The Ideal Spacing below Max calculator computes the ideal even spacing over a length or distance based an a maximum spacing.
On Center Uses
Fences
Floor Joists
Wall Studs 
Rebar Mats
INSTRUCTIONS: Choose units, and enter the following:
- (L) Length to be Spanned
- (MX) Maximum Spacing
Ideal Spacing (S): The calculator returns both the number of objects and their spacing to ideally spread between the end points.
The Math
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.
