Deck Beam Span Calculator

Beam size, post spacing and footing size from IRC span tables

Project Details
Enter your measurements and requirements
Results
Calculated estimates for your project
Beam2-ply 2x10 Douglas fir-larch, hem-fir or SPF, spans up to 7'0" (IRC Table R507.5).
Posts3 4x4 posts at 6'0" on center
Other beams that pass2-ply 2x10 (7'0"), 2-ply 2x12 (8'2"), 3-ply 2x6 (6'0"), 3-ply 2x8 (7'7"), 3-ply 2x10 (9'4"), 3-ply 2x12 (10'11")
Tributary area per interior post5.0 ft x 6.0 ft = 30 sq ft, 1,500 lb
Footings1 interior at 14 in, 2 corner at 12 in, 36 in deep
Concrete14 bags of 80 lb (6 per interior, 4 per corner hole)

Get your free build plan for these numbers

Steps, tools and a priced material list, saved to your account. Your first plan saves free. No card needed.

  • 2-ply 2x10 beam
  • 3 x 14" x 36" deep footings
  • 4x4 posts
Preview the plan first
Materials List
Recommended materials and quantities
2x10 beam plies2 at 12 ft
4x4 posts3
80 lb concrete14 bags

Pouring the footings? Get concrete per hole from the deck footing size calculator or the sonotube calculator. Joists and decking come next in the deck calculator.

Next you will need

Deck Footing Size CalculatorLumber Calculator

Show your build

Finished the deck beam span? Snap a before and after. The photo maker lines them up into one image you can post, and a free Blueprint account keeps the photos with your measurements and materials.

Make a before and afterEmbed this calculator on your blog
Project plan · built from your numbers

Deck framing plan: 2-ply 2x10 beam on 3 footings

A 12 ft 2-ply 2x10 Douglas fir-larch, hem-fir or SPF beam carrying 10 ft joists, with posts every 6 ft on 14" footings dug 36" deep. Structural work: pull a permit and get the footing holes inspected before you pour.

Materials
$482
Time
1 weekend
Level
Advanced
8 steps 9 tools 8 materials
  1. 1

    Permit, utilities and plan check

    A deck beam and footings are structural. Pull a deck permit, confirm your local frost depth (you entered 36") and the beam table your inspector uses, and call 811 to locate buried utilities before digging.

    • Apply for the deck permit
    • Call 811 and wait for marks
    • Confirm 2-ply 2x10 at 6.0 ft spacing (IRC R507.5 allows 7'0")
  2. 2

    Lay out the beam line

    Set the beam line 10 ft out from the ledger, parallel to the house. Run strings and square them with a 3-4-5 check, then mark 3 post centers no more than 6 ft apart across the 12 ft width.

    • Strings square to the house (3-4-5)
    • Mark 3 post centers
    • Measure diagonals; they should match
  3. 3

    Dig 3 footings

    Dig each hole 14" wide and at least 36" deep, below frost. Flare the bottom slightly, add a few inches of compacted gravel, and set the tube forms plumb.

    • All holes at least 36" deep
    • Bottoms undisturbed or compacted
    • Forms plumb and braced
01 · Your numbers
Beam
2-ply 2x10
Footings
3 x 14" x 36" deep
Posts
4x4
Est. cost
$482
02 · The plan

8 steps · 9 tools · 8 materials

5 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

Your plan is 60% built. Save it.

3/5

Deck framing plan: 2-ply 2x10 beam on 3 footings

Finished in 1 weekend · $482 DIY

Built for your beam of 2-ply 2x10. Your first plan saves free. No card needed.

Or save it with just your email

We send a link; open it and this plan is your first project. Your first plan saves free. No card needed.

Borrow these 9 tools from neighborsFull how-to guide: Build a deck

Joist span, beam span and post spacing

The joist span is how far the joists run to the beam. The beam span is the distance between posts, and it is what the span table limits: the longer the joists, the more load the beam carries, and the closer the posts must be. IRC R507.5 requires at least 2 plies; a single 2x is never a deck beam.

  1. Find the joist span. Measure how far the joists run from the ledger (or the other beam) to this beam. Add any joist cantilever past the beam; IRC caps it at a quarter of the joist span.
  2. Pick a post spacing. Decide the clear distance between posts. Posts = deck width / spacing, rounded up, plus 1.
  3. Read the span table. In IRC Table R507.5, find the column for your joist span and pick the smallest 2- or 3-ply beam whose span is at least your post spacing. Single-ply beams are not allowed.
  4. Size the footings. Load per interior post = (half the joist span + cantilever) x post spacing x 50 psf. Divide by your soil bearing for the footing area; corner posts carry half.

Deck beam span chart (IRC 2021 Table R507.5)

Southern pine, 40 psf live load. Max distance between posts by joist span.
Beam6 ft joists8 ft joists10 ft joists12 ft joists14 ft joists16 ft joists18 ft joists
2-ply 2x66'10"5'11"5'4"4'10"4'6"4'3"4'0"
2-ply 2x88'8"7'6"6'9"6'2"5'9"5'4"5'0"
2-ply 2x1010'4"9'0"8'0"7'4"6'9"6'4"6'0"
2-ply 2x1212'2"10'7"9'5"8'7"8'0"7'6"7'0"
3-ply 2x68'2"7'5"6'8"6'1"5'8"5'3"5'0"
3-ply 2x810'10"9'6"8'6"7'9"7'2"6'8"6'4"
3-ply 2x1013'0"11'3"10'0"9'2"8'6"7'11"7'6"
3-ply 2x1215'3"13'3"11'10"10'9"10'0"9'4"8'10"
Douglas fir-larch, hem-fir, SPF, 40 psf live load. Max distance between posts by joist span.
Beam6 ft joists8 ft joists10 ft joists12 ft joists14 ft joists16 ft joists18 ft joists
2-ply 2x65'11"5'2"4'7"4'3"3'11"3'8"3'5"
2-ply 2x87'5"6'5"5'9"5'3"4'11"4'7"4'4"
2-ply 2x109'0"7'10"7'0"6'5"5'11"5'6"5'2"
2-ply 2x1210'7"9'2"8'2"7'6"6'11"6'6"6'1"
3-ply 2x67'4"6'8"6'0"5'6"5'1"4'9"4'6"
3-ply 2x89'8"8'6"7'7"6'11"6'5"6'0"5'8"
3-ply 2x1012'0"10'5"9'4"8'6"7'11"7'4"6'11"
3-ply 2x1214'0"12'2"10'11"9'11"9'2"8'7"8'1"

No. 2 grade, wet service, 10 psf dead load. Verify against the code your building department enforces.

Worked examples

DeckBeamPostsFootings
12 x 16 ft attached, SPF, 12 ft joists3-ply 2x103 at 8'0"18 in interior, 14 in corner
16 x 20 ft freestanding, SP, 8 ft joists per beam2-ply 2x84 per beam at 6'8"14 in interior, 12 in corner

A freestanding deck has a beam at each side, so the joist span is split between two beams. Planning a hot tub? It needs its own engineered support; the tables stop at 40 psf.

Deck beam span FAQ

How far can a 2x10 deck beam span?

A 2-ply 2x10 southern pine beam spans 9'0" with 8 ft joists, 8'0" with 10 ft joists and 7'4" with 12 ft joists under IRC Table R507.5. In Douglas fir, hem-fir or SPF it is 7'10", 7'0" and 6'5".

How far can a 3-ply 2x10 beam span?

11'3" with 8 ft joists and 9'2" with 12 ft joists in southern pine; 10'5" and 8'6" in SPF or hem-fir.

How far can a double 2x12 span?

10'7" with 8 ft joists, 9'5" with 10 ft joists and 8'7" with 12 ft joists in southern pine.

What size beam for a 10, 12, 16 or 20 ft span?

Decks rarely span a beam that far between posts. With 8 ft joists in southern pine, a 3-ply 2x12 tops out near 13 ft. For 16 or 20 ft between posts you need an engineered LVL or glulam beam; otherwise add posts so each span stays under the table value.

How many posts for a 12 x 20 deck?

Along a 20 ft beam with 8 ft max spacing: 20 / 8 = 2.5, rounded up to 3 spans, so 4 posts at 6'8" on center. The beam on 12 ft joists then needs at least a 2-ply 2x10 in southern pine.

What size footing for a deck in sandy soil?

Sand or gravel bears about 3000 psf, so footings can be smaller than in clay. A post carrying 2,400 lb needs about 0.8 sq ft, a 14 in round footing, versus 18 in on 1500 psf clay. Depth is set by frost, not soil.

Can a beam cantilever past the post?

Yes, a beam may overhang its end post, generally by up to a quarter of the actual beam span, but splices must land over a post. Joists may cantilever past the beam up to a quarter of their span.

4x4 or 6x6 deck posts?

IRC R507.4 allows 4x4 posts only for short, lightly loaded decks, roughly 6 to 8 ft tall depending on load. 6x6 is required in many places, carries more and leaves room to notch the beam in. Use 6x6 for anything above about 6 ft or with a hot tub.

Can a deck hold a hot tub?

Not on the standard tables. A filled hot tub is 80 to 120 psf, far over the 40 psf the tables assume. Put it on its own slab or have the framing engineered.

Related calculators

All calculators

Deck Calculator

Full deck material list: boards, joists, beams, posts, concrete, railing and cost

Fence Picket Spacing Calculator

Calculate perfect equal spacing between fence pickets

Fence Designer

Draw your fence and get posts, materials, a stake-out sheet and a draft permit packet

Related cheat sheets

All cheat sheets

Deck Building Cheat Sheet

Deck sizing tool, IRC 2021 joist and beam span tables by species, footing size, ledger fastening, stair calculator, code diagrams and FAQs

Fence Building Guide

Post spacing, concrete footing requirements, board patterns, and gate construction

How to Refinish a Deck

Condition quiz, deck stain calculator with cost, weekend timeline, sanding and application, which-stain matrix, mistakes and FAQs

Plan the whole project

Turn this beam size into a deck framing plan

Save your spans to a project and Blueprint lists the beams, joists, posts and hardware, with the build steps in order. The calculators and cheat sheets stay free, no account required.

Plan my deck framingPlan any project with AI