Deck Post Spacing Calculator reads IRC Table R507.5 for your beam and joist span, then counts the posts per beam, spaces them evenly and checks post height, load and railing posts.
Pick the beam and deck size to get the IRC post spacing and post count.
Overhangs, Posts & Loads
Deck Plan With Posts
Post Spacing Chart by Beam Size
| Beam | Max Spacing | Posts per Beam | Even Spacing |
|---|---|---|---|
| Single 2×6 | 3 ft 3 in | 6 | 3 ft 0 in |
| Single 2×8 | 4 ft 2 in | 5 | 3 ft 9 in |
| Single 2×10 | 4 ft 11 in | 5 | 3 ft 9 in |
| Single 2×12 | 5 ft 10 in | 4 | 5 ft 0 in |
| Double 2×6 | 4 ft 10 in | 5 | 3 ft 9 in |
| Double 2×8 | 6 ft 2 in | 4 | 5 ft 0 in |
| Double 2×10 | 7 ft 4 in | 4 | 5 ft 0 in |
| Double 2×12 | 8 ft 7 in | 3 | 7 ft 6 in |
| Triple 2×6 | 6 ft 1 in | 4 | 5 ft 0 in |
| Triple 2×8 | 7 ft 9 in | 3 | 7 ft 6 in |
| Triple 2×10 | 9 ft 2 in | 3 | 7 ft 6 in |
| Triple 2×12 | 10 ft 9 in | 3 | 7 ft 6 in |
| Beam | Max Spacing | Posts per Beam | Even Spacing |
|---|---|---|---|
| Double 2×6 or 3×6 | 3 ft 10 in | 5 | 3 ft 9 in |
| Double 2×8 or 3×8 | 4 ft 10 in | 5 | 3 ft 9 in |
| Double 2×10 or 3×10 | 5 ft 11 in | 4 | 5 ft 0 in |
| Double 2×12 or 3×12 | 6 ft 10 in | 4 | 5 ft 0 in |
| Solid 4×6 | 4 ft 6 in | 5 | 3 ft 9 in |
| Solid 4×8 | 5 ft 11 in | 4 | 5 ft 0 in |
| Solid 4×10 | 7 ft 0 in | 4 | 5 ft 0 in |
| Solid 4×12 | 8 ft 1 in | 3 | 7 ft 6 in |
| Triple 2×6 | 5 ft 6 in | 4 | 5 ft 0 in |
| Triple 2×8 | 6 ft 11 in | 4 | 5 ft 0 in |
| Triple 2×10 | 8 ft 6 in | 3 | 7 ft 6 in |
| Triple 2×12 | 9 ft 10 in | 3 | 7 ft 6 in |
Posts for Common Deck Sizes
| Deck, Along Beam × Out | Posts | Even Spacing | Joist Column |
|---|---|---|---|
| 8 ft 0 in × 8 ft 0 in | 2 | 6 ft 0 in | 10 ft |
| 10 ft 0 in × 10 ft 0 in | 3 | 4 ft 0 in | 12 ft |
| 12 ft 0 in × 12 ft 0 in | 3 | 5 ft 0 in | 14 ft |
| Yours, 17 ft 0 in × 10 ft 0 in | 4 | 5 ft 0 in | 12 ft |
| 16 ft 0 in × 12 ft 0 in | 4 | 4 ft 8 in | 14 ft |
| 20 ft 0 in × 12 ft 0 in | 4 | 6 ft 0 in | 14 ft |
| 16 ft 0 in × 16 ft 0 in | 4 | 4 ft 8 in | 18 ft |
| 20 ft 0 in × 16 ft 0 in | 4 | 6 ft 0 in | 18 ft |
| 24 ft 0 in × 16 ft 0 in | 5 | 5 ft 6 in | 18 ft |
Post Positions to Mark
| Post | Center From Start | Near Edge | Far Edge |
|---|---|---|---|
| Post 1 | 1 ft 0 in | 9-1/4 in | 1 ft 2-3/4 in |
| Post 2 | 6 ft 0 in | 5 ft 9-1/4 in | 6 ft 2-3/4 in |
| Post 3 | 11 ft 0 in | 10 ft 9-1/4 in | 11 ft 2-3/4 in |
| Post 4 | 16 ft 0 in | 15 ft 9-1/4 in | 16 ft 2-3/4 in |
Step-by-Step Math
- Joist span between supports = 10 ft 0 in - 1 ft 0 in = 9 ft 0 in.
- Deck strip on each beam = span ÷ 2 + c + c² ÷ (2 × span) = 5 ft 6-11/16 in, so the table reads it as a 11 ft 1-5/16 in joist span, the 12 ft column.
- IRC Table R507.5, Southern Pine Double 2×10, 12 ft column: max beam span 7 ft 4 in.
- Beam between end posts = 17 ft 0 in - 2 × 1 ft 0 in = 15 ft 0 in; bays = ceil(15 ft 0 in ÷ 7 ft 4 in) = 3, so 4 posts per beam 5 ft 0 in apart.
- Heaviest post load = 5 ft 6-11/16 in strip × 5 ft 0 in spacing × 50 psf = 1,389 lb.
- Result: 5 ft 0 in Post Spacing OC.
How the Deck Post Spacing Calculator Sets Spacing
Deck post spacing is the span of the beam that sits on the posts. The deck post spacing calculator reads that span from IRC Table R507.5, the code table that sets how far each deck beam may reach between posts.
The span depends on the beam size, the lumber species and the length of joist the beam carries. A double 2×10 Southern Pine beam carrying 9 ft joists can span 8 ft 0 in, but the same beam under 14 ft joists only reaches 6 ft 9 in.
Joists that overhang the beam load it harder than their span alone suggests. The deck post spacing calculator turns the joist span and overhang into the strip of deck each beam carries.
Here s is the joist span between the ledger and the beam, c is the joist overhang past the beam, and w is the strip width. The table is then read at a joist span of 2w, so an overhang always pushes the beam into a longer joist column.
Posts per beam come from the beam length between the end posts and the allowed span. The count is rounded up, then the posts are spread evenly.
L is the beam length, cb is the beam overhang past each end post and Smax is the IRC span. Even spacing is then (L - 2cb) ÷ (N - 1), which is why the result is often shorter than the maximum.
A 17 × 10 ft Ledger Deck, Worked Through
The deck post spacing calculator opens on a 17 ft deck on a house ledger, 10 ft out, with a 1 ft joist overhang and a double 2×10 Southern Pine beam. The joists span 10 - 1 = 9 ft, and the beam carries a strip of 9 ÷ 2 + 1 + 1² ÷ 18 = 5.56 ft.
Twice that strip is 11.1 ft, which falls in the 12 ft joist column. There, IRC Table R507.5 lets a double 2×10 span 7 ft 4 in between posts.
With a 1 ft beam overhang at each end, the beam runs 15 ft between end posts. Fifteen divided by 7.33 rounds up to 3 bays, so the deck post spacing calculator places 4 posts at 5 ft 0 in on center.
That spacing uses 68% of the allowed span, with 54-1/2 in of clear space between 6×6 posts. Each middle post carries 5.56 × 5 × 50 psf = 1,389 lb.
Deck Post Spacing Chart by Beam Size
These spans come from IRC Table R507.5, as printed in the Town of Lancaster, NY copy of Section R507. They assume No. 2 lumber, 40 psf live load and 10 psf dead load, with the beam carrying joists from one side.
Southern Pine is the stronger species group, so its beams span farther. The deck post spacing calculator reads across to the joist span your beam carries, including any overhang.
| Beam | 8 ft Joists | 10 ft Joists | 12 ft Joists | 14 ft Joists | 16 ft Joists |
|---|---|---|---|---|---|
| Double 2×8 | 7 ft 7 in | 6 ft 9 in | 6 ft 2 in | 5 ft 9 in | 5 ft 4 in |
| Double 2×10 | 9 ft 0 in | 8 ft 0 in | 7 ft 4 in | 6 ft 9 in | 6 ft 4 in |
| Double 2×12 | 10 ft 7 in | 9 ft 5 in | 8 ft 7 in | 8 ft 0 in | 7 ft 6 in |
| Triple 2×10 | 11 ft 3 in | 10 ft 0 in | 9 ft 2 in | 8 ft 6 in | 7 ft 11 in |
| Triple 2×12 | 13 ft 3 in | 11 ft 10 in | 10 ft 9 in | 10 ft 0 in | 9 ft 4 in |
The second group covers Douglas fir-larch, hem-fir and spruce-pine-fir. A double 2×12 in this group spans about the same as a double 2×10 in Southern Pine, so pick this group in the deck post spacing calculator for any fir, hem-fir or SPF beam.
Redwood, western cedar, ponderosa pine and red pine form a separate IRC group with their own spans. Those beams are not covered here, so check them against the printed table.
| Beam | 8 ft Joists | 10 ft Joists | 12 ft Joists | 14 ft Joists | 16 ft Joists |
|---|---|---|---|---|---|
| Double 2×8 or 3×8 | 5 ft 11 in | 5 ft 4 in | 4 ft 10 in | 4 ft 6 in | 4 ft 1 in |
| Double 2×10 or 3×10 | 7 ft 3 in | 6 ft 6 in | 5 ft 11 in | 5 ft 6 in | 5 ft 1 in |
| Double 2×12 or 3×12 | 8 ft 5 in | 7 ft 6 in | 6 ft 10 in | 6 ft 4 in | 5 ft 11 in |
| Solid 4×10 | 8 ft 7 in | 7 ft 8 in | 7 ft 0 in | 6 ft 6 in | 6 ft 1 in |
| Triple 2×10 | 10 ft 5 in | 9 ft 4 in | 8 ft 6 in | 7 ft 10 in | 7 ft 4 in |
| Triple 2×12 | 12 ft 1 in | 10 ft 9 in | 9 ft 10 in | 9 ft 1 in | 8 ft 6 in |
The longest span in the table is 15 ft 3 in, for a triple 2×12 Southern Pine beam carrying 6 ft joists. Past that, or past 18 ft of joist span, the deck post spacing calculator stops and the beam needs an engineer.
Beams may also run past the end posts. The IRC caps that beam cantilever at one-fourth of the adjacent span, so posts 5 ft apart allow up to 1 ft 3 in of overhang. The deck post spacing calculator flags any end overhang past that limit.
How Many Posts a Deck Needs
A deck on a ledger needs one beam line of posts. A freestanding deck needs a second beam near the house, so the deck post spacing calculator doubles the post count for it.
A deep ledger deck can add a middle beam instead, laid out as Ledger + 2 Beams in the deck post spacing calculator. That beam carries joists from both sides, so it takes half the total joist span and sets the spacing for both beam lines.
The chart uses the deck post spacing calculator defaults of 1 ft joist and beam overhangs with Southern Pine beams. Each cell gives posts per beam and the even spacing between them.
| Deck, Along House × Out | Ledger Deck, Double 2×10 | Ledger Deck, Double 2×12 | Freestanding, Double 2×10 | Largest Footing, Double 2×10 |
|---|---|---|---|---|
| 8 × 8 ft | 2 at 6 ft 0 in | 2 at 6 ft 0 in | 4 | 14 in |
| 10 × 10 ft | 3 at 4 ft 0 in | 2 at 8 ft 0 in | 4 | 15 in |
| 12 × 10 ft | 3 at 5 ft 0 in | 3 at 5 ft 0 in | 6 | 15 in |
| 10 × 12 ft | 3 at 4 ft 0 in | 2 at 8 ft 0 in | 6 | 15 in |
| 12 × 12 ft | 3 at 5 ft 0 in | 3 at 5 ft 0 in | 6 | 16 in |
| 16 × 12 ft | 4 at 4 ft 8 in | 3 at 7 ft 0 in | 6 | 16 in |
| 12 × 16 ft | 3 at 5 ft 0 in | 3 at 5 ft 0 in | 6 | 17 in |
| 16 × 16 ft | 4 at 4 ft 8 in | 3 at 7 ft 0 in | 8 | 16 in |
| 20 × 12 ft | 4 at 6 ft 0 in | 4 at 6 ft 0 in | 8 | 16 in |
| 24 × 16 ft | 5 at 5 ft 6 in | 5 at 5 ft 6 in | 10 | 18 in |
A deeper beam can remove a post. On a 10 × 10 ft ledger deck, a double 2×10 needs 3 posts 4 ft apart, while a double 2×12 spans the full 8 ft with only 2. Try the next beam size up in the deck post spacing calculator before you add a footing.
Spacing above is set by the beam alone. To lay out posts at your own maximum, such as 6 ft, enter that limit instead of a beam size, and the deck post spacing calculator suggests the lightest IRC beam that covers it.
A 12 × 20 ft attached deck with the 20 ft side on the house needs 4 posts at 6 ft 0 in on one double 2×10 beam, with each middle post carrying about 1,964 lb.
If the 20 ft runs out from the house with a 1 ft overhang, even a middle beam carries 19 ft of joist load. That is past the 18 ft end of the IRC table, so the deck post spacing calculator stops there.
Stretch the joist overhang to 2 ft and the middle beam carries exactly 18 ft. Double 2×10 Southern Pine beams then take 3 posts each, 5 ft apart, for 6 posts on the 12 ft side.
Deck Post Size and Height
Post size does not change the allowed spacing, because IRC Table R507.5 limits the beam, not the post. Post size limits how tall the post can be under IRC Table R507.4.
A 4×4 can reach 8 ft under a one- or two-ply beam, but only 6 ft 9 in under a three-ply beam on a post cap.
A 4×6 reaches 8 ft, and a 6×6 or 8×8 reaches 14 ft, measured from the footing to the underside of the beam. The deck post spacing calculator checks your post height against these limits.
The American Wood Council deck guide, DCA 6, goes further and calls for 6×6 posts as a minimum. AWC engineer John Showalter notes that a 4×4 would often be overstressed in those details.
Actual post sizes are smaller than their names. A 4×4 measures 3-1/2 × 3-1/2 in, a 4×6 measures 3-1/2 × 5-1/2 in, a 6×6 measures 5-1/2 × 5-1/2 in and an 8×8 measures 7-1/2 × 7-1/2 in.
The deck post spacing calculator subtracts that width from the on-center spacing to give the clear gap between posts. It also picks the stock post length to buy, rounding a 6 ft post height up to 8 ft lumber.
Load on Each Deck Post and Its Footing
Each post carries the deck area between it and its neighbors. That area times 50 psf, the IRC 40 psf live load plus 10 psf dead load, gives the post load.
Middle posts carry a full bay, while end posts carry half a bay plus the beam overhang. On a 16 × 16 ft ledger deck with a double 2×12, the middle post carries about 2,987 lb.
That load sets the footing size, and the deck post spacing calculator sizes it for the busiest post.
On 1,500 psf clay, IRC Table R507.3.1 calls for a 19 in round footing under that 2,987 lb post, and a 15 in footing under the 1,389 lb post in the worked example.
The Deck Footing Calculator sizes every footing for firmer soils, snow loads and concrete bags. It uses the same beam layout, so the two tools agree on post count.
Deck Railing Post Spacing
Railing posts on a wood deck are usually set no more than 6 ft apart on center. Guards must resist a 200 lb load at the top, and wider spacing needs a stiffer rail system rated for it.
The deck post spacing calculator warns when you set a railing max above 6 ft, since few wood rail systems are rated for a longer bay.
Guards are required where the deck surface is more than 30 in above grade. Openings must stop a 4 in sphere under IRC R312.1.3, which sets the baluster count in each bay.
For railing runs, the deck post spacing calculator sets the end posts flush with the run ends, then splits the rest into equal bays. Balusters are 1-1/2 in wide in the chart below, with every gap under 4 in.
| Railing Run | Posts | On Center, 4×4 Posts | Balusters, 4×4 Posts | Balusters, 6×6 Posts |
|---|---|---|---|---|
| 6 ft | 2 | 5 ft 8-1/2 in | 12 | 11 |
| 8 ft | 3 | 3 ft 10-1/4 in | 16 | 14 |
| 10 ft | 3 | 4 ft 10-1/4 in | 20 | 18 |
| 12 ft | 3 | 5 ft 10-1/4 in | 24 | 22 |
| 14 ft | 4 | 4 ft 6-13/16 in | 27 | 27 |
| 16 ft | 4 | 5 ft 2-13/16 in | 33 | 30 |
| 20 ft | 5 | 4 ft 11-1/8 in | 40 | 36 |
In the deck post spacing calculator, a 16 ft railing run needs 4 posts at 5 ft 2-13/16 in on center with 4×4 posts. Each bay holds 11 balusters, for 33 in all, with gaps of about 3-9/16 in.
The Baluster Calculator counts balusters for a whole deck perimeter. The Spindle Spacing Calculator finds the exact gap for any spindle width.
Laying Out and Squaring Deck Posts
Mark every post center from the same end of the beam line. Measuring each bay from the last post adds up small errors, while marking 1 ft, 6 ft, 11 ft and 16 ft from one end keeps the 17 ft example exact.
The deck post spacing calculator gives each mark, plus the near and far edge of every post. Those edges are where the post bases go on the footings.
To set 4 posts square, measure both diagonals of the rectangle they form. When the diagonals match, the corners are 90°.
The 3-4-5 rule checks a single corner. Mark 3 ft along one string line and 4 ft along the other, and the two marks should sit exactly 5 ft apart.
The 6-8-10 version is the same triangle doubled, and it is more accurate on larger decks. The 3-4-5 Rule Calculator finds the diagonal for any side lengths.
Joist Spacing and Deck Boards
Joists are most often set 16 in on center. A 12 × 12 ft deck at 16 in on center needs 10 joists, counting one at each end, and the deck post spacing calculator counts 14 joists along the 17 ft example.
Joists may overhang the beam by no more than one-fourth of their back span under IRC R507.6. On a two-beam ledger deck, the back span is the joist length between the two beams.
Joist size depends on span. Under IRC Table R507.6 at 16 in on center, a Southern Pine 2×6 spans 9 ft 0 in and a 2×8 spans 11 ft 10 in.
For a deck that reaches 10 ft or more from the house, 2×8 or larger joists are the safe choice. The Floor Joist Calculator checks other sizes and spacings.
Deck boards need small gaps for drainage and wood movement. Most installers leave about 1/8 in, and composite boards follow the gap printed in the maker's instructions.
The Decking Calculator counts boards and screws from the deck's square feet. The Wood Beam Span Calculator covers beams outside the deck table.
Deck Post Spacing Calculator Answers
The answers below use the deck post spacing calculator defaults of Southern Pine beams, 1 ft joist and beam overhangs and 6×6 posts unless stated.
What is the recommended spacing for deck posts?
Most decks end up with posts 4 to 8 ft apart, but the beam sets the real limit. A double 2×10 Southern Pine beam under 12 ft joists can span 7 ft 4 in, while a double 2×12 reaches 8 ft 7 in.
How far apart can 4x4 deck posts be spaced?
As far as the beam allows, since post size does not change beam span. A 4×4 is limited in height instead, to 8 ft under a one- or two-ply beam, and many inspectors follow DCA 6 and ask for 6×6 posts.
How far can you space 6x6 posts for a deck?
Up to the beam's IRC span. In the deck post spacing calculator, a triple 2×12 Southern Pine beam with 8 ft joists lets 6×6 posts sit 13 ft 3 in apart, and the posts themselves can be up to 14 ft tall.
How many posts for a 10x10 deck?
A 10 × 10 ft deck on a ledger needs 2 posts with a double 2×12 beam, or 3 posts 4 ft apart with a double 2×10. A freestanding 10 × 10 ft deck needs 4 posts on two beams, the same count the deck post spacing calculator gives with either beam.
How many posts do I need for a 12x20 attached deck?
Run the 20 ft side along the house and the deck needs 4 posts 6 ft apart on one beam, each on a 16 in footing in clay. Turned the other way, it needs a middle beam and a 2 ft joist overhang, which gives 6 posts 5 ft apart on two double 2×10 beams.
What size footing for a 4x4 deck post?
The footing is sized by load, not by post size. The IRC minimum is a 14 in round or 12 × 12 × 6 in square footing, which carries up to 20 sq ft of deck on 1,500 psf soil.
What is the best way to set deck posts?
Set each post on a concrete footing with a metal post base, which gives the lateral restraint IRC R507.4.1 asks for. The other option is to embed the post at least 12 in into soil or a concrete pier.
How deep should deck posts be set?
Embedded posts go at least 12 in deep, and the footing under them must sit below the frost line. In cold regions that means 36 to 48 in, so check your local frost depth.
Can deck posts be notched for the beam?
Only to an approved detail. Showalter notes that notching a post to seat a 3×, 4× or two-ply beam goes past the notching limits for wood, so a post cap connector is the simpler route.
Do deck posts need to be pressure-treated?
Posts near or in the ground must be pressure-treated or naturally decay-resistant wood. Buy lumber rated for ground contact, which carries a heavier preservative than above-ground stock.
Will I need additional posts for a raised deck?
Height does not change post spacing, but it changes post size. Above 8 ft, switch from 4×4 to 6×6 posts, and past 14 ft the posts need an engineered design.
Can I add more posts later if needed?
Yes, and each new post shortens the beam span. Every added post needs its own footing below the frost line and a connector at the beam. Enter the new post count in the deck post spacing calculator to see the shorter spacing.
What is the 30cm rule for decking?
It is a UK planning rule, not a US code. Decking more than 30 cm above the ground needs planning permission in England, while US decks follow the IRC 30 in guard rule instead.
Should you leave gaps between decking?
Yes, so water drains and the boards can move. Wet pressure-treated boards are often laid tight because they shrink, while dry and composite boards get a set gap.
Is the deck post spacing calculator a structural design?
It applies the IRC prescriptive tables for typical decks at 40 psf live load and 10 psf dead load. Hot tubs, roofs and snow over 40 psf fall outside those tables and need an engineer's design.
How does local regulation affect post spacing?
Most US towns adopt the IRC with local changes, often for snow load and frost depth. Snow above 40 psf shortens beam spans, and the deck post spacing calculator warns when your ground snow load passes that basis.
Do I need a permit for a deck?
Most decks need one. The IRC exempts only decks under 200 sq ft that sit no more than 30 in above grade, are not attached to the house and do not serve a required exit door.
Does the deck post spacing calculator work for Canada?
The layout math and metric units work anywhere, but the spans are from the US IRC. Canadian decks follow provincial codes based on the National Building Code of Canada, which have their own span tables.
Is there a deck post spacing calculator app?
The deck post spacing calculator runs in any phone browser with no install. Tape marks print in feet, inches and sixteenths, so you can lay out posts on site from your phone.