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
