, Total Load, Tributary Width, Beam Width, and Beam Depth -- the same demand numbers apply to every grade being compared, since none of the 9 combinations changes the beam's own geometry or the load it carries. What differs per combination is only the allowable stress (Fb, from 375 psi for Eastern White Pine No.2 up to 1750 psi for Southern Pine Select Structural) and stiffness (E), which determine each combination's own bending ratio, deflection ratio, and pass/fail.
Grades That Pass counts how many of the 9 clear both checks -- as Beam Span grows, the fixed moment demand rises with the square of span, so progressively fewer (and eventually zero) of the 9 combinations can carry it without a bigger cross-section. Because Southern Pine's design values run highest and Eastern White Pine's lowest, the combinations most likely to fail first as span grows are always the Eastern White Pine grades.
Inputs
Results
Grades that pass
5
How to Use This Calculator
- Enter Beam span, Total load, and Tributary width.
- Set Beam width and Beam depth.
- Review the Grades that pass result.
- Use Max bending moment (lb·ft) and Section modulus provided (in³) to inform your decision.
How the result changes with Beam span
| Beam span | Grades that pass |
|---|---|
| 8 | 9 |
| 12 | 7 |
| 24 | 0 |
| 40 | 0 |
What each input means
- Beam span
- Clear span of the beam.
- Total load
- Combined dead + live load.
- Tributary width
- Tributary width for the beam.
- Beam width
- Width of the timber beam.
- Beam depth
- Depth of the timber beam.
What each result means
- Grades that pass
- Number of the 9 species/grade combos that pass both checks.
- Max bending moment
- M = wL²/8.
- Section modulus provided
- S = bd²/6.
- Uniform load on beam
- w = load_psf × tributary_width.
- DF-L Select Struct. bending ratio
- Bending demand/capacity for Douglas Fir-Larch Select Structural.
- DF-L Select Struct. defl. ratio
- Deflection demand/capacity.
- DF-L Select Struct. pass
- 1 = pass, 0 = fail.
- DF-L No.1 bending ratio
- Bending ratio for DF-L No.1.
How this is calculated
Formula
M = wL²/8 | f_b = M / S | Deflection = 5wL⁴/(384EI) | Utilization = f_b / F_bEngine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does the beam's width or depth affect the Max Bending Moment shown?
No. Max Bending Moment (M = wL^2/8) depends only on Beam Span, Total Load, and Tributary Width -- it is the demand the beam must resist, computed before any grade comparison happens. Beam Width and Beam Depth determine Section Modulus Provided (the beam's own capacity contribution), which is compared against each of the 9 species/grade allowable stresses separately to produce each combination's bending ratio.
Why does increasing the beam span reduce the number of Grades That Pass?
Max Bending Moment grows with the SQUARE of Beam Span (M = wL^2/8), so every one of the 9 species/grade bending ratios rises together as span increases -- Grades That Pass can only fall or stay the same as span grows across its full 4-40 ft range, never increase, since a longer span never makes a fixed cross-section easier to carry.
Which species/grade combinations are the first to fail as span increases?
Eastern White Pine's grades carry the lowest allowable bending stress of the three species (Fb from 375 psi at No.2 up to 775 psi at Select Structural, versus up to 1750 psi for Southern Pine Select Structural), so its bending ratios cross 1.0 at the shortest spans. Southern Pine's grades, with the highest design values in the comparison, are typically the last to fail as span keeps increasing.
What does the Section Modulus Provided number depend on?
Section Modulus Provided (S = b x d^2/6) depends only on the Beam Width and Beam Depth you entered -- it is the SAME value used to compute the bending ratio for every one of the 9 species/grade combinations, since all 9 are evaluated against the identical cross-section. Only each combination's allowable stress (Fb) changes between rows, not the section modulus itself.
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