Math July 13, 2026 · 8 Min Read

Beam Size Calculator – Guide & Formulas

Calculate residential wood or steel beam load distributions, tributary areas, and bending moment capacities.

Estimate wood header and support beam sizes for structural framing. Calculate tributary areas, point loads, uniform line loads, and compare recommended solid timber or multi-ply LVL/engineered dimensions.

Key Takeaway

Use the free Beam Size Calculator to calculate residential wood or steel beam load distributions, tributary areas, and bending moment capacities. Get instant results with step-by-step explanations.

How to Use the Beam Size Calculator

  1. Enter the spans of joists supported on each side of the beam.
  2. Enter the clear span length of the beam between load-bearing posts.
  3. Select building codes or design values (dead load and live load specifications).

The Formula

Line Load = TributaryWidth * (LiveLoad + DeadLoad)

Variable Definitions

  • JoistSpan: Lengths of clear floor joists supported by the beam
  • LiveLoad: Occupancy load in lbs per sq ft (typically 30-40 psf for residential)
  • DeadLoad: Self-weight of building materials (typically 10-20 psf)

Sizing a Support Beam

A basement beam spans 10 feet between posts, supporting 14-foot joists on one side and 12-foot joists on the other. Code requires 40 psf live load and 10 psf dead load.

  1. Step 1: Calculate tributary width: (14 / 2) + (12 / 2) = 7 + 6 = 13 feet.
  2. Step 2: Sum loads: 40 psf (live) + 10 psf (dead) = 50 psf.
  3. Step 3: Line load on beam: 13 ft × 50 psf = 650 lbs per linear foot (plf).
  4. Step 4: Beam total load: 650 plf × 10 ft span = 6,500 lbs. Check bending capacity against wood species tables.

Frequently Asked Questions

What is tributary width?

Tributary width is the portion of a floor or roof that transfers its weight directly to a specific supporting beam or column. Typically, it is half the distance to the next parallel support on each side.

What is LVL?

LVL stands for Laminated Veneer Lumber. It is an engineered wood product that is significantly stronger, straighter, and less prone to warping than solid sawn lumber beams.

What is the standard deflection limit for floor beams?

The standard deflection limit for floors is L/360, which means the beam should bend no more than the span length (in inches) divided by 360 under active load.