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Beam Size Helper

Rough beam sizing helper using span and load per foot with headroom.

Results

Required section modulus (in³, approx)
1.56
Suggested depth (in, 1.5 in width)
1.84

How to use this calculator

  1. Enter the clear span of the beam in feet. This is the distance between supports, not including bearing length on top of posts or walls.
  2. Enter the uniform load per linear foot the beam is expected to carry. This is usually derived from tributary area times design loads (live + dead) in psf.
  3. Choose a safety/headroom percentage to add above the basic calculated requirement (for example, 25% or 40%) to account for uncertainties and conservatism.
  4. Review the approximate required section modulus and the suggested beam depth assuming a 1.5 in width.
  5. Use these outputs as a planning cue to decide which member sizes you might ask an engineer to check or which lines of a span table you should study.
  6. Always confirm final beam sizing using official span tables, manufacturer literature, or stamped engineering before you build.

Inputs explained

Beam span (ft)
The clear distance in feet between the beam’s supports (such as posts or bearing walls). Use the free span, not including bearing length, for a realistic bending moment estimate.
Load (plf)
The uniform load the beam must support, in pounds per linear foot. This usually comes from tributary width multiplied by design live and dead loads in pounds per square foot.
Safety/headroom %
An extra percentage added to the minimum required section modulus to provide margin for unknowns and conservatism. Higher percentages give a deeper, more conservative suggested beam.

How it works

The calculator treats your beam as a simple, uniformly loaded span and uses the classic bending stress relationship: maximum moment M ≈ wL²/8, where w is load per foot and L is span.

Using an assumed allowable bending stress (Fb) of roughly 1,000 psi for dimensional lumber, we estimate the required section modulus S from S ≈ M ÷ Fb, then bump that value by your chosen safety/headroom percentage.

With an approximate required section modulus in hand and assuming a nominal 1.5 in width (typical for many dimensional lumber members), we back-calculate a suggested depth that would provide that section modulus.

The result is a sense of how deep a beam in a given width class might need to be for the input span and uniform load. It’s intentionally conservative and simplified—not a full structural design or a replacement for span tables.

Internally, the math is done in consistent units and rounded for display; real lumber sizes, species, grades, and engineered products will have their own published properties that should be used for final design.

Formula

For a simply supported, uniformly loaded beam:\n\n1. Maximum moment M ≈ wL² ÷ 8, where w is uniform load (lb/ft) and L is span (ft).\n2. Assume allowable bending stress Fb (psi).\n3. Base section modulus S_base ≈ M ÷ Fb (converted to consistent units).\n4. Apply safety/headroom factor: S_required ≈ S_base × (1 + headroom%).\n5. Assuming width b ≈ 1.5 in (typical lumber), approximate depth d from S_required ≈ (b × d²) ÷ 6 and solve for d.

When to use it

  • Early sizing conversations with clients or team members before requesting stamped engineering or detailed span calculations.
  • Comparing how changes in span or load (for example, adding tributary width or heavier finishes) might push you toward deeper or engineered beams.
  • Quick checks for planning material takeoffs and budget estimates when you need a ballpark beam depth but not a final design yet.
  • Evaluating whether a proposed beam will likely be in the realm of conventional dimensional lumber or if engineered lumber (LVL, PSL, glulam) is more realistic.
  • Teaching junior designers or students how span, load, and allowable stress interact in a very simplified beam sizing context.

Tips & cautions

  • Use conservative load assumptions when in doubt—slightly overestimating load is safer for planning than underestimating it.
  • Remember that different species and grades of lumber have different allowable bending stresses (Fb). This tool assumes a generic value; your engineer may use different numbers that change the required section modulus.
  • For long spans or heavy loads, expect the suggested depth to push you toward engineered products even if the calculator shows a dimensional lumber depth that seems close.
  • Pair this helper with joist span and tributary width tools to estimate the total uniform load feeding into a given beam from floor or roof framing.
  • Always cross-check any rough sizing against local code requirements, manufacturer span tables, and professional judgment before ordering materials.
  • Assumes a simply supported beam with uniform load and no overhangs, point loads, or complex loading patterns.
  • Uses a generic allowable bending stress and simple wL²/8 bending model; it does not check shear, deflection, vibration, or bearing.
  • Applies a uniform safety/headroom factor but does not incorporate full code load combinations or load duration factors.
  • Does not differentiate between wood species, grades, or engineered products—all of which have unique design values and span capabilities.
  • Not suitable for final design, code compliance, or any situation where failure could cause injury or significant property damage.

Worked examples

10 ft span, 100 plf, 25% headroom

  • Span L = 10 ft, load w = 100 plf, headroom = 25%.
  • Compute maximum moment M ≈ wL²/8 ≈ 100 × 10² ÷ 8 = 1,250 ft‑lb (converted internally to in‑lb).
  • Estimate base section modulus from M and Fb, then apply 25% headroom to get S_required.
  • Back‑calculate an approximate depth assuming 1.5 in width. The helper suggests a depth that gives a rough sense of beam size to discuss with your engineer.

12 ft span, 150 plf, 30% headroom

  • Span L = 12 ft, load w = 150 plf, headroom = 30%.
  • Enter these values and review the larger required section modulus and deeper suggested beam.
  • Interpretation: the jump in span and load significantly increases beam demand, which may point you toward engineered products.

Sensitivity to headroom percentage

  • Keep span and load fixed and change headroom from 15% to 40%.
  • Observe how required section modulus and suggested depth increase as you add more safety margin.
  • Use this to understand how conservative assumptions can impact rough beam sizing, then confirm details with formal design.

Deep dive

This beam size helper gives you an approximate section modulus and suggested beam depth from span, load per foot, and a safety margin so you can quickly gauge how big a beam might need to be before consulting detailed span tables or an engineer.

Enter beam span, uniform load, and headroom percentage to see a rough sizing cue for dimensional lumber width, then follow up with official design resources for final selection.

Ideal for builders, designers, and students who want a fast, conceptual feel for how span and load affect beam depth without performing full structural calculations by hand every time.

FAQs

Can I use this instead of span tables or an engineer?
No. This tool is for rough planning only. Building codes, safety, and liability require that you follow official span tables, manufacturer guidance, or stamped engineering for actual construction decisions.
Does it handle point loads, cantilevers, or multi-span beams?
No. It assumes a simple span with uniform load only. Point loads, cantilevers, and multi-span conditions require more advanced analysis and should be designed by a qualified professional.
What about deflection limits and serviceability?
This helper focuses on bending strength via section modulus. It does not check deflection, which is often the controlling factor for floors and roofs. Always verify deflection criteria with span tables or engineering.
Can I use this for steel or engineered lumber?
The general concepts carry over, but steel and engineered lumber have very different section properties and allowable stresses. Use manufacturer literature or steel design manuals for those materials.
How accurate are the suggested depths?
They are order‑of‑magnitude estimates only. Real beam sizes may end up smaller or larger once full design checks are done. Treat the output as a conversation starter, not a final answer.

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This beam size helper provides a simplified, approximate estimate of required section modulus and beam depth for conceptual planning only. It does not perform full structural design checks, does not account for all load combinations, deflection limits, or material properties, and is not a substitute for stamped engineering, official span tables, or local building code requirements. Always consult a qualified engineer, architect, or building official before relying on any beam size for actual construction.