Wood Beam Span Calculator for preliminary span comparison, tributary floor or roof loading, uniform beam load, support reactions, maximum bending moment, and estimated beam project cost.
Home & Construction Calculators
Compare a measured clear span with an allowable span obtained for the exact beam from an approved table, manufacturer, architect, or structural engineer, then calculate simple-span planning loads.
Do not guess the allowable reference span. It must match the exact sawn-lumber, LVL, PSL, glulam, or other beam product; species, grade, width, depth, plies, moisture condition, load case, deflection limit, bearing, and connection details.
About the Wood Beam Span Calculator
This wood beam span calculator compares the project’s clear span with an allowable reference span supplied by the user. It also converts area loads over a tributary width into a uniform line load and estimates total uniform load, equal end reactions, maximum simple-span bending moment, and project cost. It does not select a beam or determine code compliance.
Wood beam formulas and assumptions
Reference-span utilization = clear span ÷ allowable reference span × 100. Span reserve = allowable reference span − clear span. Uniform line load, w = tributary width × (dead load + live or snow load) + beam self-weight. Total uniform beam load, W = w × clear span. For a simply supported beam with a uniform load, reaction at each end = W ÷ 2, and maximum midspan moment = w × span² ÷ 8.
The load formulas assume one simple span, two supports, and a uniform load. They do not model point loads, multiple spans, cantilevers, unbalanced snow, drift, wind uplift, seismic effects, load combinations, notches, holes, lateral instability, or connection eccentricity.
Worked wood-beam example
A 12-ft clear span compared with a 14-ft approved reference uses 85.7% of that reference and leaves 2 ft of span reserve. A 10-ft tributary width carrying 15 psf dead load and 40 psf live load creates 550 lb/ft; adding 12 lb/ft for beam weight gives 562 lb/ft. The uniform beam load is 6,744 lb, each simple support carries 3,372 lb, and maximum moment is 10,116 lb-ft. One $650 beam plus $500 in labor, delivery, hardware, and extras totals $1,150.
Beam measurement checklist
- Measure clear span between inside support faces.
- Trace the actual tributary area delivering load to the beam.
- Separate dead, live, roof, and snow loads correctly.
- Identify point loads from posts, girders, hips, and headers.
- Confirm beam product, plies, grade, bearing, and connections.
Understanding the result
A utilization at or below 100% only means the entered project span does not exceed the entered reference span. It does not prove that the reference is correct or that bending, shear, deflection, bearing, vibration, fire resistance, connections, posts, or foundations are adequate. An over-limit result means the chosen reference cannot be used for that span without redesign.
Manual calculation steps
- Measure the clear support-to-support span.
- Find an allowable span for the exact beam and load case.
- Multiply area load by tributary width.
- Add the beam self-weight line load.
- Multiply line load by span for total uniform load.
- Divide by two for equal simple-support reactions.
- Calculate wL² ÷ 8 for maximum simple-span moment.
- Have the complete load path and member checked professionally.
Frequently asked questions
Does this calculator choose a wood beam size?
No. The allowable reference span must come from an approved source for the exact member. The calculator compares and summarizes planning loads.
What is tributary width?
It is the width of floor or roof area whose load flows to the beam. For regular framing, it is commonly half the supported joist span from each side, but the actual load path controls.
Why are the two reactions equal?
Equal reactions apply only to a simply supported beam with a uniform load and no additional point loads. Real projects can have unequal reactions.
Can I use the result for permits or construction?
No. Permit and construction documents must follow local requirements and use approved span tables, manufacturer design data, or engineering.
Common wood-beam mistakes
- Using overall beam length instead of clear span.
- Guessing an allowable span from a different product or load.
- Missing point loads or using the wrong tributary width.
- Checking bending but ignoring deflection, shear, and bearing.
- Ignoring posts, footings, hangers, fasteners, and lateral bracing.