What is the L/360 deflection limit?

L/360 means the maximum allowable deflection is the span length (L) divided by 360. For a 10-foot (120-inch) floor beam, the deflection limit is 120/360 = 0.33 inches. This is the standard for floors under live load to prevent bouncy floors and cracked finishes.

How do I calculate beam deflection?

For a simple span with uniformly distributed load (UDL), the formula is: Deflection = (5 × w × L^4) / (384 × E × I), where w = load per inch, L = span, E = modulus of elasticity, and I = moment of inertia.

What is the difference between LVL and dimensional lumber?

LVL (Laminated Veneer Lumber) is an engineered wood product that is stronger and more consistent than solid dimensional lumber. A 1.75" x 11.25" LVL beam can often replace a triple 2x12 (4.5" x 11.25") while being lighter and straighter.

Can I use this for steel beams?

Yes, but this calculator uses simplified formulas. For critical steel beam design, you should use AISC tables or consult a structural engineer. This tool is best for preliminary sizing and wood beam analysis.

What is Section Modulus (S)?

Section Modulus (S) is a geometric property that indicates a beam's resistance to bending. For a rectangular beam: S = (b × h²) / 6. Higher S means the beam can handle more bending moment before exceeding allowable stress.

📏 Beam Load Calculator

Calculate Deflection, Load Capacity & Code Compliance

Beam Material

Beam Width (inches)

Actual width (e.g., 2x8 = 1.5")

Beam Depth (inches)

Actual depth (e.g., 2x10 = 9.25")

Span (feet)

Total Load (psf or lbs)

Uniform load per sq ft or total lbs

Tributary Width (feet)

Width of floor/roof carried by beam

Deflection Limit

📐 Beam Deflection Formulas

Deflection is the amount a beam sags under load. Excessive deflection causes bouncy floors, cracked drywall, and structural concerns.

Simple Span - Uniformly Distributed Load

δ = (5 × w × L⁴) / (384 × E × I)

δ = Deflection (inches)
w = Load per inch (lbs/in)
L = Span (inches)
E = Modulus of Elasticity (psi)
I = Moment of Inertia (in⁴)

Moment of Inertia (Rectangular Beam)

I = (b × h³) / 12

For a 2x10 (1.5" x 9.25"): I = (1.5 × 9.25³) / 12 = 98.9 in⁴

💡 From James Liu, Structural Engineer (PE, 12 years):

"The most common mistake is ignoring live load deflection separately from dead load. Building codes require L/360 for LIVE load only, not total load. Also, remember that doubling the beam depth increases stiffness by 8x, while doubling the width only increases it by 2x. Go deeper, not wider, when fighting deflection."

⚠️ Common Mistakes

1. Confusing Nominal vs Actual Sizes: A '2x10' is actually 1.5" x 9.25". Using 2" x 10" in your calculation will give you completely wrong answers.

2. Forgetting the Tributary Width: A beam supporting a 10ft wide floor doesn't carry 50 psf—it carries 50 psf × 10 ft = 500 lbs per linear foot!

3. Using Total Load for Deflection Check: Live load deflection is checked separately. A floor with 10 psf dead + 40 psf live should be checked at L/360 for the 40 psf live load only.

Reviewed by James Liu, PE

Licensed Professional Engineer, 12 years specializing in residential and commercial structural design.

📐 Beam Deflection Formulas

Simple Span - Uniformly Distributed Load

Moment of Inertia (Rectangular Beam)

⚠️ Common Mistakes

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