Beam Size Calculator — Size Your LVL, Glulam & Dimensional Beams
Calculate the required beam size for floor and roof loads. Find the right LVL, glulam, or dimensional lumber beam for your span and load.
How to Use This Calculator
- Select the beam type — LVL is the most common for residential construction
- Enter the clear span (distance between supports) in feet
- Enter the tributary width — the total width of floor or roof area the beam carries
- Select the load type based on use (floor or roof with appropriate snow load)
- Enter the dead load (weight of structure itself — 10 psf is typical for light framing)
- Click "Calculate Beam Size" to see the recommended minimum beam depth and key structural values
Disclaimer: This calculator provides estimates for preliminary planning only. All structural beams should be verified by a licensed structural engineer before construction.
How the Calculation Works
w = tributary_width × (live_load_psf + dead_load_psf) // lbs per linear foot
Mmax = w × L² / 8 // max bending moment (lb-ft)
Mmax_in_lbs = Mmax × 12 // convert to lb-in for section modulus
S_required = Mmax_in_lbs / Fb // required section modulus (in³)
deflection_limit = L × 12 / 360 // L/360 for floors (inches) The beam sizing calculation starts by computing the total uniform load (w) from the tributary width and combined live plus dead load. The maximum bending moment for a simple span beam is w×L²/8. The required section modulus S is found by dividing the moment by the allowable bending stress (Fb) of the beam material. The calculator then finds the smallest standard LVL depth whose capacity exceeds both the bending and deflection requirements.
Variables:
- w: Total uniform load in lbs per linear foot — tributary width times total psf load
- L: Clear span of the beam in feet — measured between supports
- Mmax: Maximum bending moment at mid-span for a simple span beam (lb-ft)
- Fb: Allowable bending stress for the beam material — 2600 psi for LVL 1.9E
- S_required: Required section modulus in cubic inches — determines minimum beam depth
- tributary_width: Width of floor or roof area contributing load to the beam — typically half the joist span on each side
Pro Tips
- Always have a structural engineer review beams carrying more than 2 stories or unusually large spans
- LVL beams are more consistent and stronger than dimensional lumber of the same size
- Use multiple plies of LVL rather than going to the next width — easier to handle on site
- Beam bearing must be adequate — minimum 3" on each end for LVL
- This calculator provides estimates only — always verify with a licensed engineer for structural applications
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Frequently Asked Questions
What size LVL beam do I need for a 20-foot span?
It depends on the load. A 20-ft span carrying a typical floor load (40 psf live + 10 psf dead) with a 12-ft tributary width typically needs a 3.5" × 14" or 3.5" × 16" LVL. Always verify with a structural engineer or the manufacturer's span tables.
Can I use dimensional lumber instead of LVL?
Yes, for shorter spans and lighter loads. Dimensional lumber has more natural variability in strength, so LVL is preferred for longer spans and critical structural applications where consistent performance is required.
How many plies of LVL do I need?
Most residential beams use 1–3 plies of 1.75"-wide LVL to match wall thickness. A 2-ply LVL is 3.5" wide (matching a 2×4 wall), and a 3-ply LVL is 5.25" wide. Plies are fastened together with nails or bolts per the manufacturer specification.
Does my beam need to be engineered?
Any beam carrying a significant structural load — especially supporting floor or roof loads over occupied spaces — should be verified by a licensed structural engineer or through the manufacturer's published span tables. Most jurisdictions require stamped drawings for large beams.
What is tributary width?
Tributary width is the total width of floor or roof area that contributes load to the beam. For a beam in the middle of a floor, it is typically half the joist span on each side. For example, if joists span 12 ft on each side of the beam, the tributary width is 12 ft (6 ft each side).