Type your deck's size, where the beam sits and the beam you plan to use. You get the most its posts may be apart from the building code's own beam tables, how many posts that is, every other beam size for the same deck, and whether your joists span it. Your city has the last word on the snow load.
Your deck's posts
Posts up to 8 ft 7 in apart3 posts on a 16 ft beam, 8 ft apart · 2-ply 2×10 southern pine · 40 psf · IRC 2021 Table R507.5(1)
Quick answer: how far apart deck posts can be depends on the beam they hold up, the lumber, the snow load and how long the joists are. On a 16 × 12 ft deck with joists 1 ft past the beam, a beam of two southern pine 2×10s may span 8 ft 7 in between posts (IRC 2021 Table R507.5(1)). Three posts, 8 ft apart, carry it.
How to use the deck span calculator
- Type the deck’s width along the beam and its depth out from the house (feet, or meters with the toggle), and whether it hangs on a ledger board or stands on its own beams.
- Where the beam sits: how far the joists run past it. 0 puts the beam at the edge of the deck.
- The beam you plan to use: one, two or three boards nailed together, and their size, in southern pine, fir or cedar. Then your snow load.
- Your joists and their spacing, so the calculator can check they reach the beam too. You get the post spacing, how many posts, every other beam for the same deck, and a link that hands the spacing to the deck footing calculator. Copy link keeps your deck.
The code’s rule: the beam span depends on the joists it carries
The IRC 2021 says “Maximum allowable spans for wood deck beams, as shown in Figure R507.5, shall be in accordance with Tables R507.5(1) through R507.5(4).” There is one table per load: 40 psf, then 50, 60 and 70 psf of ground snow, because “Decks shall be designed for the live load required in Section R301.5 or the ground snow load indicated in Table R301.2, whichever is greater.”
Each table is read by the joists the beam holds up. Longer joists put more deck on the beam, so its posts come closer. The tables use an effective joist span: “For calculation of effective deck joist span, the actual joist span length shall be multiplied by the joist span factor in accordance with Table R507.5(5).” That factor is 0.66 when the joists stop at the beam and rises to 1.00 when they run past it by a quarter of their span. A 12 ft joist with no overhang is read as about 8 ft; with 3 ft past the beam it is read as 12 ft. The calculator takes the factor of the row at or above your deck, never a smaller one.
| Beam | 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|---|
| 1 – 2×6 | 4-7 | 4-0 | 3-7 | 3-3 | 3-0 | 2-10 | 2-8 |
| 1 – 2×8 | 5-11 | 5-1 | 4-7 | 4-2 | 3-10 | 3-7 | 3-5 |
| 1 – 2×10 | 7-0 | 6-0 | 5-5 | 4-11 | 4-7 | 4-3 | 4-0 |
| 1 – 2×12 | 8-3 | 7-1 | 6-4 | 5-10 | 5-5 | 5-0 | 4-9 |
| 2 – 2×6 | 6-11 | 5-11 | 5-4 | 4-10 | 4-6 | 4-3 | 4-0 |
| 2 – 2×8 | 8-9 | 7-7 | 6-9 | 6-2 | 5-9 | 5-4 | 5-0 |
| 2 – 2×10 | 10-4 | 9-0 | 8-0 | 7-4 | 6-9 | 6-4 | 6-0 |
| 2 – 2×12 | 12-2 | 10-7 | 9-5 | 8-7 | 8-0 | 7-5 | 7-0 |
| 3 – 2×6 | 8-6 | 7-5 | 6-8 | 6-1 | 5-8 | 5-3 | 4-11 |
| 3 – 2×8 | 10-11 | 9-6 | 8-6 | 7-9 | 7-2 | 6-8 | 6-4 |
| 3 – 2×10 | 13-0 | 11-2 | 10-0 | 9-2 | 8-6 | 7-11 | 7-6 |
| 3 – 2×12 | 15-3 | 13-3 | 11-10 | 10-9 | 10-0 | 9-4 | 8-10 |
| Beam | 6 ft | 8 ft | 10 ft | 12 ft | 14 ft | 16 ft | 18 ft |
|---|---|---|---|---|---|---|---|
| 1 – 2×6 | 4-1 | 3-6 | 3-0 | 2-8 | 2-5 | 2-3 | 2-1 |
| 1 – 2×8 | 5-6 | 4-8 | 4-0 | 3-6 | 3-2 | 2-11 | 2-9 |
| 1 – 2×10 | 6-8 | 5-10 | 5-1 | 4-6 | 4-1 | 3-9 | 3-6 |
| 1 – 2×12 | 7-9 | 6-9 | 6-0 | 5-6 | 5-0 | 3-9 | 3-6 |
| 2 – 2×6 | 6-1 | 5-3 | 4-9 | 4-4 | 3-11 | 3-7 | 3-3 |
| 2 – 2×8 | 8-2 | 7-1 | 6-4 | 5-9 | 5-2 | 4-8 | 4-4 |
| 2 – 2×10 | 10-0 | 8-7 | 7-9 | 7-0 | 6-6 | 6-0 | 5-6 |
| 2 – 2×12 | 11-7 | 10-0 | 8-11 | 8-2 | 7-7 | 7-1 | 6-8 |
| 3 – 2×6 | 7-8 | 6-8 | 6-0 | 5-6 | 5-1 | 4-9 | 4-6 |
| 3 – 2×8 | 10-3 | 8-10 | 7-11 | 7-3 | 6-8 | 6-3 | 5-11 |
| 3 – 2×10 | 12-6 | 10-10 | 9-8 | 8-10 | 8-2 | 7-8 | 7-2 |
| 3 – 2×12 | 14-6 | 12-7 | 11-3 | 10-3 | 9-6 | 8-11 | 8-5 |
The table prints a single fir 2×12 at 16 and 18 ft as 3-9 and 3-6, the same as a single 2×10; the calculator keeps the code’s number and never lengthens it. The tables’ own notes: “Interpolation permitted. Extrapolation not permitted.” Between two columns the calculator interpolates and rounds down to the inch; under an effective 6 ft it reads the 6 ft column, and past 18 ft there is no answer. The tables assume “No. 2 grade, wet service factor included.” and “Beams supporting a single span of joists with or without cantilever.” The snow tables (50, 60 and 70 psf) and the cedar and redwood column are in the calculator.
How many posts: at the ends, or set in
Divide the deck’s width by the beam’s span and round up: that many gaps, one more post. The code also lets the beam run past its end posts: “Beams shall be permitted to cantilever at each end up to one-fourth of the actual beam span.” Setting the end posts in can save a post. A 12 ft beam that may span 8 ft 3 in needs 3 posts at the ends, or 2 posts 8 ft apart with the beam running 2 ft past each one.
The joists: their own span chart
“Maximum allowable spans for wood deck joists, as shown in Figure R507.6, shall be in accordance with Table R507.6.” A joist’s span depends on its size and how far apart the joists are, and “The maximum joist spacing shall be limited by the decking materials in accordance with Table R507.7.”
| Joist | Southern pine | Fir, hem-fir, SPF | Cedar, redwood |
|---|---|---|---|
| 2×6 | 9-11 / 9-0 / 7-7 | 9-6 / 8-4 / 6-10 | 8-10 / 8-0 / 6-10 |
| 2×8 | 13-1 / 11-10 / 9-8 | 12-6 / 11-1 / 9-1 | 11-8 / 10-7 / 8-8 |
| 2×10 | 16-2 / 14-0 / 11-5 | 15-8 / 13-7 / 11-1 | 14-11 / 13-0 / 10-7 |
| 2×12 | 18-0 / 16-6 / 13-6 | 18-0 / 15-9 / 12-10 | 17-5 / 15-1 / 12-4 |
The same table limits how far a joist may run past the beam, by its back span: a 2×8 of southern pine at 40 psf may run 2 ft past the beam on an 8 ft back span, and not at all on a 14 ft one (“NP = Not Permitted.”). The cantilever is checked with “L/Δ = 180 at cantilever with a 220-pound point load applied to end.” The calculator reads it for your joists.
Worked examples
- A 16 × 12 ft deck on the house, the beam 1 ft in from the edge, two southern pine 2×10s: the joists span 11 ft, read as 8.8 ft → posts up to 8 ft 7 in apart, 3 posts 8 ft apart.
- A 12 × 10 ft free-standing deck, two beams 1 ft in from each edge, two southern pine 2×8s each: the joists span 8 ft between the beams → posts up to 8 ft 3 in apart. That is 3 posts on each beam, or 2 posts 8 ft apart with the beam running 2 ft past each.
- A 20 × 14 ft deck on the house in 50 psf snow country, the joists 2 ft past the beam, three fir 2×10s: the joists span 12 ft, read as 10.8 ft → posts up to 9 ft 2 in apart, 4 posts 6.7 ft apart, or 3 posts 8 ft apart set 2 ft in from each end.
- A 24 × 16 ft deck on the house, the beam at the edge, three southern pine 2×12s and 2×12 joists at 16″: the joists span 16 ft, read as 10.56 ft → posts up to 11 ft 6 in apart, 4 posts 8 ft apart.
Common mistakes
- Using one post spacing for every beam. Posts every 8 ft is a habit, not a rule: at a 12 ft effective joist span a single southern pine 2×8 may span 4 ft 2 in, three 2×12s 10 ft 9 in.
- Reading the table by the deck’s depth. The tables read the joists’ span, corrected for how far they run past the beam. A 12 ft deck with the beam 1 ft in has 11 ft joists.
- Leaving the boards of the beam loose. “Beam plies shall be fastened together with two rows of 10d (3-inch × 0.128-inch) nails minimum at 16 inches (406 mm) on center along each edge.”
- A beam shallower than the joists, set flush. “Beam depth shall be equal to or greater than the depth of intersecting joist for a flush beam connection.”
- Forgetting the snow. In 60 psf country the same beam spans less, and its posts move closer.
Deck spans: questions people ask
How far apart should deck posts be?
As far as the beam on top of them may span for your deck, from the IRC’s beam tables: from under 3 ft for a single 2×6 to over 13 ft for three 2×12s on short joists. A double 2×10 of southern pine on 11 ft joists is about 8 ft 7 in.
How far can a deck beam span?
Tables R507.5(1) to (4) give it by the beam’s boards and size, the wood, the load and the effective joist span. At 40 psf the longest is 15 ft 3 in (three southern pine 2×12s on a 6 ft effective joist span); the beam may also run past its end posts by a quarter of the span next to it.
What size beam do I need for my deck?
The smallest one whose span reaches your post spacing. Pick the spacing your footings allow, then read the calculator’s every-beam chart for your deck: each cell is the most that beam may span.
How far can a 2×10 deck joist span?
At 40 psf, Table R507.6 allows a southern pine 2×10 14 ft at 16 inches on center (16 ft 2 in at 12 inches, 11 ft 5 in at 24 inches). Fir, hem-fir and SPF reach 13 ft 7 in at 16 inches, cedar and redwood 13 ft.
How far apart should 4×4 deck posts be?
The post size does not set the spacing; the beam does. A 4×4 post is limited by its height (Table R507.4) and has to be wide enough for the beam to bear on it; the deck footing calculator checks the post for its height.
Can deck joists overhang the beam?
Yes, by the cantilever Table R507.6 allows for the joist’s size and back span: a southern pine 2×10 at 40 psf may run 2 ft past the beam on an 8 ft back span and 3 ft 4 in on a 14 ft one. The beam tables stop at an overhang of a quarter of the joist span.
Read next
- Low deck ideas at ground level
- Patio deck designs
- Deck railing designs
- Deck skirting ideas
- Under-deck patio designs
With the posts spaced, the free deck footing calculator sizes each one’s footing and counts the concrete, the deck permit checker tells you if it needs a permit, and the 2D planner lays out its furniture.
Sources, read October 16, 2026: the International Residential Code 2021 — R507.1, R507.5 and Tables R507.5(1) to R507.5(5), R507.6 and Table R507.6 — in the ICC’s own code viewer and as published by UpCodes for Colorado’s DFPC Residential Code 2021, which adopts it without amendments. Three cells printed as typos in both (cedar and redwood three-ply 2×8 at 50 and 60 psf) are read from the IRC 2024’s reprint of the same table. Your city’s code and snow load may differ: this page is not engineering advice, and it is re-checked every six months.