Deck railing post spacing calculator
Post spacing is not a span question, it is a division question: given a run and a maximum bay, how many posts and how do you make the bays equal. The trap is counting posts as bays.
Railing run
How this is calculated
- Bays = ⌈run ÷ max spacing⌉. Bays, not posts.
- Posts from division = bays + 1, because a run with n bays has n + 1 posts including both ends.
- Actual bay = run ÷ bays, which is always ≤ the maximum.
- Total = posts from division + corner posts + (2 × gates) − any post shared with an existing corner.
Worked example
A 48 ft railing run with a 6 ft maximum, one corner and no gates:
- Bays = ⌈48 ÷ 6⌉ = 8
- Posts = 8 + 1 = 9
- Actual bay = 48 ÷ 8 = 6.0 ft — exactly at the limit, so no adjustment needed
- With the corner post: 10 posts
Change the run to 50 ft and the arithmetic forces 9 bays, an actual bay of 5.56 ft, and 10 posts before the corner. The extra post is not optional — 8 bays of 6.25 ft would exceed the maximum, and a wood top rail at that spacing sags visibly within a season.
The post that is not on the run
Three post types never appear in a simple division and are the usual source of a short material list:
- Corner posts carry two rails at 90°, so they see roughly double the load and must be anchored as such.
- Gate posts carry the hinge and latch loads, which are lateral and cyclic. A gate post that is only bolted through the rim will loosen.
- Stair posts at the top and bottom of a flight, which are separate from the level run and often need to be longer to take the rake rail.
On a deck with a stair, a gate and two corners, it is normal to add five posts to the division result. That is 20–25% more posts than the arithmetic suggests, and it is the difference between a railing that is solid and one that moves when someone leans on it.
What this does not check
- It does not set the maximum spacing. That comes from the rail system: wood top rails typically allow 6 ft, aluminum and cable systems often require 4 ft or less, and glass requires manufacturer-specific framing.
- It does not check post anchorage. A post that is spaced correctly but bolted with two lag screws into a rim joist will still fail the 200 lb load test.
- It treats the run as straight. An angled bay needs its own division and a longer rail.
- It does not include stair rails, which have their own post spacing along the rake.
Questions people actually ask
How far apart should deck railing posts be?
Six feet is the common maximum for a wood top rail, four feet for many aluminum and cable systems. The limit comes from the rail's deflection under a 200 lb concentrated load, not from the post itself.
How many posts for a 48 foot railing?
Nine if the run divides evenly at 6 feet: eight bays, nine posts. Add one for each corner and two for each gate opening.
Do I count posts or spaces when dividing a railing run?
Divide to get spaces (bays), then add one for the posts. Eight bays need nine posts. Counting posts first is the single most common layout error on a railing.
Should corner posts be larger?
Often yes, and they always need better anchorage. A corner post carries two rails meeting at 90 degrees, so the load path is not symmetrical — through-bolts or a manufactured post base are usual.