Wood Beam Span Calculator — Size, Load & Span Charts

Size a single beam the way an engineer would: tributary load in, bending / deflection / shear checks out. Sawn lumber, built-up 2× beams, 4× and 6× timbers — plus the span charts and the answer to where sawn lumber stops and LVL begins.

Last updated: October 25, 2026 · Reviewed by the Joist Calculator framing team

Wood Beam Span Calculator

One beam, one verdict — bending, deflection and shear in a single pass.

Beam build (plies × size)

Species & grade (from the stamp)

Tributary width (load it carries)

Live load (+10 psf dead)

Beam span you want to use

✓ preview: 10' 0" (120″)

Allowable beam span

8' 8"

Governing check: bending · (2) 2×10 SYP · 300 plf (40+10 psf × 6 ft tributary)

Section modulus S

42.8 in³

I = 197.9 in⁴ · Fb′ = 798 psi

Your 10' 0" span

FAIL

115% of capacity

Method: AWC No. 2 design values (same engine as every joist page), size factor by depth, repetitive-member factor 1.0, uniform load, simple span, L/360. 4× and 6× timbers use a conservative size factor of 1.0 — published timber values are typically a little higher. For point loads, irregular framing, or long spans, have the final beam sized by a licensed professional.

Engineering scope: this wood beam span calculator covers uniform-load simple spans in sawn lumber — the common residential case. Structural ridge beams, long spans, point loads, and any beam supporting a second story deserve a licensed engineer's or your local building department's review. Use the outputs for planning and sanity checks, then verify against your code's published tables.

This wood beam span calculator extends Joist Calculator's joist tools to the member every joist eventually lands on. Begin at the main calculator homepage for joist sizing, or browse the complete calculator hub for the span, load, and cost tools that surround this one. The beam check uses the same AWC design values and the same load assumptions as every joist page on the site, so a beam sized here is consistent with the joists sized there.

Beams and joists share material but not behavior. A joist is one of many: it is braced by the subfloor, shares load with neighbors through the deck or sheathing, and gets a repetitive-member bonus for it. A beam stands alone: no bonus, deeper section, concentrated reactions at its posts, and a failure mode — rolling or buckling sideways — that joists never see. That is why the same 2×10 that spans 14 ft as a floor joist is good for roughly half that as a beam carrying the same strip of floor, and why the sections below walk the beam math step by step instead of borrowing the joist table.

Why You Can't Read a Beam Span Off a Joist Table

Four structural differences separate a beam from the joists it carries. Each one pushes the beam's allowable span down — or its required section up.

Difference	Joist	Beam	Effect
Repetition	One of many at 12–24″ o.c.	A single member (or a single built-up)	Joists earn the 1.15 repetitive factor; beams get 1.0
Load width	Carries its 16″ strip	Carries half the span of every joist that lands on it	Beam load is tributary width × psf — often 300–800 plf
Bracing	Subfloor braces the compression edge	Top edge braced by joists, bottom edge bare	Beam stability depends on the connection detail
Reactions	Distributed into walls and beams	Concentrated into posts and footings	Bearing length and post caps become part of the design

Step by step

How to Size a Beam: Four Steps From Load to Lumber

The calculator above runs this exact sequence — follow it by hand once and the tool's verdicts will always be readable.

Convert the load it carries into plf

Foundation of every later step

A beam never carries the whole floor — it carries a tributary strip. Multiply the design load by the tributary width: 40 psf live + 10 psf dead on a beam with 6 ft of floor feeding each side is 50 × 6 = 300 plf. Deck beams work the same way: joists on both sides each deliver half their span to the beam.

Read the species and grade off the stamp

Decision point: the stamp is the fact

The stamp, not the receipt, decides the design values. SYP, DF-L, SPF and Hem-Fir differ by up to 35% in bending strength — a beam sized from the wrong species column is quietly undersized from day one. No. 2 grade is the table default; No. 1 and Select Structural raise capacity, No. 3 lowers it.

Check bending with the section modulus

Calculator does this live

Bending capacity is Fb′ × S, where S = b·h²/6 for the whole built-up section. Depth matters quadratically — a 2×12 carries about 4× the load of a 2×6 at the same span — while plies matter linearly. That is why the first upgrade is always depth, the second is a wider built-up.

Cap it with deflection and shear

Decision point: which check governs

The bending check passes first; L/360 deflection and the 1.5V/(b·h) shear check usually trim the answer back. The calculator reports which check governs — if deflection governs, an extra ply (which adds I) buys stiffness faster than anything else; if shear governs, the span is already generous.

Wood Beam Span Chart: SYP at 300 plf

Allowable simple spans for Southern Pine under 300 plf — that is 40 psf live + 10 psf dead spread over a 6 ft tributary, the most common deck and floor-beam case. Computed live by the same engine as the calculator, so the chart and the tool can never disagree.

Beam build	Allowable span (L/360)	Typical use
2×8	5' 0"	short openings, plate beams
2×10	6' 1"	short openings, plate beams
2×12	7' 1"	short openings, plate beams
(2) 2×8	7' 1"	standard deck & floor beams
(2) 2×10	8' 8"	standard deck & floor beams
(2) 2×12	10' 1"	standard deck & floor beams
4×6	5' 3"	low platforms, pergolas
4×8	7' 0"	short openings, plate beams
6×6	6' 8"	low platforms, pergolas

Scale the load first

Spans scale with √(1/load): double the plf and every span shrinks to about 71%. A 10 ft span at 300 plf is roughly an 8'-5″ span at 500 plf — do this division before abandoning a size.

Species moves it 20%+

These rows are Southern Pine. SPF and Hem-Fir beams span shorter at the same load; Douglas-Fir often longer. Re-run the calculator on your species before ordering — species is a bigger lever than most first-time builders expect.

Don't span to the limit

An allowable span is a code floor, not a comfort target. Beams over a living space feel best built 10–15% short of the limit — sag you can see in a ceiling line is a callback no calculation forgives.

Built-up wooden beam of three plies bolted together spanning between posts on a deck frame

Built-up plies, bolted pattern, rated caps — the beam before the joists arrivePhoto: Unsplash

Four Beam Scenarios, Four Different Load Paths

The same section behaves differently depending on what feeds it and what it feeds. Find your scenario, then carry its tributary rule back to the calculator.

🪟Most common

Header over a wall opening

A header carries the floor or roof load above a window or door — its tributary width is half the span of whatever bears on it, narrowed by the opening. Interior 2×6 and 2×8 headers cover most 3–6 ft openings; the real errors are bearing length (need solid trimmer studs, not nails) and forgetting the load is doubled when the floor above is a second story.

Shimming a header to width with OSB strips instead of ordering the right build-up — the bearing rides on the shims.

🏠Different animal

Ridge beam (structural)

A structural ridge beam runs at the roof peak and supports rafter ends, halving rafter spans and killing ceiling-joist tie requirements — but every ridge beam load lands on posts that must run to foundation. Ridge beams see concentrated loads and need engineered sizing; the ridge board (non-structural) is a different, much lighter part.

Setting a structural ridge on posts that stop at a floor — the load path must reach the ground or it is not a ridge beam.

🌿Deflection rules

Pergola & patio beams

Pergola beams carry light dead load plus occasional snow — strength is rarely the issue, sag and appearance are. Size them one stiffness class tighter than the numbers require (L/240 or better visually), and remember a 6×6 or doubled 2×10 that looks right often is right: pergola spans of 10–14 ft are routine.

Routing joists to hang beside the beam instead of on top — the drop-hanger detail needs a flush beam sized for the hanger's bearing.

🪚Pairs with the deck page

Deck beam under joists

A deck beam is a floor beam with wet service and no ceiling to hide the sag. Built-up treated 2× lumber (two or three plies) is the default answer; 4×6 and 6×6 appear on low platforms. Cap every ply-to-post connection with a rated post cap and bolt the plies together so they act as one section.

Nailing plies with a few 16d nails instead of the two-row staggered bolt pattern — unconnected plies each carry load as separate, weaker beams.

Where Sawn Lumber Stops: Reading an LVL Span Table

Every sawn chart has a wall — for residential floor loads it lands between 12 and 16 ft. Past that wall the material changes: LVL (laminated veneer lumber) and glulam carry spans sawn lumber cannot, and they are sized from manufacturer tables, not universal code tables.

Find the load table first

LVL tables are printed per application — floor, roof, flush header — and per design value (E, Fb). Grab the table that matches your job before reading any span; the same LVL prints different numbers per table.

Match span row to load column

Spans run down the left, loads across the top in plf (or psf-plus-tributary). Your 300 plf from Step 1 reads straight into the grid; the intersection gives a depth × ply answer like 11⅞″ × 2.

Check bearing and holes

The fine print governs: minimum end bearing (usually 3–4½ in.), bearing length at intermediate posts, and the allowable hole chart for plumbing runs. The span number is meaningless without those two footnotes.

Why no LVL numbers are printed here: LVL span tables are manufacturer-specific — the same nominal 11⅞″ beam varies 10–20% between brands and product lines. Any generic LVL span chart is a guess with a logo on it. Take your span, load, and bearing conditions to a supplier's table (free at any lumberyard counter) and use those values.

Beam Installation Checklist

0/8 confirmed

Sizing is half the job. Tap each item as you verify it — the list matches the assumptions the calculator makes about your beam.

Beam Span Questions, Answered With Numbers

Q1What size beam do I need for a 20 foot span?▼

For a 20 ft span under ordinary floor or deck loads, sawn dimensional lumber is out: even a 3-ply 2×12 SYP build-up tops out around 11–13 ft at 300 plf, and no standard depth covers 20 ft. The answer is an engineered beam — typically an LVL or glulam 14–18 in. deep, sized from the manufacturer's span tables with your exact plf. As a sizing exercise: 300 plf over 20 ft puts 180,000 lb-in of moment on the beam, demanding roughly 226 in³ of section modulus in sawn- lumber terms — two and a half times the strongest build-up on the chart above — which is LVL territory. Take the 20 ft span and your load to an LVL supplier's table (or their counter) — the wood beam span calculator above will show you precisely where sawn lumber stops.

Q2How far can a 4×6 or 6×6 beam span?▼

At 300 plf (40+10 psf over a 6 ft tributary), the chart above computes a 4×6 around 5 ft and a 6×6 around 6 ft before deflection pulls them back — honest numbers for deck beams at that load. The 6×6's real advantage is post continuity, not span: the same 6×6 that spans 6 ft as a beam is the post you want under it. Pergolas carry far less load (often under 100 plf), which is why 4×6 and 6×6 pergola beams routinely span 10–14 ft — same beam, one third the load.

Q3How do I read a beam span table?▼

Every beam span table is three inputs deep: load (psf or plf), species/grade, and section (size or build-up). Find your load column first — most residential tables print 40/10 psf values with a tributary-width note; match your tributary or convert to plf. Then read down the species column to your section row. The printed span assumes a simple beam, uniform load, and the deflection limit the table states (L/360 standard). If any of those assumptions fail — point loads, continuous spans, roof snow — the table no longer applies as printed.

Q4Is a doubled 2×10 stronger than a single 2×12?▼

In bending, yes — and the section modulus explains it. (2) 2×10: S = 3 × 9.25²/6 = 42.8 in³. Single 2×12: S = 1.5 × 11.25²/6 = 31.6 in³ — the pair wins even before its higher size factor applies, and in stiffness it keeps winning: I of 198 in⁴ for the doubled 2×10s against 178 in⁴ for the lone 2×12. The doubled 2×10 is both stronger and stiffer. That is the general rule — two plies of the next size down beat one ply of the next size up, which is why built-up beams dominate residential framing.

Q5What is the difference between a ridge board and a ridge beam?▼

A ridge board is a spacing element: rafters from opposite sides meet against it and it carries essentially no load — a 1× or 2× board set on edge does the job. A ridge beam is structural: it supports the upper ends of rafters, which is why it needs posts at each end carrying to the foundation and why its sizing is a real beam calculation. If someone tells you to 'just size the ridge like a beam' without asking whether it bears on posts, they are describing a ridge board — and no beam sizing is needed.

Q6Can I use the calculator for point loads or a beam with a post mid-span?▼

Not directly, and that is by design. The calculator solves the uniform-load simple-span case, which covers the overwhelming majority of residential floor, deck, and pergola beams. A concentrated point load (a stair stringer landing, another beam framing in) produces roughly twice the bending moment of the same total load spread evenly — so a beam passing at 100% uniform utilization can genuinely fail a point-load check. For point loads, intermediate posts, or continuous multi-span beams, the mid-span post actually helps: it splits the span, and each segment can be checked with the tool using that segment's share of the load.

Q7How much bearing does a beam need on its posts?▼

The working minimum is 1.5 in. of bearing per ply on wood, which is why rated post caps exist: a (2) 2×10 beam has 3 in. of bearing area, a (3) ply has 4.5 in., and the cap transfers all of it to the post without relying on end-grain crushing. Never rest a built-up beam on top of a post through toenails, and never let a single ply overhang the post edge — both details concentrate the entire reaction onto a fraction of the intended bearing.

With the beam sized, the supporting cast follows: the deck footing & post calculator sizes what a deck beam stands on, and the joist span table covers the members it carries. Keep this wood beam span calculator open while you compare bids — a contractor who cannot name the tributary width behind their beam choice is guessing at the most expensive member in the frame. The Joist Calculator homepage links every tool in the suite, and the calculator hub groups them by task so the beam check always has its neighbors one click away.