Deck Beam Span Calculator
This free deck beam span calculator sizes the beam the way a plans examiner checks it — from the joist span the beam supports, not a rule of thumb. Tell it the species, the joist backspan (or both sides for a center beam), and the post spacing you want, and it returns the smallest built-up beam (2-ply or 3-ply of 2×6 through 2×12) that reaches that spacing, using the IRC Table R507.5 / DCA 6 Table 3A span table for a 40 psf live plus 10 psf dead load.
It goes past the span table to the things that actually get flagged: the beam cantilever (up to one-fourth of the actual span you build), the tributary area that sets the post size and footing, and the beam-to-post connection — a notched 6×6 with through-bolts for a 2-ply beam, or a required post cap for a 3-ply. It also warns when a beam crosses three or more posts (no code credit for continuity), enforces the flush-beam depth rule, and flags freestanding and snow-load conditions.
Everything is grounded in IRC R507.5 (beams), R507.5.1 (bearing and cantilever), R507.5.2 (connection), R507.4 (posts), and R507.3.1 (footings), plus AWC DCA 6. It reuses the exact, verified span tables from our deck calculator. Sizing only — no pricing, no signup. For a full deck materials list, use the Deck Calculator.
Deck Beam Span Calculator
Size a deck beam and post spacing from the joist span it carries — beam size and ply, post size, footing, cantilever, and the post connection, per IRC R507.5 / DCA 6. Free, no signup.
What are you solving for?
Loads on the beam
For a ledger deck, enter the ledger-to-beam distance. For a center beam with joists both sides, enter each side — the calculator sums the half-spans per the IRC.
Beam & framing
Deck beam span table — Southern Pine #2 (PT)
Maximum beam span between posts (ft-in) by beam size and the joist span it supports, per IRC R507.5 / DCA 6 Table 3A (40 psf live + 10 dead, No. 2 grade). Change the species above to see its table.
| Beam | 6′ joist | 8′ joist | 10′ joist | 12′ joist | 14′ joist | 16′ joist | 18′ joist |
|---|---|---|---|---|---|---|---|
| 2-ply 2×6 | 6'-8" | 5'-8" | 5'-1" | 4'-7" | 4'-3" | 4'-0" | 3'-9" |
| 2-ply 2×8 | 8'-6" | 7'-4" | 6'-6" | 5'-11" | 5'-6" | 5'-1" | 4'-9" |
| 2-ply 2×10 | 10'-1" | 8'-9" | 7'-9" | 7'-1" | 6'-6" | 6'-1" | 5'-9" |
| 2-ply 2×12 | 11'-11" | 10'-4" | 9'-2" | 8'-4" | 7'-9" | 7'-3" | 6'-9" |
| 3-ply 2×6 | 7'-11" | 7'-2" | 6'-5" | 5'-10" | 5'-5" | 5'-0" | 4'-9" |
| 3-ply 2×8 | 10'-7" | 9'-3" | 8'-3" | 7'-6" | 6'-11" | 6'-5" | 6'-1" |
| 3-ply 2×10 | 12'-9" | 11'-0" | 9'-9" | 8'-9" | 8'-3" | 7'-8" | 7'-3" |
| 3-ply 2×12 | 15'-0" | 13'-0" | 11'-7" | 10'-6" | 9'-9" | 9'-1" | 8'-7" |
Simple-span values, joists from one side. Cantilever the beam up to ¼ of the actual span. Verify your locally adopted code edition.
What sizes a deck beam
A beam is sized by the load it carries — and that comes from the joist span feeding into it. This cross-section shows where the beam sits in the load path, and what “joist span” actually means when you enter it above.
Posts set the beam span. Past the end post, the beam may cantilever up to one-fourth of that span — and this is a completely separate limit from the joist cantilever (¼ of the joist span, past the beam). Mixing up the two is the most common beam-sizing mistake.
How the beam meets the post matters as much as the span. A 2-ply beam can sit in a notched 6×6 with two through-bolts; a 3-ply beam must use a post cap — notching isn't allowed for three plies. Either way a positive connection is required; bearing alone never is.
Calculation Formulas
The allowable beam span between posts is read from IRC Table R507.5 / DCA 6 Table 3A, indexed by species group, beam size and ply, and the joist span the beam supports. Round the joist span up to the next table column.
Example:
Southern Pine, 2-ply 2×12, 12-ft joist span → 8'-4" between posts.
The table assumes joists load the beam from one side. For a center beam with joists framing from both sides, sum the two joist spans and divide by two (the sum of the half-spans).
Example:
8 ft one side + 8 ft the other → look up the 16-ft column (not 8).
A beam may overhang its end post by up to one-fourth of the actual span you build between posts (2021/2024 IRC R507.5.1). Each ply must run continuous across a post that carries a cantilever.
Example:
7'-9" beam span → 1'-11" maximum cantilever past the end post.
Each post carries half the joist span times the beam span between posts. This drives the post size and footing. A center beam uses the full effective span instead of half.
Example:
12-ft joist span, 8-ft post spacing → (12 ÷ 2) × 8 = 48 sq ft per post.
IRC Table R507.4 gives the maximum post height by size and tributary area. DCA 6 requires a 6×6 minimum; the IRC still allows 4×4/4×6 at limited heights.
Example:
48 sq ft tributary, 8-ft post → 4×4 (Southern Pine) meets R507.4.
IRC Table R507.3.1 sizes the footing from the tributary area on 1,500 psf soil. Larger tributary areas need a wider footing.
Example:
48 sq ft tributary → a 23"-diameter round footing.
Standard Constants
| Constant | Value | Description |
|---|---|---|
| Table load | 40 psf live + 10 psf dead | IRC R507.5 / DCA 6 Table 3A base load. 2021+ adds 50/60/70 psf snow sub-tables (use the greater of live or snow). |
| Deflection limit | L/360 span, L/180 cantilever | Main span at L/360; cantilever at L/180 with a 220-lb point load. Same basis as the IRC tables. |
| Beam cantilever | ¼ of actual beam span | 2021/2024 IRC (R507.5.1). 2015/2018 said '¼ of the allowable span' — use the actual span you build in practice. |
| Bearing length | 1½" wood/metal, 3" concrete | Minimum beam bearing at each support, full beam width (R507.5.1). |
| Ply fastening | 2 rows 10d @ 16" o.c. | Built-up plies nailed with two rows of 10d nails at 16 inches on center along each edge; splices land over posts, staggered. |
| 3-ply connection | Post cap required | A 3-ply built-up beam must connect with a post cap — notching is not permitted for 3-ply (DCA 6). |
| Notched-post bolts | Two ½" through-bolts | A notched 6×6 carrying a 2-ply beam uses two ½-inch through-bolts with washers; field-treat the cut with preservative. |
| Continuous-beam limit | Simple span only | The table has no credit for a beam continuous over 3+ posts — size each span as an independent simple span; continuous beams need engineering. |
Note: All calculations include appropriate waste factors based on project complexity and material type. Results are estimates and should be verified by professionals before purchasing materials.
IRC R507.5 — Deck Beams(IRC R507.5 / Table R507.5)
View StandardThe prescriptive span table for wood deck beams. Gives the maximum span between posts by species, beam size/ply, and supported joist span, for a 40 psf live / 10 psf dead load.
Key Requirements:
- •Beam size and ply from Table R507.5 by supported joist span
- •Simple-span basis; joists loading from one side (sum half-spans for a center beam)
- •2021+ adds 50/60/70 psf snow tables and a joist-span-factor table R507.5(5)
- •Interpolation permitted in 2021/2024; drop to the next lower value in 2015/2018
IRC R507.5.1 — Beam Bearing & Cantilever(IRC R507.5.1)
View StandardGoverns the beam's bearing at each support and how far it may cantilever past the end post.
Key Requirements:
- •Minimum bearing 1½" on wood/metal, 3" on concrete/masonry, full beam width
- •Beam may cantilever up to ¼ of the actual beam span (2021/2024)
- •Beams and each ply continuous between bearings and across a cantilever bearing
- •Flush beams: beam depth not less than the joist depth
IRC R507.5.2 — Beam-to-Post Connection(IRC R507.5.2)
View StandardRequires a positive beam-to-post connection to prevent lateral displacement — a notched post with through-bolts or a sized post cap.
Key Requirements:
- •A positive connection is always required — bearing alone is not enough
- •Notched 6×6 (through-bolted) permitted up to a 2-ply beam; 3-ply requires a post cap
- •Manufactured post caps sized for the post and beam; bolts get washers under head and nut
- •Side-bolting an un-notched post is prohibited without engineering
AWC DCA 6 — Prescriptive Residential Wood Deck Guide(AWC DCA 6)
View StandardThe American Wood Council's deck-construction guide that the IRC deck provisions are built from. Its Table 3A matches the IRC beam table; it adds detailing DCA 6 requires beyond the code.
Key Requirements:
- •6×6 minimum posts
- •Beam tables valid for single- and two-span conditions only
- •Scope: attached, single-level decks, ≤ 40 psf snow, deck length ≤ width
- •Hot tubs, large point loads, and full freestanding lateral design are out of scope
Standards Disclaimer: Standards and codes are subject to periodic updates. Always verify current requirements with local building authorities and professional engineers before beginning construction. Links provided are for reference only.
Code Edition
2015/2018 vs. 2021/2024
The beam provisions were reorganized and refined across editions. The cantilever wording changed from '¼ of the allowable span' to '¼ of the actual span,' and 2021 added snow sub-tables and a joist-span factor.
Regional Examples:
Snow Load
Above 40 psf changes the table
The base table is 40 psf live load. In snow country, the ground snow load can exceed that, and the 2021+ IRC provides 50/60/70 psf beam sub-tables. Use the greater of live or snow.
Regional Examples:
Posts & Notching
IRC vs. DCA 6 vs. NADRA
The IRC allows 4×4/4×6 posts at limited heights and permits notching; DCA 6 requires 6×6 minimum posts; NADRA prohibits notching entirely and requires a sized connector.
Regional Examples:
Ledger vs. Freestanding
Lateral load path
A ledger deck ties to the house with hold-down devices; a freestanding deck needs a second beam line and, above 30 inches, diagonal bracing. Full freestanding lateral design is outside the prescriptive code.
Regional Examples:
Beyond the Prescriptive Table
When you need an engineer
Continuous beams over three or more supports, joists continuous over more than two supports, hot tubs and other concentrated loads, single solid 4× beams, and side-mounted beams without notching all fall outside the prescriptive table.
Regional Examples:
Before You Build
- •Contact your local building department for specific requirements
- •Verify frost line depths, wind zones, and seismic requirements for your area
- •Check if permits are required and schedule required inspections
- •Consult with a local contractor familiar with local codes
Related Calculators
Deck Calculator
Code-compliant deck framing, decking, footings, ledger, stairs & railing — built on 2021 IRC R507 and AWC DCA 6 tables. Free, no signup.
Joist Span Calculator
Free joist span calculator: max spans for floor and deck joists by species, grade, size, and spacing per IRC R502.3.1 and R507.6. No signup.
Stud Wall Framing Calculator
Size studs, plates, headers, cripples & nails for 2x4 / 2x6 walls. Per 2021 IRC R602.3, R602.7 header spans & R602.3(1) nailing. Free, no signup.
How to Use This Calculator
- Choose a mode: size a beam from the post spacing you want, or find the max post spacing for a beam you already have.
- Set the beam position — one side (ledger deck or outer beam) or a center beam with joists from both sides — and enter the joist span the beam supports.
- Pick your wood species, code edition, and beam style (drop or flush).
- Enter the post spacing you want (or the beam you have), the post height, and optionally the beam run length for a post count.
- Click Calculate for the beam size and ply, post spacing, cantilever, post size, footing, and the required beam-to-post connection.
- Check the span table below for the full IRC R507.5 values for your species.
Why the Joist Span Sizes the Beam
A deck beam is sized by how much roof — er, deck — drains onto it, and that load is set by the joist span it carries. The IRC beam table is entered with the joist span supported, because a longer joist span puts more tributary load on the beam and shortens how far it can reach between posts. A 2-ply 2×10 in Southern Pine spans about 7 feet between posts when it carries a 12-foot joist span, but only about 6 feet when it carries a 16-foot span. Two things trip people up. First, for a center beam with joists framing from both sides, you don't use one joist span — you sum both and divide by two, because the beam carries half of each side. Second, the beam cantilever and the joist cantilever are separate limits: the beam may overhang its end post by up to one-fourth of the actual beam span, while the joists may overhang the beam by up to one-fourth of the joist span. The connection matters too: a 2-ply beam can sit in a notched 6×6 post with two through-bolts, but a 3-ply beam has to use a post cap — notching is not allowed for three plies. And the prescriptive table is simple-span only: run a beam continuously over three or more posts and you get no credit for the continuity, so each span is sized on its own.
Frequently Asked Questions
How far can a deck beam span?
It depends on the beam size and ply, the wood species, and the joist span the beam carries. Per IRC Table R507.5 / DCA 6 Table 3A (40 psf live + 10 dead), a Southern Pine 2-ply 2×10 spans about 7 feet between posts at a 12-foot joist span, a 2-ply 2×12 about 8 feet, and a 3-ply 2×12 about 10.5 feet at a 6-foot joist span. Douglas Fir-Larch and cedar/redwood span a bit less. The longer the joist span the beam supports, the shorter it can reach between posts. This calculator reads the exact value for your inputs.
What size beam do I need for my deck?
Enter the joist span the beam carries and the post spacing you want, and the calculator picks the smallest built-up beam that works — 2-ply or 3-ply of 2×6 through 2×12. As a rough guide in Southern Pine at a 12-foot joist span: a 2-ply 2×10 reaches ~7 ft between posts, a 2-ply 2×12 ~8 ft, and stepping to a 3-ply buys you 2–3 more feet of spacing. Bigger joist spans, heavier snow loads, and weaker species all push you to a larger beam or closer posts.
How do I size a beam for joists coming from both sides (a center beam)?
Don't use one joist span — the IRC table assumes joists load the beam from one side only. For a center beam with joists framing from both sides, sum the two joist spans and divide by two (the sum of the half-spans), then look up that number. For example, 8 feet of joist on each side means you look up the 16-foot column, not the 8-foot column, because the center beam carries the load from both sides. Set the beam position to 'center' and the calculator does this for you.
How far can a deck beam cantilever past the post?
Up to one-fourth of the actual beam span between posts (2021/2024 IRC R507.5.1). So a beam spanning 8 feet between posts can overhang the end post by up to 2 feet. The 2015/2018 IRC worded it as one-fourth of the allowable span, but you should use the actual span you build. This is a separate limit from the joist cantilever (joists can overhang the beam by up to one-fourth of the joist span), and the beam and each ply must run continuous across any post that supports a cantilever.
How do I connect a deck beam to the post — notch or post cap?
A 2-ply beam can sit in a notched 6×6 post (leave about 2½ inches of post, use two ½-inch through-bolts with washers, and field-treat the cut), or on a post cap. A 3-ply beam must use a post cap — notching is not permitted for three plies (e.g. a Simpson BCS2-3/6 on a 6×6, rated about 895 lb uplift / 1,330 lb lateral). Bolting a beam to the side of an un-notched post is prohibited without an engineer. A positive connection is always required; bearing alone never suffices. DCA 6 also requires 6×6 minimum posts.
Can a deck beam run continuously over three or more posts?
You can build it that way, but the IRC/DCA 6 span table gives no credit for the continuity — the tables are for a simple span. Size each span between posts as an independent simple span (which this calculator does), and land any built-up ply splices directly over a post, staggered between plies, never mid-span. A true continuous-beam design that takes advantage of the extra stiffness has to be engineered. Joists continuous over more than two supports are also outside the prescriptive tables.
Is this the same as the deck calculator?
No. This calculator focuses on beam sizing — beam size and ply, post spacing, cantilever, post size, footing, and the beam-to-post connection, with the code details spelled out. The full Deck Calculator is a complete materials take-off: decking boards, joists, beam, posts, footings, ledger fasteners, hangers, and screws for the whole deck. Both use the same verified IRC span tables. Use this one to nail down the beam, then the deck calculator for the full build list.