On Center Spacing

The On Center Spacing calculator computes the even spacing between a number (n) of objects (e.g. posts, studs, joists, rafters, trusses, lights, rebars) over a specified length (L) or distance.

INSTRUCTIONS: Choose units and enter the following:

• (L)  Length over which the objects are spread (e.g., a fence that is 240 feet long)
• (n) Number of objects being spread over the distance (e.g., 31 posts)

On Center Spacing (oC): The calculator returns the even separation in inches.  However, this can be automatically converted to other length units (e.g. feet or meters) via the pull down menu.

The Math / Science

The on center calculation is fairly easy.   However, people typically fail to account for the first object.  So, if you want to spread 4 posts (n) out to create a fence 24 feet long (L), you can not spread them out where spread = 24 ft/4.  This forgets that the first post is at position zero.    You a have to reduce the number of posts by one for the first post.  In this case, the calculation would be 24 ft/3 (3 = n-1), which 8 ft separation.  The formula for on center spacing is

oC = L / (n-1)

where:

• oC = on center spacing
• L = length 
• n = number of objects

Many applications of spacing can be seen in life.  Four common spacing examples are fences, floor joist, the spacing of studs and the spacing of rebar in a slab.

Spacing of joists

Spacing of studs

Spacing of rebar