Calculate how many deck posts you need and how far apart to space them along a beam, using a typical 8 ft support or 6 ft railing limit or your own.

Typical limits. Your beam size, wood and joist span set the real support spacing (IRC Table R507.5 or your plans).

Cantilever and display unit
Blank means 0. Measure from the end-post center; verify actual support-face cantilever limits separately.

Related Calculators

Deck Post Spacing Formula

The following formula is used to calculate the deck post spacing:

S = (L - 2c) / (N - 1) N = ceil((L - 2c) / M) + 1

Variables:

  • S is equal center-to-center spacing along one beam
  • L is total beam length
  • c is the equal beam overhang beyond each end-post center
  • N is the number of posts along this beam; M is the approved maximum layout spacing

Subtract both end overhangs from the beam length, then divide by the number of intervals between posts (one fewer than the post count). For count mode, round the supported center distance divided by the approved maximum layout spacing upward, then add one end post.

What is a Deck Post Spacing?

Deck post spacing is the distance between support locations along a beam. This calculator arranges post centers; it does not select a safe span. Beam material, size, grade, loads, bearing, connections and footing conditions govern the approved design. Structural clear spans and cantilevers are measured at support faces, so account for post and bearing dimensions when supplying a center-to-center layout limit.

How to Calculate Deck Post Spacing?

The following steps outline how to calculate the Deck Post Spacing.

  1. First, determine total beam length and any equal overhang beyond the end-post centers.
  2. Next, decide on the maximum spacing between the deck posts (in inches).
  3. Next, convert the maximum spacing from inches to feet.
  4. Subtract both end overhangs, divide by the approved maximum spacing and round upward to obtain the number of intervals.
  5. Add one to the interval count to include both end posts. Check the resulting equal center spacing against the approved beam layout.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Length of the deck (in feet) = 12

Approved maximum layout spacing = 6 ft; no end overhang. Count = ceil(12/6) + 1 = 3 posts at 0, 6 and 12 ft.